Elements, mixtures, reactions and acids. Attnom Academy

S3.6.1 Chapter 1

Elements and compounds

Gold, water and air are all made of tiny particles. Are the particles inside them all the same kind?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Some materials have one kind of atom. Others have different kinds of atom joined together.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Every material has its own kind of tiny bit, and no two materials share one.
You picked B. Read on. At the end, the chapter comes back to your guess.
C All materials are made from the very same atoms, only arranged in different ways.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain that an element has only one kind of atom
Name common elements, such as oxygen, carbon, iron and gold
Explain that a compound has elements joined together, such as water

A gold ring, a drink of water and the air you breathe are very different. Yet all three are made of tiny particles. In this chapter you look at the particles themselves.

The smallest bits of most materials are called atoms. There are only about a hundred kinds of atom. Everything around you is built from them, a little like words built from letters.

Draw it first

Draw an element and a compound

1 Draw circles that are all the same. Write Au inside each one. This is gold. It has one kind of atom.
2 Draw a big circle with O joined to two small circles with H. Draw more groups like it. This is water. It has two kinds of atom.

Gold is an element. An element has only one kind of atom. Every atom in a gold ring is a gold atom.

Iron has one kind of atom. Oxygen gas also has one kind of atom, though they are joined in pairs. Both are elements.

Look at oxygen. Its atoms are joined in pairs. But every atom is still an oxygen atom. So oxygen is an element. Joined atoms do not make a compound. Different kinds of atom joined together do.

Each element has a symbol, one or two letters. Oxygen is O and carbon is C. Iron is Fe and gold is Au. Those two come from old Latin names. A symbol is a short way to write the name.

Four elements you meet every day.

Now think about water. Its atoms are not all alike. Each small group has one oxygen atom joined to two hydrogen atoms. Water is a compound. A compound has atoms of two or more elements joined together.

Two compounds. Water is a liquid and carbon dioxide is a gas. Each is made of small groups that are all alike. Some compounds, such as salt, are one big block of joined atoms instead.

A formula shows the atoms in one group. The small number says how many.

We write water as H₂O. The H is hydrogen and the small 2 means two atoms. The O is oxygen, one atom. Carbon dioxide is CO₂: one carbon atom and two oxygen atoms.

A compound is not like the elements inside it. Hydrogen is a gas that burns. Oxygen is a gas that helps things burn. Water, made from the two, is a liquid that puts fires out.

See it three ways

With things

Think of table salt. It is made from sodium, a metal that reacts fiercely with water, and chlorine, a poisonous gas. Yet you sprinkle it on your food. A compound can be very different from its elements.

As a picture

In words

Water is made of hydrogen and oxygen. Yet it is nothing like either. When atoms join, they make a new substance with its own properties.

Try it

Build water from paper circles

You need: paper, scissors, a pencil.

Be safe: Take care with scissors. Ask a grown up to cut if you are unsure.

Cut out 2 big circles and 4 small circles from paper. Write O on each big one and H on each small one.
Make one water group. Lay 1 big circle on the table. Put 2 small circles touching it.
Make a second group the same way. Count the circles of each kind. Then count all the atoms.

What you will see: Each group has 1 oxygen and 2 hydrogen, which is 3 atoms. Two groups use 2 oxygen and 4 hydrogen, which is 6 atoms. Every group is the same.

If you cannot try it, this is what happens.

A wrong idea many people have

Water is just hydrogen and oxygen mixed together.

Why it feels right: Water is made of hydrogen and oxygen. So it seems the two gases must sit in there side by side.

The test that breaks it: Hydrogen burns. Oxygen helps things burn. If water held the two gases side by side, it would feed a fire. But water puts fires out.

What is true: In water the atoms are joined in groups of H₂O. That makes a new substance. It does not behave like hydrogen or oxygen.

Key words

atoms: the smallest bits that most materials are made of.
element: a substance with only one kind of atom.
symbol: one or two letters that stand for an element.
compound: a substance with atoms of two or more elements joined together.

Back to your guess

The answer is A: Some materials have one kind of atom. Others have different kinds of atom joined together.

Gold has one kind of atom. Water has two kinds joined together. There are only about a hundred kinds of atom, and they build everything.

Keep it

Copy this into your exercise book.

An element has one kind of atom. A compound has different atoms joined, such as water, H₂O. A compound is not like the elements in it.

Draw: Draw one water group: a big circle O joined to two small circles H. Label it H₂O.

S3.6.2 Chapter 2

Pure substances and mixtures

A bottle says pure spring water, and the water is clear. Would a scientist call it pure?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Not always. In science, pure means only one substance, and clear water can hold others.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes. If water looks clean and clear, there is nothing else in it.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Yes. Water from the ground is always one substance.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Tell a pure substance from a mixture
Explain why sea water and air are mixtures

In shops, pure often means clean or natural. A scientist uses the word in a different way. This chapter shows you how.

Draw it first

Draw a pure substance and a mixture

1 Draw water groups, all the same. Nothing else is in the box. This is a pure substance.
2 Draw water groups, and add single circles, Na and Cl, for the dissolved salt. They are not joined to the water. This is a mixture.

A pure substance is only one substance. It is either an element, such as gold, or a compound, such as water. All its groups of atoms are the same.

A mixture has two or more substances that are not joined together. The substances in a mixture keep their own properties.

Two pure substances. Gold is an element. Water is a compound. Each box has only one kind of group.

Pure does not mean clean. Pure also does not mean natural or safe. A glass of clear tap water is a mixture. It has dissolved salts in it that you cannot see.

Air is a mixture of gases. About 78 of every 100 parts are nitrogen. About 21 parts are oxygen. The rest, about 1 part, is argon, carbon dioxide and a few other gases.

A small sample of air: 8 pairs of nitrogen and 2 pairs of oxygen. The gas particles are far apart, and the two gases are mixed, not joined.

Each gas in the air keeps its own properties. The oxygen in the air still helps things burn. The nitrogen does not. The gases are side by side, so air is a mixture.

Sea water is a mixture too. It is mostly water, with salt dissolved in it, and a little of other things. When salt dissolves, it splits into separate particles among the water groups. Its parts are not joined.

Sea water. The dissolved salt is split into separate Na and Cl particles among the water groups. This is a small drawing: real sea water has many more water groups for each Na and Cl.

See it three ways

With things

Think of a bowl of rice and beans. You can see two things, and each is still rice or beans. A compound is different. Its parts are joined, and it has new properties.

As a picture

In words

In a compound, the atoms are joined and the amounts are always the same. In a mixture, the substances are not joined, and you can have more or less of each.

Try it

Mix and take apart

You need: a bowl, a handful each of dried rice, dried beans and small stones, a spoon.

Be safe: Wash the stones and your hands afterwards. Do not eat anything from the bowl.

Pour the rice, the beans and the stones into the bowl. Stir with the spoon.
Look at the bowl. Can you still tell the three things apart?
Pick out the stones by hand. Then pick out the beans. Rice is left in the bowl. Wash your hands afterwards.

What you will see: Each thing is still itself. The rice is still rice. The beans and stones are unchanged. You can take them apart again, because a mixture has no joined parts.

If you cannot try it, this is what happens.

A wrong idea many people have

Clear water must be pure.

Why it feels right: You can see through it, and nothing in it shows. So it seems there is nothing else in it.

The test that breaks it: Sea water is clear, yet it is salty. Leave a dish of it in the sun until the water has gone. A white crust of salt is left. The salt was in the water all the time.

What is true: Pure means only one substance. Many clear liquids are mixtures, because dissolved things cannot be seen.

Key words

pure substance: a substance that is only one thing, an element or a compound.
mixture: two or more substances together that are not joined.

Back to your guess

The answer is A: Not always. In science, pure means only one substance, and clear water can hold others.

Pure means only one substance. Clear water can still have salts dissolved in it, so it is a mixture. Air and sea water are mixtures too.

Keep it

Copy this into your exercise book.

A pure substance is only one thing. A mixture has two or more things not joined. Clear does not mean pure. Air and sea water are mixtures.

Draw: Draw a box of water groups, and a second box with water groups and salt pairs. Label them pure water and mixture.

S3.6.3 Chapter 3

Separating mixtures

How can you get salt back out of sea water, or take the mud out of muddy water?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Leave the water in the sun. The water goes into the air and the salt stays behind.
You picked A. Read on. At the end, the chapter comes back to your guess.
B You can only get salt back by pouring the water through paper.
You picked B. Read on. At the end, the chapter comes back to your guess.
C You cannot get it back. Once salt dissolves, it is gone for ever.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Choose sieving, filtering or evaporating to separate a mixture
Get salt back from salty water by leaving it in the sun
Separate the colours in ink using paper and water

The parts of a mixture are not joined. That is the good news. It means you can separate them again, without any chemistry. You choose a method that suits the mixture.

Each method uses a difference between the parts. They may be different in size. One may dissolve and one may not. Or the liquid may turn to a gas while the other part stays.

Draw it first

Draw salt coming back from salty water

1 Draw a shallow dish of salty water in the sun.
2 Draw it later. The water is lower. Draw arrows rising from it. The water is leaving as water vapour.
3 Draw it later still. The water has all gone. The salt is left in the dish.

To evaporate is to turn from a liquid into a gas. The water in the dish evaporates, and its vapour goes into the air. Salt does not evaporate in the sun, so it stays in the dish. This is how salt is made from sea water in hot places.

Now a different mixture: small stones and sand. They do not dissolve, and they are different in size. A sieve is a bowl with a mesh of holes. It lets small pieces fall through and holds big pieces back.

The holes are wider than a grain of sand and narrower than a stone. The stones stay on top.

Next, muddy water. Mud does not dissolve, but it is too small for a sieve. Use a filter. Filter paper has holes so small that mud cannot get through. The mud stays on the paper.

The solid left on the paper is the residue. The clear liquid that passes through is the filtrate.

Filtering takes out a solid that does not dissolve. It cannot take out dissolved salt. Salt particles are so small that they pass through the paper with the water. The filtrate of salty water is still salty.

See it three ways

With things

Think of what each method uses. A sieve uses size. A filter uses the fact that a solid does not dissolve. Evaporating uses the fact that water becomes a gas and salt does not.

As a picture

In words

Match the method to the mixture. Sieve when the pieces are different sizes. Filter when a solid is in a liquid. Evaporate when a solid is dissolved.

One more mixture is a drop of ink. The black ink in a felt-tip pen is often a mixture of several coloured substances. Chromatography separates them. A strip of paper and a little water are all you need.

The ink spot starts above the water. If the spot were under the water, the ink would wash into the glass.

The water soaks up the paper and carries the ink with it. Each colour is carried a different distance. Some colours cling to the paper and travel only a little. Others dissolve well in water and travel far.

The colours have separated. Yellow travelled 4 cm, red 2.5 cm and blue 1 cm from the start line.

Try it

Get salt back from salty water

You need: a dark plate or a flat dish, a cup of water, a spoon of salt, a sunny, safe place.

Be safe: Do not taste the salt. Keep the plate away from small children and pets.

Stir a spoon of salt into half a cup of water until it has all dissolved.
Pour a thin layer of the salty water onto the plate. Put the plate in a sunny place where nobody will knock it.
Wait until the water has all gone. This may take a day or two. Look at the plate.

What you will see: The water disappears into the air. A white crust of salt is left on the plate. The salt was in the water all the time.

If you cannot try it, this is what happens.

Try it

Split an ink into colours

You need: a strip of coffee filter paper or kitchen paper, about 3 cm wide, a pencil, a felt-tip pen with washable ink, a tall glass, water.

Be safe: Use only washable pens. Do not drink the water.

Draw a pencil line across the strip, 2 cm from one end. Use a pencil, not a pen. Pencil does not dissolve in water, so the line stays put. Put a dot of ink on the line.
Pour water into the glass, about 1 cm deep. Hang the strip so its end dips in the water. The ink spot must stay above the water.
Wait about 10 minutes. Take the strip out and lay it flat. Look at the colours, and measure how far each one went from the line.

What you will see: The water climbs the paper. A black or brown ink often splits into several colours. Each colour stops at its own height. Your colours will differ from the picture.

If you cannot try it, this is what happens.

A wrong idea many people have

Filtering salty water takes the salt out.

Why it feels right: Filter paper takes the mud out of muddy water. So it seems it should take salt out of salty water too.

The test that breaks it: Pour salty water through filter paper. Taste is not a safe test, so leave the filtrate in a dish in the sun. A white crust of salt is left. The salt went through the paper.

What is true: A filter holds back only solids that are too big to pass through. Dissolved salt is far smaller, so it passes through. To get salt back, evaporate the water.

Key words

evaporate: turn from a liquid into a gas.
sieve: a mesh of holes that lets small pieces fall through.
filter: paper with tiny holes that holds back solid pieces.
residue: the solid left on the filter paper.
filtrate: the liquid that passes through the filter.
chromatography: a way to separate coloured substances using paper and water.

Back to your guess

The answer is A: Leave the water in the sun. The water goes into the air and the salt stays behind.

The water evaporates into the air as vapour. The salt does not evaporate, so it is left behind. Dissolved salt is still there, and you can get it back.

Keep it

Copy this into your exercise book.

Sieve to separate by size. Filter to remove a solid. Evaporate to get a dissolved solid back. Chromatography splits an ink into its colours.

Draw: Draw a funnel with paper in it, mud on the paper, and clear water in a beaker below. Label residue and filtrate.

S3.6.4 Chapter 4

Physical and chemical changes

Ice melting and wood burning are both changes. Are they the same kind of change?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Yes. In both, the stuff stays the same stuff and only its looks change.
You picked A. Read on. At the end, the chapter comes back to your guess.
B No. Melting makes no new substance. Burning makes new substances that you cannot easily turn back.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Yes. In both, a new substance forms because the thing looks different afterwards.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Sort changes into physical and chemical
List signs of a chemical change, such as gas, heat or colour

Things change all the time. Ice melts. Milk turns sour. Paper burns. Sugar vanishes in tea. In science we sort these changes into two kinds. The question to ask is whether a new substance is made.

Draw it first

Draw ice melting

1 Draw the particles of ice, in neat rows. They hold each other in place.
2 Draw the same particles as water. They are jumbled and slide past each other. They are the same particles.

Melting is a physical change. The particles are the same before and after. No new substance is made. Only the way the particles are arranged has changed.

Every change of state can go both ways. Melting can be reversed by freezing.

A physical change can often be turned back, so we say it is reversible. Freeze the water and you get ice again. Dissolve sugar in water and you can get the sugar back by evaporating. Crushing a can or cutting paper are also physical changes.

In a chemical change, the atoms are joined in new ways, and a new substance is made. The new substance has new properties. A chemical change is not easy to turn back.

Hydrogen burns in oxygen and makes water. There are 6 atoms before and 6 atoms after. The atoms are not lost. They join in new groups.

How can you tell if a change is chemical? Look for signs. These are clues that a new substance may be forming.

Four signs of a chemical change. A cut apple turns brown because it reacts with oxygen in the air.

But a sign alone is not proof. Some physical changes show the same signs. The real test is whether a new substance has been made.

A kerosene lamp burns its fuel to give light and heat, and that is a chemical change. An electric bulb gives light and heat with no new substance, and that is physical.

Each of these looks like a sign, but no new substance is made.

See it three ways

With things

Ask two questions. Is a new substance made? Can it be turned back easily? Melting ice: no new substance, and it turns back. Burning wood: new substances, and no way back.

As a picture

In words

In a physical change no new substance is made, and it can often be reversed. In a chemical change a new substance is made, and it is hard to reverse.

Try it

Fizz or no fizz

You need: two large cups, a plate, a spoon of sugar, a spoon of baking soda, water, vinegar.

Be safe: Use a big cup, because the fizz can spill over. Keep your face back from the cup. If any splashes in your eyes, rinse them with plenty of clean water. Do not taste anything. Do not seal the mixture in a bottle.

Cup A: stir a spoon of sugar into half a cup of water. Watch for bubbles and for any change in heat.
Cup B: put a spoon of baking soda in a large cup. Slowly pour in a little vinegar.
Watch cup B. Then pour a thin layer of the sugar water from cup A onto the plate. Leave it in the sun until the water has gone. What is left?

What you will see: In cup A the sugar vanishes with no bubbles. The water evaporates and a sticky layer of sugar is left on the plate. So dissolving is a physical change. In cup B a gas forms and fizzes. The gas is carbon dioxide, a new substance, so this is a chemical change.

If you cannot try it, this is what happens.

A wrong idea many people have

If something changes colour, it must be a chemical change.

Why it feels right: Burning wood goes black and rust is brown. So colour seems to be the sign that matters.

The test that breaks it: Stir a drop of food colouring into a glass of water. The water changes colour. Now leave it in the sun until the water has gone. The colouring is left, unchanged. Nothing new was made.

What is true: A colour change is only a clue. Ask if a new substance has been made, and if the change can be turned back.

Key words

physical change: a change where no new substance is made.
reversible: able to be turned back.
chemical change: a change where atoms join in new ways and a new substance is made.

Back to your guess

The answer is B: No. Melting makes no new substance. Burning makes new substances that you cannot easily turn back.

Melting is a physical change. The water is still water. Burning is a chemical change. It makes new substances, such as ash and gases, that you cannot easily turn back.

Keep it

Copy this into your exercise book.

A physical change makes no new substance. A chemical change makes a new substance. Gas, heat or light, colour or a new solid are clues, not proof.

Draw: Draw two boxes, before and after, with atoms joined in new groups. Label it chemical change.

S3.6.5 Chapter 5

Rusting and other reactions

Why do iron nails turn orange-brown outside, but stay shiny in a dry drawer?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A Only water makes iron rust. The air does nothing.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Iron rusts when it has both air and water. A dry drawer has no water.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Rust is just dirt that sticks to wet iron.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Write word equations for rusting and other reactions
Test what iron nails need to rust, using three jars

An iron gate that stays wet goes brown and flaky. A bicycle chain left in the rain does the same. The brown stuff is rust. It is not iron any more, and it is not dirt. It is a new substance.

Draw it first

Draw the word equation for rusting

1 Write the starting substances, with plus signs between them: iron, oxygen and water.
2 Draw an arrow. It means makes. Here it points down.
3 Write what is made after the arrow: hydrated iron oxide. This is rust.

A chemical change is also called a reaction. The substances that react are the reactants. The new substances are the products. We write a reaction as a word equation, with an arrow that means makes.

Reactants come before the arrow, and products come after it. In our pictures the arrow points down, so the reactants are above it and the products are below.

Rust is hydrated iron oxide. It is iron joined with oxygen and water. It is crumbly and weak, and it is nothing like strong iron.

Does iron need air, or water, or both? You can test it. Put iron nails in three jars. In jar A there is air and water. In jar B there is water but no air. In jar C there is air but no water.

After some days, only the nails in jar A are rusty.

Jar B has water, but a layer of oil on top keeps air out. Boiled water has lost most of the air that was in it. Jar C has air, but the rice keeps it almost dry and the lid keeps wet air out. Only jar A has both. So iron needs both oxygen and water to rust.

The word equation for rusting.

Salt in the water makes iron rust faster. That is why iron rusts so fast near the sea. Paint, oil and a coat of zinc keep air and water away from iron.

A nail gets heavier when it rusts. The oxygen and water come from outside and join the iron. Their mass is added. Nothing is lost. The gain is small, and it takes many months outside, with every flake kept, to weigh it.

A nail before and after many months outside. Its mass went from 20 g to 22 g. The bars are close, but the gain is real.

Rusting is one reaction. There are many more, and each can be written as a word equation. Hydrogen burns in oxygen to make water. Baking soda and vinegar make a gas, carbon dioxide, with water and a salt.

Hydrogen burns in oxygen. The product is water.

Baking soda and vinegar. The fizz is carbon dioxide gas. Sodium acetate is a salt.

See it three ways

With things

Think of a recipe. Flour, eggs and milk go in. Pancakes come out. A word equation is a recipe for a reaction. The ingredients come before the arrow. What you make comes after it.

As a picture

In words

Rust forms only when iron has air and water together. Take away either one, and the iron does not rust.

Try it

What do nails need to rust?

You need: 3 jars, 6 plain iron nails, water, cooking oil, dry rice, a lid for one jar.

Be safe: A grown up must boil the water and let it cool. Use plain iron nails, not shiny coated ones. Rusty nails can cut, so do not squeeze them. Wash your hands afterwards.

Jar A: put 2 nails in. Pour in water until it covers half of each nail. Leave the jar open.
Jar B: a grown up boils water and lets it cool all the way. Put 2 nails in the jar and fill it with the cool water. Pour a layer of oil on top.
Jar C: put dry rice in the bottom. Stand 2 nails on it. Screw the lid on tight.
Put the jars somewhere safe. Look at them each day for a week or more. Write down what you see.

What you will see: The nails in jar A turn orange-brown, most of all where air meets water. The nails in B and C stay much cleaner. The rice in C cannot dry the air fully, so a little change is possible. The test is a fair one, because only air and water change.

If you cannot try it, this is what happens.

A wrong idea many people have

Iron rusts because of water alone.

Why it feels right: Rust shows up where things are wet, such as gates and bicycles left in the rain.

The test that breaks it: In jar B the nails sit in water, but oil stops air reaching them. They stay clean. In jar C the air reaches the nails, but there is almost no water. They stay almost clean too.

What is true: Iron needs oxygen from the air and water together. If either is missing, it will not rust.

Key words

reaction: a chemical change, where substances join in new ways.
reactants: the substances that react.
products: the new substances that are made.
word equation: a way to write a reaction in words, with an arrow.

Back to your guess

The answer is B: Iron rusts when it has both air and water. A dry drawer has no water.

Iron needs oxygen from the air and water together. Rust is a new substance made when the iron joins with them. It is not dirt.

Keep it

Copy this into your exercise book.

Iron + oxygen + water → hydrated iron oxide. Iron needs air and water to rust. Reactants go before the arrow, and products go after it.

Draw: Draw three jars: air and water, water with oil on top, and dry with a lid. Mark which nails rust.

S3.6.6 Chapter 6

Burning and fire safety

Why does a candle go out under a jar, and why is water wrong for some fires?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A A fire needs only something to burn. Take that away and the fire goes out.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A fire needs fuel, oxygen and heat. Take away any one of them and the fire goes out.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Water puts out every fire, because water is wet.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Name the three things a fire needs: fuel, oxygen and heat
Explain how each way of putting out a fire works
Write a word equation for burning a fuel

Fire cooks food and gives light. It also burns down homes. To use it safely, you need to know what a fire is made of and how it stops.

Draw it first

Draw the fire triangle

1 Draw a triangle. Write fuel, the thing that burns, on one side. Write oxygen on another side and heat along the bottom. Write fire in the middle.
2 Draw it again. Take away the fuel. There is no fire.
3 Draw it again. Take away the oxygen. There is no fire.
4 Draw it again. Take away the heat. There is no fire.

This is the fire triangle. A fire needs fuel, oxygen and heat, all three at once. Take away any one side, and the fire goes out.

Burning is a chemical change. A fuel is something that burns, such as wood, wax or gas. It joins with oxygen from the air and gives out heat and light. The gases it makes float away.

A jar over a candle holds only so much air. The flame uses the oxygen, and when too little is left it goes out.

A bigger jar holds more air, and more oxygen. So the flame lasts longer. This is a fair test, because only the jar changes.

Each way of putting out a fire takes away one side of the triangle. Water takes away heat. A lid or sand smothers a fire. It covers it so no air reaches it, and that takes away oxygen. Turning off the gas takes away the fuel.

Three ways to put out a fire, and the side of the triangle each one takes away.

But water is not right for every fire. Oil is lighter than water, so it floats on it. Water poured on burning oil sinks, turns to steam and throws burning oil about. For an oil fire in a pan, turn off the heat. Slide a lid or a baking tray over it. Never use water. Leave the lid on until the pan is cold, or the oil may flare up again. If the fire is big, get out and call for help.

Water also carries electricity. Never pour water on an electrical fire. If it is safe, switch off the power first. Otherwise get away and get help.

Two fires where water is the wrong choice.

Now the reaction. Wax and cooking gas are made of carbon and hydrogen. Wood is made of carbon, hydrogen and oxygen. When these burn, the carbon joins with oxygen to make carbon dioxide. The hydrogen joins with oxygen to make water vapour.

Burning a fuel made of carbon and hydrogen.

A candle burning. The wax seems to vanish, but it has become gases.

These equations hold when there is plenty of air. With too little air, a fire makes soot and carbon monoxide. Carbon monoxide is a poison gas, and you cannot see or smell it. Never burn charcoal, wood or gas in a closed room. Always let fresh air in.

A burning candle gets shorter. The wax has not been destroyed. It has become carbon dioxide and water vapour, which go into the air. Burning adds carbon dioxide to the air.

See it three ways

With things

Cover a burning candle with a jar. The flame goes out after a few seconds. The jar did not cool it or remove the wax. It kept fresh air away.

As a picture

In words

The flame uses oxygen. Under a jar there is only a little. When too little is left, the flame goes out.

Try it

Time a flame under two jars

You need: a candle on a saucer, two glass jars, one small and one big, a clock or a way to count seconds, a grown up with matches.

Be safe: Only a grown up lights the candle. Do not touch the jar while it is hot. Never leave a flame alone. Keep paper and cloth away.

A grown up lights the candle. Everyone else stays back. Tie long hair back.
Lower the small jar over the flame. Count the seconds until the flame goes out. Write it down.
Wait until the jar has cooled. A grown up lights the candle again. Do the same with the big jar. Compare the times.

What you will see: The flame lasts longer under the big jar, because there is more air and so more oxygen. Your times will differ from the picture.

If you cannot try it, this is what happens.

A wrong idea many people have

Water puts out every fire.

Why it feels right: Water is what we throw on a campfire, and fire engines carry it.

The test that breaks it: Water takes away heat. It does not stop an oil fire from burning. Burning oil floats, and hot oil and water spit and splash. Water also carries electricity.

What is true: Pick the way that takes away one side of the triangle safely. A lid for an oil fire. Switch off the power for an electrical fire.

Key words

fuel: something that burns, such as wood, wax or gas.
fire triangle: the three things a fire needs: fuel, oxygen and heat.
smothers: covers a fire so no air reaches it.

Back to your guess

The answer is B: A fire needs fuel, oxygen and heat. Take away any one of them and the fire goes out.

A fire needs fuel, oxygen and heat. A jar takes away oxygen. Water takes away heat, but it makes some fires worse, so it is not right for every fire.

Keep it

Copy this into your exercise book.

A fire needs fuel, oxygen and heat. Take away one and it goes out. Never use water on oil or electrical fires. Fuel + oxygen → carbon dioxide + water.

Draw: Draw the fire triangle with fuel, oxygen and heat on its sides.

S3.6.7 Chapter 7

Acids and alkalis

Lemon juice is sour and soap feels slippery. Can you sort them without tasting them?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A No. The only way to tell is to taste them.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Yes. A plant dye can show which is an acid and which is an alkali.
You picked B. Read on. At the end, the chapter comes back to your guess.
C No. All liquids at home are the same, so you cannot sort them.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Make an indicator from red cabbage, turmeric or hibiscus flowers
Sort lemon juice, vinegar, soap and baking soda into acids and alkalis
Place common substances on the pH scale from 0 to 14

Some liquids are sour, like lemon juice. Some feel slippery, like soapy water. Scientists sort these into three groups. They are acids, neutral liquids and alkalis.

An acid tastes sour. Lemon juice, vinegar and the juice in your stomach are acids. An alkali is the opposite of an acid. Soap and baking soda are alkalis. Strong ones feel slippery and burn.

Tasting is not a safe test. Instead we use an indicator. This is a dye that has one colour in an acid and another in an alkali. A liquid that is neither is neutral. Pure water is neutral.

Draw it first

Draw the pH scale

1 Draw a strip and number it from 0 to 14. Mark 7 in the middle as neutral. Write acid on the left and alkali on the right.
2 Add the acids. Lemon juice goes at 2 and vinegar at 3.
3 Add pure water at 7, the neutral place, and soap at 10 on the alkali side.

The pH scale runs from 0 to 14. Below 7 is acid, and the lower the number, the stronger the acid. 7 is neutral. Above 7 is alkali, and the higher the number, the stronger the alkali.

The pH of some things at home. These values are about, and real ones vary. Stomach acid is about 1 and lemon juice about 2.

Baking soda in water is a weak alkali, at about 8. Soap is stronger, at about 10.

Red cabbage makes a good indicator. Its juice is red or pink in an acid. It is purple in a neutral liquid. It turns blue, then green, in an alkali, and yellow in a very strong one.

Red cabbage indicator in three liquids. The word under each cup says the colour.

Turmeric is a yellow spice. It is yellow in acids and neutral liquids. It turns red-brown in an alkali. Hibiscus flowers also work, as red or pink in an acid and green or blue in an alkali. Colours vary a little with the plant and the water.

To make a turmeric indicator, stir a pinch of turmeric into a cup of water. To make a hibiscus indicator, a grown up pours hot water over a few dried petals. Wait until the water is coloured.

Turmeric stays yellow in an acid. It changes only in an alkali.

See it three ways

With things

Each liquid gives a pair of colours. Together they say what the liquid is. You can check one indicator with the other.

As a picture

In words

An acid turns cabbage pink and leaves turmeric yellow. A neutral liquid turns cabbage purple. An alkali turns cabbage blue or green, and turmeric red-brown.

Four things from the kitchen, sorted into acids and alkalis.

Try it

Make a red cabbage indicator

You need: 3 leaves of red cabbage, a bowl, hot water, poured by a grown up, a spoon, 4 clear cups, vinegar, baking soda, a little bar soap or laundry soap, scraped into flakes, water.

Be safe: A grown up pours the hot water. Use only these kitchen liquids. Never use bleach or toilet cleaner. Do not taste the liquids. Wash your hands when you finish.

Tear the cabbage into small pieces and put them in the bowl. A grown up pours hot water over them.
Wait about 15 minutes until the water is deep purple. Let it cool. Pour the purple liquid into a cup and throw the leaves away.
Set out 4 clear cups. Put vinegar in the first. Put plain water in the second. Stir a spoon of baking soda into water in the third. Stir a little scraped bar soap into water in the fourth. Liquid hand soap and dish liquid are often close to neutral, so they may show no change.
Add 2 spoons of the purple liquid to each cup. Watch the colours. Write them down.

What you will see: The vinegar turns pink. The plain water stays purple. The baking soda turns blue. The soap turns green. Your colours may be a little different.

If you cannot try it, this is what happens.

A wrong idea many people have

Acids are always strong and dangerous.

Why it feels right: We hear about acids that burn through metal, and we learn to keep away from them.

The test that breaks it: You eat acids every day. Lemon juice and vinegar are acids. They are weak enough to eat. Your stomach makes a strong acid to digest food. Mucus on its lining protects it. Soap and baking soda are alkalis you handle at home too.

What is true: Strength varies. A strong acid or strong alkali can burn. The pH scale tells you how strong, as well as which kind.

Key words

acid: a substance with a pH below 7. Many taste sour.
alkali: a substance with a pH above 7. Soap is one.
indicator: a dye that changes colour in acids and alkalis.
neutral: neither an acid nor an alkali, with a pH of 7.
pH scale: a scale from 0 to 14 for how acid or alkali something is.

Back to your guess

The answer is B: Yes. A plant dye can show which is an acid and which is an alkali.

An indicator is a dye that changes colour. It tells acids, neutral liquids and alkalis apart, with no tasting at all.

Keep it

Copy this into your exercise book.

Acids are below 7 on the pH scale, neutral is 7 and alkalis are above 7. Cabbage is pink in acid, purple in neutral and blue or green in alkali.

Draw: Draw a strip numbered 0 to 14. Write acid, neutral and alkali. Mark lemon juice, water and soap.

S3.6.8 Chapter 8

Neutralisation

Why does a chalky tablet ease a sore stomach, and why do farmers spread white powder on soil?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

A The tablet adds more acid, so the stomach pain goes away.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The tablet soaks up all the acid, so the stomach cannot digest food.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The tablet is a weak alkali. It cancels some of the extra acid.
You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain neutralisation as an acid and an alkali cancelling out
Explain how an antacid eases indigestion
Explain why farmers add lime to sour soil

An acid and an alkali are opposites. When you mix them, each one weakens the other. This is useful for a sore stomach and for sour soil.

Draw it first

Draw a neutralisation

1 Write acid and alkali, with a plus sign between them. These are the substances you start with.
2 Draw an arrow. It means makes.
3 Write what they make: a salt and water. The acid and the alkali are used up.

When an acid and an alkali react, they cancel each other out. We call this neutralisation. The reaction makes a salt and water.

Here a salt means any substance made when an acid and an alkali react. Table salt is one salt, but there are many others.

The word equation for neutralisation.

You can watch it with an indicator. Put red cabbage indicator in vinegar, an acid. It turns pink. Add baking soda a little at a time. At first it stays pink, because some acid is left. With enough soda it turns purple, which is neutral. Add too much, and it turns blue.

Too little alkali leaves some acid. Enough makes the liquid neutral. Too much leaves an alkali.

Baking soda also makes gas as it reacts with vinegar. That is the fizz. The fizz is carbon dioxide.

Vinegar is the acid. Sodium acetate is the salt. The gas makes the fizz.

Your stomach makes acid to break down food. Sometimes it makes too much, and your stomach hurts. This is called indigestion. An antacid is a medicine that eases it. An antacid is a weak alkali, often a chalky tablet. It cancels some of the extra acid.

See it three ways

With things

Think of a bath that is too hot. You add cold water, and it cools. An antacid does the same job for acid. It cancels some, so the stomach is less sour.

As a picture

In words

The stomach acid is at about pH 1. An antacid raises the pH to about 3. It does not go all the way to 7, because the stomach still needs some acid.

In symbols

3 − 1 = 2

An antacid must be a weak alkali. A strong alkali would burn the inside of the body. Only use medicine that a grown up or a health worker says is right for you.

Soil can be acid too. Rain, and some plant waste, make it acid. Many crops grow poorly in sour soil. Farmers fix this with lime. Lime is made from limestone, and it is an alkali.

Sour soil is about pH 5. Lime raises it, here to about 6, which is closer to neutral. Most crops grow best near neutral soil.

Two problems of too much acid, and the alkali that fixes each.

Farmers wear gloves and masks when they spread lime. It is an alkali and it can burn skin and eyes. They also test the soil first, so they add only as much as the soil needs.

Try it

Cancel out vinegar

You need: red cabbage indicator from the last chapter, vinegar, baking soda, a large cup, a spoon.

Be safe: Use a large cup, because the fizz can spill over. Keep your face back from the cup. If any splashes in your eyes, rinse them with plenty of clean water. Do not taste anything. Wash your hands afterwards.

Put 2 spoons of vinegar in a large cup. Add 2 spoons of cabbage indicator. It turns pink.
Add a pinch of baking soda. Stir. It fizzes. Watch the colour.
Keep adding one pinch at a time. Stir each time. At first the colour stays pink. Stop when it turns purple. Then add more, and see it turn blue.

What you will see: The colour stays pink at first, because there is still acid. It turns purple when the acid has been cancelled. A little more soda turns it blue, because now there is too much alkali. The fizz is carbon dioxide gas.

If you cannot try it, this is what happens.

A wrong idea many people have

Adding more alkali always makes an acid safer.

Why it feels right: An alkali cancels an acid. So it seems that more alkali must cancel more.

The test that breaks it: Add baking soda to cabbage indicator in vinegar, a pinch at a time. The colour goes pink, then purple, then blue. After purple, more soda only leaves an alkali.

What is true: Neutralisation needs the right amount. Too little leaves some acid. Too much leaves an alkali. This is why farmers test their soil.

Key words

neutralisation: a reaction where an acid and an alkali cancel each other out.
salt: a substance made when an acid reacts with an alkali.
antacid: a weak alkali that cancels some stomach acid.
lime: an alkali that farmers add to sour soil.

Back to your guess

The answer is C: The tablet is a weak alkali. It cancels some of the extra acid.

An antacid is a weak alkali. It cancels, or neutralises, some of the extra acid. Farmers use lime for the same job in soil.

Keep it

Copy this into your exercise book.

Acid + alkali → salt + water. This is neutralisation. An antacid eases stomach acid. Lime raises the pH of sour soil. Too much alkali leaves an alkali.

Draw: Draw three cups of cabbage indicator: pink for acid, purple for neutral, and blue for too much alkali.

Draw an element and a compound

Build water from paper circles

Draw a pure substance and a mixture

Mix and take apart

Draw salt coming back from salty water

Get salt back from salty water

Split an ink into colours

Draw ice melting

Fizz or no fizz

Draw the word equation for rusting

What do nails need to rust?

Draw the fire triangle

Time a flame under two jars

Draw the pH scale

Make a red cabbage indicator

Draw a neutralisation

Cancel out vinegar

Key words, A to Z