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.
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.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Which jar has nails that rust?
See the answer
Jar A, with air and water.
Only jar A has both air and water. Jar B has no air, and jar C has no water.
If you chose A: The oil keeps air out. Iron has water but no oxygen, so it does not rust. Go to S3.6.5.
If you chose B: The jar has air but no water. Iron needs both to rust. Go to S3.6.5.
The nails in the jar with the oil layer stayed clean. Why?
See the answer
The oil kept air out, so the iron had water but no oxygen.
Boiled water has lost most of its air, and the oil stops new air getting in. Iron needs oxygen to rust.
If you chose A: Oil is only a cover. It stops air reaching the iron. It does not clean it. Go to S3.6.5.
If you chose B: The jar was full of water. It was the air that was missing. Go to S3.6.5.
Which word equation is right for rusting?
See the answer
iron + oxygen + water → hydrated iron oxide
Iron, oxygen and water react, so they come before the arrow. The new substance, hydrated iron oxide, comes after it.
If you chose A: The arrow is backwards. The reactants come before the arrow, and the product comes after it. Go to S3.6.5.
If you chose C: Iron cannot be on both sides if it changes. It joins with oxygen and water to make a new substance. Go to S3.6.5.
In a word equation, what is written before the arrow?
See the answer
The reactants, which are the substances that react.
Read the equation as: reactants make products. The reactants go before the arrow.
If you chose B: The products come after the arrow. The arrow means makes. Go to S3.6.5.
If you chose C: The products come after the arrow. The substances that react come first, and they are the reactants. Go to S3.6.5.
Baking soda and vinegar fizz. The gas that bubbles is carbon dioxide. Which are the products?
See the answer
carbon dioxide, water and sodium acetate
Baking soda and vinegar react to make carbon dioxide, water and sodium acetate. The fizz is the gas.
If you chose A: These are the reactants. They go before the arrow. The products are the new substances. Go to S3.6.5.
If you chose B: The gas is not the only new substance. Water and a salt, sodium acetate, are made too. Go to S3.6.5.
A nail has a mass of 20 g. After many months outside, the nail and all its rust have a mass of 22 g. How many grams of oxygen and water joined the iron?
See the answer
2 g
The extra mass came from outside. 22 − 20 = 2.
From an earlier chapter
Rust forms on an iron nail, and the nail gets weaker. What kind of change is this?
See the answer
A chemical change, because a new substance forms.
Rust has new properties. It is crumbly and brown, and it is not iron. A new substance means a chemical change.
If you chose A: The iron has joined with oxygen and water to make rust. Rust is a new substance, so this is chemical. Go to S3.6.4.
If you chose C: Speed does not decide. A slow change can still make a new substance. Go to S3.6.4.
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.
