Why does ice melt on a hot day, and steam turn to drops on a cold lid?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Explain melting, boiling and condensing as particles gain or lose energy
Use melting and boiling points to find the state at room temperature
Ice melts on a hot day. Steam turns back to drops on a cold lid. In both, a material turns from one state to another. This is a change of state.
Heat gives particles energy. Energy is what makes particles move. The more energy they have, the faster they move.
Heat a solid. Its particles vibrate harder and harder. At last they shake free of their places and begin to slide. The solid has melted. The temperature where this happens is the melting point.
Keep heating the liquid. Its particles move faster and faster. At the boiling point they have enough energy to break free of each other. Bubbles of gas form all through the liquid. It boils.
Cooling does the opposite. Energy leaves the material. A gas cools, and its particles slow down. They are pulled together, and the gas condenses into a liquid. Cool the liquid more. The particles slow until they lock in place. It freezes.
Draw it first
Draw the changes of state
1 Draw three boxes of particles in a row: solid, liquid and gas.
2 Draw arrows above them, pointing right. Heating puts energy in. Write melting and boiling.
3 Draw arrows below them, pointing left. Cooling takes energy out. Write condensing and freezing.
While a pure solid melts, its temperature stays the same. Ice stays at 0 °C until all of it has gone. The energy goes into freeing the particles, not into making them hotter. The same is true while a liquid boils.
Boiling makes bubbles all through the liquid. Evaporation happens only at the surface.
Evaporation is different. It happens at the surface of a liquid, at any temperature below the boiling point. A few particles near the surface have more energy than the rest, and they escape as a gas. Boiling happens all through the liquid, and only at the boiling point.
A pure material, such as water or iron, has its own melting point and boiling point. They tell you its state at any temperature. Below the melting point it is a solid. Between the two points it is a liquid. Above the boiling point it is a gas. These points are for normal air pressure, near sea level.
Melting points and boiling points in °C, at normal air pressure near sea level. High up, water boils at a lower temperature. Mercury is a poisonous metal. Never touch it.
Room temperature is about 20 °C. Oxygen melts at −219 °C and boils at −183 °C. 20 °C is far above both, so oxygen is a gas in a room. Iron needs 1538 °C to melt, so it is a solid.
See it three ways
As a picture
In words
Below the melting point, a material is a solid. Between its melting point and its boiling point, it is a liquid. Above its boiling point, it is a gas.
In symbols
At sea level, water is solid below 0 °C. It is liquid from 0 °C to 100 °C. It is gas above 100 °C.
Try it
Turn steam back into drops
You need: a pot with a little water, a grown up to heat it, a cold metal plate or lid, an oven glove.
Be safe: A grown up must do the heating. Steam and hot water burn badly. Keep your face and hands away from the pot.
A grown up heats the water until bubbles rise all through it. Then they turn the heat off.
The grown up holds the cold plate, tilted, about 20 cm above the pot, wearing the oven glove.
Wait a few seconds. Look at the underside of the plate. Do not touch the pot or the steam.
What you will see: Drops of water form on the underside of the cold plate. The vapour gave energy to the plate and turned back into a liquid. The plate also gets warm.
If you cannot try it, this is what happens.
A wrong idea many people have
When ice melts, the particles themselves turn runny.
Why it feels right: The ice turns soft and runny, so it seems each little bit has softened.
The test that breaks it: Freeze the water again and you get ice again, the same as before. If the particles had changed into something else, the ice could not come back.
What is true: The particles do not change. Heat gives them more energy, so they move more and break free of their places.
Key words
change of state
a material turning from a solid, liquid or gas into another of the three.
energy
what makes particles move. More energy means faster particles.
melting point
the temperature at which a solid melts. It is also where the liquid freezes.
boiling point
the temperature at which a liquid boils, with bubbles of gas all through it.
Back to your guess
The answer is A: Heat gives the particles energy. They move more and break free of their places.
The particles do not change. Heat gives them energy, so they vibrate harder and slide free. Cooling takes the energy away again.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
Ice is heated. What happens to its particles as it melts?
See the answer
They gain energy and break free of their fixed places.
Heat gives the particles energy. They vibrate harder until they slide free, and the solid is a liquid.
If you chose A: The particles stay the same size. Only their energy and the way they are held change. Go to S3.5.3.
If you chose C: Heating makes particles move more, not less. Go to S3.5.3.
Bubbles of gas form all through a pan of water. What is happening?
See the answer
The water is boiling.
Boiling makes bubbles all through the liquid, at the boiling point. Evaporation happens at the surface, at any lower temperature.
If you chose B: Evaporation happens only at the surface. Bubbles all through the water mean it is boiling. Go to S2.4.4.
If you chose C: Melting turns a solid into a liquid. This water is already a liquid. Go to S3.5.3.
The scale is for iron. Is iron a solid, a liquid or a gas at room temperature, about 20 °C?
See the answer
A solid. 20 °C is below the melting point of 1538 °C.
Room temperature is far below the melting point, so the iron is a solid. It would need 1538 °C to melt.
If you chose A: Zero degrees is the melting point of water. Iron has its own melting point, and it is much higher. Go to S3.5.3.
If you chose B: Metals do not melt easily. Iron needs 1538 °C to melt, and a room is far cooler than that. Go to S3.5.3.
Oxygen melts at −219 °C and boils at −183 °C. What state is oxygen at room temperature, about 20 °C?
See the answer
A gas. 20 °C is above the boiling point of −183 °C.
20 °C is above the boiling point, so the particles have broken free of each other. Oxygen is a gas.
If you chose A: It has melted, but it is also far above its boiling point of −183 °C. So it has boiled and is a gas. Go to S3.5.3.
If you chose C: A solid melts when it gets warmer than its melting point. At 20 °C oxygen is far warmer than −219 °C. Go to S3.5.3.
Steam touches a cold window and turns into drops of water. What happens to the particles?
See the answer
They lose energy, slow down and are pulled together.
The cold glass takes energy from the vapour. The particles slow down until they are held close together. That is a liquid, so the vapour condenses.
If you chose A: That is what happens when a liquid boils. Steam turning into drops is the opposite. Go to S3.5.3.
If you chose B: The particles stay water particles. Only their energy and the way they are held change. Go to S3.5.3.
From an earlier chapter
What is it called when particles spread out by themselves?
See the answer
Diffusion
Diffusion is the spreading of particles by their own movement, such as a smell crossing a room.
If you chose A: Freezing is a liquid turning into a solid. Spreading out by themselves has another name. Go to S3.5.2.
If you chose C: Melting is a solid turning into a liquid. Spreading out by themselves has another name. Go to S3.5.2.
Keep it
Copy this into your exercise book.
Heat gives particles energy. At the melting point a solid melts. At the boiling point a liquid boils. Cooling takes energy away: gas condenses, liquid freezes.
Draw: Draw solid, liquid and gas boxes in a row. Add arrows above, pointing right, and arrows below, pointing left. Name each one.
