ChemistryIce Melts, Water Freezes, and It Can Go Back
Atlas the friendly robot stands in a bright kitchen, holding an ice cube over a clear bowl as a puddle of water grows below it, smiling and pointing at the melting ice near a warm sunny window.
- Name heating and cooling as two ways to change matter between solid and liquid.
- Predict that an ice cube melts into water when it gets warm.
- Predict that liquid water freezes into ice when it gets very cold.
- Explain that melting and freezing are reverse changes that can repeat over and over.
- Tell what makes a change reversible using ice and water as examples.
Key terms
- solid
- matter that is firm and keeps its own shape, like an ice cube
- liquid
- matter that flows and pours and takes the shape of its container
- melting
- when heat turns a solid into a liquid, like ice into water
- freezing
- when cold turns a liquid into a solid, like water into ice
- reversible change
- a change you can undo and repeat over and over again
Heat Makes Ice Melt
Ice is a solid, which means its tiny bits hold still and stay packed closely together, giving the ice a firm shape. When you add heat, the bits start moving faster and begin to slide past each other. As they break free from their fixed spots, the hard ice turns into liquid water. This change is called melting, and it always needs warmth to happen.
Cold Makes Water Freeze
Liquid water flows because its tiny bits can slide around. If you take heat away by putting the water in a very cold freezer, the bits slow down more and more. Eventually they lock together and stop sliding, so the water becomes solid ice again. This change is called freezing, and it is exactly the opposite of melting.
Going Back and Forth
The amazing part is that melting and freezing can happen again and again with the same water. You can melt an ice cube, freeze the water back into ice, then melt it once more. Because you can undo the change every time, we call it a reversible change. The water is never lost or made into something different; it just keeps switching between solid and liquid.
Worked examples
Predict what happens to an ice cube left on a warm plate.
- The plate and air around the ice are warm, so heat moves into the ice.
- Heat makes the tiny bits move faster and slide past each other.
- The solid ice slowly turns into liquid water, which is called melting.
Answer: The ice cube melts and becomes liquid water on the plate.
How can you turn that puddle of water back into ice?
- Put the liquid water somewhere very cold, like a freezer.
- The cold takes heat away, so the bits slow down and lock together.
- When the bits stop sliding, the water freezes into solid ice.
Answer: Put the water in the freezer; the cold makes it freeze back into solid ice.
Activity
Look at each thing below. Predict whether heat or cold will change it, and whether the change can be reversed.
Practice
Predict what happens to a snowman on a warm spring day and why.
Explain why melting and freezing are called reversible changes.
Common mistakes to avoid
- Once water freezes it stays ice forever.Freezing is reversible, so warming the ice will melt it back into liquid water again.
- Melting ice makes the water disappear into the air.Melting turns the ice into liquid water you can see and touch; it does not vanish, it just changes form.
Check your understanding
What happens to an ice cube when it gets warm?
How can you turn liquid water back into solid ice?
Atlas says melting is a reversible change. What does reversible mean?
If water freezes into ice in the freezer, what will happen if you take it out and leave it somewhere warm?
Recap
Heat melts solid ice into liquid water, and cold freezes liquid water back into solid ice. Because you can melt and freeze the same water over and over, this is called a reversible change, and the water is never lost.
Reflect
When have you watched something melt or freeze at home?