Grade by Grade
C. Matter and Energy · Properties of Liquids and Solids

Melting, Freezing, and Staying the Same

C2.4 C2.3 A1.2 A1.3 A1.4 A1.5About 40 minutes
Learning goals
  • I can name solids that melt when they are heated, like butter, chocolate and ice cream.
  • I can name liquids that freeze when they are cooled, like juice and water.
  • I can tell when a solid or liquid will stay the same.
  • I can design and test a way to keep an ice cube from melting.

Words to know

heat
To make something warmer.
cool
To make something colder.
melt
To change from a solid to a liquid when heated.
freeze
To change from a liquid to a solid when cooled.
room temperature
How warm it usually is inside a home, about 20°C.
design
A plan for making something to solve a problem.
insulate
To wrap or cover something to slow down heat moving in or out.

The ice cream problem

It is a hot July day at the beach. You buy an ice cream cone. A few minutes later... drip, drip, drip! Sticky ice cream is running down your hand.

But the ice cream in the freezer at home is still hard. Why does one melt and the other stay the same?

A vanilla ice cream cone melting and dripping onto a picnic table at a sunny beach.
On a hot day, the solid ice cream warms up and melts into a liquid that drips down the cone. AI-generated image

Many solids melt

In the last lesson you learned that ice melts when it gets warm. Ice is not the only solid that melts! When these solids are heated enough, they melt into liquids:

  • ice cream and popsicles (they melt on a warm day)
  • chocolate (it can melt in a warm hand)
  • butter (it melts in a warm pan)
  • candle wax (it melts near the flame)

Each solid melts at its own temperature. Ice melts just above 0°C. Butter and chocolate need to be warmer. A metal spoon or a rock needs to be much, much hotter than anything in your home. Even rock can melt deep inside the Earth, where it is extremely hot. Melted rock is called magma. When it pours out of a volcano, we call it lava!

🍦🍫🧈🕯️🧊
Solids that melt when heated: ice cream, chocolate, butter, candle wax and ice.

Many liquids freeze

When these liquids are cooled enough, they freeze into solids:

  • water freezes into ice cubes
  • juice freezes into popsicles
  • milk freezes too

And some liquids that were once solids become solid again when they cool. Melted chocolate gets hard again in the fridge. Melted butter turns solid again when it cools.

Staying the same

Most of the time, solids and liquids do not change state. They stay the same when the temperature is right for them:

  • A rock, a wooden block and a metal spoon stay solid at room temperature, and on the hottest summer day.
  • Ice cream stays solid in the freezer, because it is kept very cold.
  • Milk stays liquid in the fridge. The fridge is cold, but not cold enough to freeze milk.
  • Water in a glass stays liquid at room temperature (about 20°C).
  • Snow stays solid outside when it is colder than 0°C.

So the condition that matters most is temperature. To change a state, you have to heat it or cool it enough.

To make something melt (heat it)To make something freeze (cool it)To keep it the same
sunshinea freezerkeep the temperature steady
warm handscold winter air below 0°Cfrozen foods stay in the freezer
warm waterice mixed with salt around it (like an old-fashioned ice cream maker)ice cream goes back in the freezer quickly
a stove or oven (grown-ups only!)a popsicle mould in the freezera cooler keeps cold things cold
Conditions that change the state of matter, or keep it the same

Not every change is melting

Some things do not melt when they get hot. Wood and paper burn. Bread turns into toast. These are a different kind of change, and they cannot be changed back. Melting and freezing are special because they can go back and forth.

Worked example

Let's do a fair test: Where does ice melt fastest?

We put one ice cube in three places: in a cup of warm tap water, on a plate on the kitchen counter, and on a plate in the fridge. Which melts first?

  1. Make it fair: Use ice cubes from the same tray, so they are the same size. Start them all at the same time. We only change one thing: the place.
  2. Predict: The warm water is the warmest place. The fridge is the coldest place (but still above 0°C).
  3. Observe: Check every 5 minutes. The cube in warm water gets smaller fast. The cube on the counter melts more slowly. The cube in the fridge melts very slowly.
  4. Record: Warm water: melted first. Counter: second. Fridge: last.

The ice in warm water melted fastest because it was heated the most. The ice in the fridge melted slowest because it was kept cold. The warmer the place, the faster ice melts.

warm watermelted firstkitchen countermelted secondcoldfridgemelted lastsame-sized ice cubes, checked after the same time
The warmer the place, the faster the ice melts. After the same time, the cube in warm water is smallest and the cube in the fridge is biggest.
Worked example

Let's sort: Will it change or stay the same?

For each one, say if it will change state or stay the same: (a) a chocolate bar in a hot car, (b) a rock in the sun, (c) a glass of milk in the fridge, (d) a juice box left outside at −15°C.

  1. (a) A hot car is very warm. Chocolate melts when heated. It will change (melt).
  2. (b) A rock needs to be far hotter than any sunny day to melt. It will stay the same.
  3. (c) The fridge is cold but not below 0°C, so the milk will stay the same (liquid).
  4. (d) −15°C is much colder than 0°C. The juice will change (freeze).

Change: the chocolate (melts) and the juice (freezes). Stay the same: the rock and the milk.

Just for fun

Why did the chocolate bar stay out of the hot car?

It didn't want to lose its cool and turn into a puddle!

Be an engineer: The ice cube keeper

Engineers solve problems by designing and building things. Here is a problem: How can we keep an ice cube frozen for as long as possible, without a freezer?

One idea is to insulate it. Wrapping something in a material like cloth, newspaper or cotton balls slows down heat moving in. That is why coolers and winter coats work!

Hands-on activity

Design an ice cube keeper

You need: ice cubes from the same tray, small plates or zip-top bags, and some materials to choose from: a sock, a mitten, newspaper, aluminum foil, cotton balls, a small box, a plastic container with a lid, bubble wrap.

  1. Plan: Choose materials and draw your ice cube keeper. Why do you think it will work?
  2. Build: Put one ice cube in a small bag (to hold the water as it melts). Wrap and pack it in your design.
  3. Test: Put a second ice cube in a bag on a plate with nothing around it. This is your control, the one we compare with.
  4. Leave both in the same place. Check every 15 minutes. Which ice cube is bigger?
  5. Improve: What could make your design even better? Try it again!
  6. Share: Tell or draw what worked best.

Safety: A grown-up does any heating. Never use the stove, oven, candles or a hair dryer yourself. Wipe up drips so no one slips.

Your turn

1

Name two solids that melt when they are heated.

2

Name one liquid that freezes when it is cooled.

3

What do you need to do to make butter melt?

4

Why does ice cream stay solid in the freezer?

5

Will a wooden block melt on a hot summer day?

6

Can melted chocolate become solid again? How?

7

What is the most important condition that makes solids melt or liquids freeze?

More practice: warm-up

1

Is a freezer a place to melt things or freeze things?

2

Does a popsicle melt on a hot day?

3

Will milk in the fridge freeze?

4

Does a rock melt in your hand?

5

Who should use the stove?

More practice: on your own

1

Why does chocolate melt in your hand but a coin does not?

2

In the ice fair test, why did we use ice cubes from the same tray?

3

It is −10°C outside. You leave a snowman outside. Will it melt? Explain.

4

How could you turn apple juice into a popsicle?

5

Why do people bring a cooler with ice on a summer picnic?

Challenge

1

A cube of butter and an ice cube sit on a counter at room temperature (about 20°C). After a while, the ice is water but the butter is only soft. Why?

2

Kim wrapped her ice cube in a wool sock. Jay left his on a bare plate. Which ice cube will last longer? Why?

3

Is turning bread into toast the same kind of change as melting butter? Explain.

Check your understanding

Quiz

1.

Which solid melts most easily in a warm hand?

2.

To make juice freeze, you should...

3.

Which one will stay the same for a whole hot summer day outside?

4.

Ice cream stays solid when it is kept...

5.

In a fair test about melting ice in different places, what should change?

6.

Which change can go back again?

For parents and teachers

Big idea: Temperature is the main condition that changes a state (C2.4). Heating can melt solids; cooling can freeze liquids; and at the right temperature, solids and liquids stay the same. Different materials melt at different temperatures. Melting and freezing can be reversed.

How to help: Notice changes of state while cooking: butter softening, chocolate chips melting in warm muffins, juice turning into popsicles. Ask, "What made it change? Can it change back?"

Engineering (Strand A): The ice cube keeper uses the design process: plan, build, test, improve. The bare ice cube is a control for comparison. Celebrate improvements, not just the "winner".

Common difficulties:

  • Thinking all solids melt at the same temperature. Compare ice and butter on a counter.
  • Thinking the fridge freezes things. The fridge is cold but above 0°C; the freezer is below 0°C.
  • Calling burning or toasting "melting". Use the test "Can it change back?"

Safety: Children should never use the stove, oven, candles or a hair dryer to melt things. A grown-up does any heating.