Here is a sealed tub of ice. Soon it will be a puddle of water inside the tub. Will it then weigh more, less, or exactly the same?

Hands up your guess.

When ice melts, does the amount of material change, or just its shape?

Each group has a sealed tub of ice and a set of scales. If the class shares one balance, we weigh each tub at the front bench together. Your teacher also has a second sealed container: a clear zip bag with a little warm water trapped inside it. The steps for your tub are on the board, and so is one person's prediction — read it and decide for yourself whether you agree, because nobody knows yet which guess the balance will back up.

One of the steps asks your group to choose one more thing to predict and test. You can only use what is already here: your sealed tub, the scales, the teacher's sealed bag of warm water, the cold metal tray or window, and the dry cloth. Pick one of these, or think of your own and check it with your teacher before you try it:

• Does a sealed tub left in the warmest spot in the room melt faster than one left on a cool bench?
• Does the sealed bag of warm water read the same on the scales after droplets have formed on its cold side?
• Does wiping the outside of the tub dry with the cloth change the number you read?

Whichever you choose, say what you expect first, then write down the reading you actually get, even if it surprises you.

While your tub sits melting, watch that clear bag of warm water. The warm water turns into an invisible gas (water vapour). On the cold side of the bag it turns back into tiny droplets you can see. It is the same water, still trapped in the bag.

The particles in your tub are far too small to see, so the screen gives you a model of them instead: one material in a container, with the temperature in your hands. The state name only appears after you have watched what the particles do, so watch first and read the name second.

Now finish your Investigation Journal page on your own, answering the questions on the board.