Why Does Freezing Water Expand?

Have you ever put a full bottle of your favorite beverage into the freezer just to cool it off — and then forgotten it there?  If so, you’ve probably ended up with a broken bottle, and maybe a mess. It’s all about the hydrogen bonds. Your elementary students aren’t ready to contemplate hydrogen bonds, but they’ll be interested to learn about the behavior of ice. Activity 1 is good for young students, who can just be amazed. Go on to Activity 2 if your students are ready to think about the behavior of molecules.

Activity 1: Ice Expands

Fill a jar halfway with water and secure a lid on it. Mark the level of the water and ask students if they think it will go up, go down, or stay the same when it’s frozen. Put the jar in the freezer and check it the next day. The line will be lower than the level of ice. Ask students why they think that the level of water went up. Tell students you didn’t add anything to the jar. Nothing changed in the amount of water that was inside the jar so how come the level is higher?

Take another empty jar and put it into a tray. Add water to the jar until the jar overflows. Add a few ice cubes and show students that more water has come out of the jar so the level of the water is at the very top of the jar. As the ice melts, ask your students to look at the level of water in the jar—did it go down? Ask students what this means and why they think it changed.

Activity 2: Molecular Structure

Tell students that water is made up of one oxygen atom and two hydrogen atoms — that’s why H2O is another name for water.

Explain that molecules have positive or negative charges, just like magnets. Use classroom magnets with north and south poles marked to demonstrate how the sides which are the same repel one another, but the sides which are different attract.

Call on three students, one big student and two smaller students who are about the same size, and ask them to help you with the demonstration. Explain to students that the larger student is oxygen and has a double negative charge. Give the larger student a hat with a minus sign written on it and ask the student to wear the hat. Explain that the two smaller students have positive charges and give them each a hat with a positive sign on it.

Ask the oxygen student to hold hands with one hydrogen student and explain that the other hydrogen wants to join oxygen too but doesn’t want to hang out with the hydrogen because they’re alike in charge. Explain that because the hydrogens are small, they’re also similar to a double negative charge. Ask students how they can arrange the three atoms so that all three are as far apart as possible while still holding hands. Students should arrange the three example students into a V-shape. Explain that no matter what form water is in, liquid, solid, or gas, one water molecule will always stay in this shape.

Invite three more students to join in the example and give them each corresponding hats. Next, take the students through the different states of matter, starting with liquid. Tell the molecule students that they are a liquid and are able to move around each other’s groups, getting close to each other but slowly moving around. Remind students they must keep their V-shape. Now tell students you are turning up the heat and they now have more energy because it is warmer. Tell the molecule students that they need to move faster. They might bump into each other, even, because they are moving so quickly. Now tell students you are turning the heat down so it is getting very cold.

Ask the class how the two molecules must now arrange around each other, reminding them that opposite charges attract. Position the molecule students so one hydrogen is close to the other oxygen. Explain that now they are frozen and can only shake in place. Show students that the molecule students now take up a lot of space because of how they need to stand compared to the other temperatures.

Ask molecule students to go back to their seats and ask the group what they learned and compare it to what they saw happen in the first activity. Finish by letting all the students be molecules heating up and then cooling down. Crank up the music and shake off the cobwebs!

Resources