How does liquids freeze

This is why ice floats in water. The molecules in ice are in fixed positions but still vibrate. Students do not need to orient the molecules exactly as they are in the space-filling model but they should try to have a hydrogen atom of one molecule near an oxygen atom of another.

Ask students to handle their models gently because they will need them for other lessons. Tell students that the temperature at which a substance freezes is called the freezing point. Corn oil and isopropyl alcohol have lower freezing points than water. This means that they need to be cooled to lower temperatures to make them freeze. Tell students that the air around them is made of different kinds of gases.

The attractions between the molecules of gases in air except water vapor are so weak that they need to be cooled to very low temperatures in order to condense to a liquid or freeze to a solid. If nitrogen is made cold enough, the weak attractions between its molecules can cause it to condense to a liquid.

Tell students that under some conditions a gas can turn directly to a solid without going through the liquid phase.

Explain that this process is called deposition. Some of the ice that formed on the outside of the can may have been a result of deposition. Read more about how changes of state relate to the weather in the teacher background section. Tell students that the frost that forms on the ground, windows, or grass in winter is formed by deposition. Give students time to answer questions about freezing points, nitrogen, and deposition to complete their activity sheets for this lesson.

You could also show students images of snowflakes and videos of a snowflakes forming. The American Chemical Society is dedicated to improving lives through Chemistry. Skip Navigation. Lesson 2. Engage Show students that liquid water expands when it freezes to become solid ice. Teacher Preparation Place 50 milliliters of water into a plastic ml graduated cylinder and place it in the freezer over night.

The next day, bring it into class and show students that the level of ice is higher than the level of water you started with. Explain to students that as water freezes, it expands and takes up more space than it did as liquid water. Ask students: Why do you think freezing water in the metal container caused it to burst? Water molecules move further apart when water freezes. This movement caused the metal container to burst. Why are roads likely to develop potholes during cold winters?

Hint : Think about what happened to the metal container. When water gets in small cracks in the road and freezes it expands and breaks the asphalt. When this continues to happen below the surface, it eventually forms a pothole. Ask students: What do you think happens to water molecules when liquid water changes to solid ice? Students learned that when water vapor is cooled, attractions between water molecules cause them to condense and become liquid water.

Students may say that the water molecules slow down enough that their attractions hold them together as ice. Give each student an activity sheet. Explore Have students chill a metal can so that ice forms on it. Question to investigate How can you make the water vapor in air condense and then freeze? Materials for each group Empty clean metal soup can Salt Ice Metal spoon or sturdy stick Teaspoon Paper towel Materials for the teacher Pliers Duct tape Teacher preparation Use pliers to bend sharp edges on the can down.

Procedure Dry the outside of a can with a paper towel. Why does water expand when it freezes? Why does liquid water have a density maximum? Most liquids have a quite simple behavior when they are cooled at a fixed pressure : they shrink. The liquid contracts as it is cooled; because the molecules are moving slower they are less able to overcome the attractive intermolecular forces drawing them closer to each other.

Then the freezing temperature is reached, and the substance solidifies, which causes it to contract some more because crystalline solids are usually tightly packed. Water is one of the few exceptions to this behavior. The new freezing point depends on the added substance and how much you mix with water, and this is why cities put salt on the roads in some states to remove ice and snow in the winter.

As another example, vodka, a mixture of water and alcohol, stays liquid for an extended period when kept in a freezer. The alcohol in the vodka lowers the freezing point significantly. Most substances contract, or shrink, in volume as they get cold. Water only contracts until it is lowered to 39 degrees; at colder temperatures, it begins to expand.

As the water gets colder, its molecules slow down and arrange themselves such that gaps exist between groups of molecules. As they get colder, the molecules form hexagonal patterns that eventually become snowflakes and related crystals.

If you fill a bottle completely filled with water, then seal it with a lid before putting it into a freezer, the water expands as it gets colder. Eventually, the ice will burst the bottle.

This is true even for containers made of strong materials such as iron; the pressure exerted by freezing water is as high as 40, psi at minus 7.