When you wake to find frost has covered the ground, that means the night temperature dropped below 0°C. Cold temperatures cause water vapor in the air to freeze instantly on surfaces such as grass or the top of a car. You can make frost happen inside the classroom to demonstrate this principle. Take an empty coffee can and fill it with ice and one-half cup of salt. Let students stir the ice and salt mixture with a spoon, and put on the lid. In just a few minutes, the can will be covered in frost!
Why does a lake often remain frozen, even though the temperature is above freezing? Simply put, it takes quite a lot of energy for ice to thaw, especially over a large surface. Show your students how much effort it takes to turn ice to water by having an ice-cube-melting race. Give each child a sealed zipper-style sandwich bag with an ice cube inside. Challenge them to come up with the quickest way to melt the ice cubes. They can try rubbing the bag, sitting on it, or blowing on it.
Hint: The fastest way of melting it is usually by smashing the ice, as the more surface area in contact with the air, the faster it will melt.
Expanding Ice Trick
As water turns to ice, it expands as much as nine percent. This is why, when the temperature drops below 0°C, "frost wedging" can occur. Water trapped in tiny cracks expands, causing sidewalks to split and potholes to form. Show your students frost wedging in action with this experiment. Fill a plastic soda bottle to the brim, and screw the cap on tight. Place the bottle in a freezer overnight, and the next day you'll find that the bottle has split open. The ice has expanded outside the bottle!
Cold Water Sinks, Warm Water Rises
Why does lake water feel colder near the bottom than at the top? Sun and air help to warm the water near the surface. As water warms, it expands and becomes less dense-lighter. Cold water is denser and heavier. To demonstrate this, fill a large container half full with cold water, 12 ice cubes, and some blue food coloring. Stir to melt the ice. Fill an aquarium half full of warm water (about 50°C) and add red food coloring. Slowly pour the cold water down one side of the aquarium. You'll see the blue water sink to the bottom and form a layer under the warmer red water. How long do students predict it will take for the two waters to mix on their own, without stirring