Phases of matter
Do you sometimes dump ice cubes into a drink to help keep cool on a hot summer day? Have you ever watched the ice cubes melt and wondered how you could make them melt more slowly—or even faster? In this science activity you will get to try some different, common household substances to try and answer this question: What will help a solid ice cube turn into a liquid puddle the fastest?
Temperature isn't the only thing that affects how a liquid freezes—and melts. If you've ever made homemade ice cream the old-fashioned way using a hand-crank machine, you probably know that you need ice and salt to freeze the cream mixture. Similarly, if you live in a cold climate, you've probably seen the trucks that salt and sand the streets after a snowfall to prevent ice from building up on the roads. In both of these instances salt is lowering the freezing point of water, which means that the water needs to be colder to turn from liquid into ice. For the ice cream maker, the temperature of the ice–salt mixture can get much lower than if just using normal ice, and this makes it possible to freeze the ice cream mixture. For the salt spread on streets, lowering the freezing point means that ice can melt even when the outdoor temperature is below water’s freezing point. Both of these events demonstrate “freezing point depression.”
Salt mixed with water is an example of a chemical solution. In a solution there is a solute (salt in this example) that gets dissolved in a solvent (water in this case). When other substances are mixed with water they may also lower its freezing point. In this science activity you'll investigate how salt, sand and sugar affect water's freezing point.
- Four ice cubes (They should all be the same size and shape.)
- Four drinking glasses (They should all be identical.)
- Table salt
- One-quarter teaspoon measuring spoon
- Prepare (or purchase) some ice cubes if you do not have any ready. They should all be the same size and shape.
- Into each drinking glass place one ice cube. Make sure the ice cube is oriented the same way in each glass. (Tip: If you are using ice cubes from a tray, it helps to let the tray sit at room temperature for about five minutes so that the ice cubes more easily come out of the tray cups and do not break into pieces.)
- Carefully sprinkle one-quarter teaspoon (tsp.) of salt over the ice cube in one drinking glass. Then sprinkle one-quarter tsp. of sugar over the cube in another glass and one-quarter tsp. of sand over the ice in the third. Do not sprinkle anything over the ice cube in the fourth glass. (It will be your control.) How do you think the salt, sugar and sand will affect how quickly the ice cubes melt?
- Set the drinking glasses aside somewhere indoors, out of direct sunlight.
- Watch the ice cubes over time, checking on them every five to 10 minutes. After around 30 minutes, which cube has melted the most? Which is the first one to melt completely? Which is the last?
- Overall, how do you think added salt, sugar or sand affects how quickly the ice melts? Can you explain why this might be?
- Extra: You could try this activity at different temperatures, such as in the refrigerator or outside on a hot day. How does adding salt, sugar or sand to the cubes affect how quickly they melt when exposed to a hotter or colder environment?
- Extra: In this activity you used one-quarter tsp. of each substance, but you could try adding more or less. Does the melting rate depend on the amount of the substance added?
- Extra: Identify some other substances to test on the ice cubes. Do other substances help melt the cubes more quickly or do they end up melting more slowly?