Although pure water freezes at 0°C (32°F), water that has salt dissolved in it has to be colder before it freezes. If the water has as much salt dissolved in it as it can hold (that's called a saturated solution of salt), so that any further salt would just come out as crystals, the freezing temperature is around -21 °C, or about -6 °F. If your freezer isn't colder than that, the part of the ice touching the salt will start to melt. If you've put so much salt on the ice that the water can all melt and form a saturated solution, and still leave some salt crystals, then it will all melt. If you've put only a little salt on, it will melt some ice until the salt crystals are gone. Now as more ice melts the solution becomes less salty, more like pure water. So its freezing temperature goes up. At some point its freezing temperature will be the same as the freezer temperature, so the freezing will stop. You'll have some ice left, and some salty water.
What is interesting is that this effect is used all over the place. Often, salt is put on roads to melt ice. If there's a lot of ice, you need a lot of salt. If the temperature drops below -21°C, it won't work at all.
You also wanted to know why it works, why saltwater has to get colder than pure water before it freezes. We've got some other answers on that, which you can find by searching this site for "saltwater". Briefly, the ice is a crystal, an almost perfect array of pure water molecules with almost no salt in it. To make that out of pure water requires limiting the ways the water molecules move around. To make that out of salt water requires BOTH limiting the ways the water molecules move around AND limiting the ways the salt can move around (it's stuck in the liquid, or in separate crystals), which is harder to do.
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(published on 10/22/2007)
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?
Observations and results
Did the ice cube sprinkled with table salt melt the fastest?
In this activity you tried adding salt, sugar or sand to ice to see whether the substance would help melt the ice. In other words, you wanted to test whether these substances could demonstrate freezing point depression, or the lowering of the ice's freezing point so that it melted into a liquid at a lower temperature than normal. You should have seen that the ice cube with salt sprinkled on it melted faster than any of the other cubes. This is because the amount by which the freezing point is lowered depends on the number of molecules dissolved, but not their chemical nature. (This is an example of what's called a "colligative property.") In the same volume there are more molecules of salt than there are of sugar or sand because the chemical components that make up the salt are much smaller than the sugar or sand.
Be sure not to pour the sand down a sink drain or garbage disposal! Instead, throw out the damp sand outside or in a trash can.
More to explore
Salt and Icy Roads, from Science Kids
How Does Salt Melt Ice and Snow?, from Highlights
What Makes Ice Melt Fastest?, from Science Buddies
Make Ice Cream in a Bag, from Science Buddies
This activity brought to you in partnership with Science Buddies