How and why salt melts ice
Why do people pour salt on icy sidewalks to melt the snow? Usually, the result is a big pile of slush made of melted snow and ice crystals. And why do the lakes and streams freeze over solid while the ocean always remains flowing? Is there something magical about salt? Let’s find out!
The melting property of salt demonstrated
In this experiment, we’ll explore the melting properties of salt in two steps or “tests”.
- Take 2 cups of water.
- Place about a tablespoon of salt in one of the cups.
- Place both cups in the freezer.
- Check each cup about every 10 minutes. Can you guess which one will freeze first?
- Now grab an ice cube out of the freezer.
- Place the ice cube on a plate and begin sprinkling salt on the ice cube. Melt Down! Now you can understand why people put salt on their icy driveways.
Salt lowers the freezing point of water. Normally, water freezes when the temperature reaches about 32 degrees Fahrenheit (0 degrees Celsius). When you mix salt with water, you actually lower this freezing point.
Salty water will still freeze, but the temperature has to be colder than it would for regular water. How much colder? That depends on how much salt you put in the water. The more salt you add, the colder it must get before the water freezes. For instance, ocean water freezes at about 28.4 degrees Fahrenheit. When seawater freezes, however, the ice contains very little salt because only the water part freezes, leaving the salt behind. In fact, frozen ocean water can be melted down for drinking water.
Can the whole ocean freeze solid?
It is doubtful that the entire ocean could freeze solid. If it did, the required temperatures would ensure nobody was around to witness it (does the refrigerator light go off when we close the door?).
At an average salinity of about 3.5%, the ocean would begin freezing at about -2 degrees Celsius. But since only the water freezes, the salt would form ice crystals that would be added to the remaining water, raising the salinity of the rest of the ocean. With an increased saltiness, the temperature required to freeze it would be even lower (the freezing point would decrease by .28 degrees Celsius for every parts per thousand of salinity increase). This would continue on and on and on meaning, it would be very hard to freeze the entire ocean solid.
In-Article Image CreditsA single grain of table salt taken using a scanning electron microscope via Wikimedia Commons with usage type - Public Domain. June 4, 2009
A close up of salt crystals via Wikimedia Commons by Mark Schellhase with usage type - Creative Commons License. July 25, 2008
Featured Image CreditA close up of salt crystals via Wikimedia Commons by Mark Schellhase with usage type - Creative Commons License. July 25, 2008