Freezing
ANTARCTICA IS THE “COOLEST” PLACE ON EARTH!
Antarctica is the coldest, driest, windiest continent on the planet. Temperatures can plummet to -58°F, which is 90°F below freezing (32°F is freezing). Antarctica is so cold that most of the ice there never melts; the continent is permanently covered in ice. Yet, Weddell seals can live there even in the middle of winter, when the water is at its coldest. They can do this because some water remains unfrozen, and they can dive and re-surface through these holes in the ice. How do these holes stay open? How come they don’t immediately freeze over? Well, the answer is, it’s a joint effort between the seals and the properties of the water.
Weddell seals have specialized teeth that allow them to chew through the ice and keep their breathing holes open when they begin to freeze closed. But the temperature at which these holes freeze is influenced by how much salt is in the water. Pure water (water with no salt in it) freezes at 32°F. As you begin adding salt to the water, the freezing point of the water goes down. The more salt you add, the lower the temperature at which the water will freeze. Antarctica has some of the saltiest ocean water on Earth. But how did it get that way?
Antarctica ‘s temperatures are so cold that glaciers form around the continent. Glaciers are HUGE pieces of ice that form from old snow. Icebergs are the smaller chunks of ice that break off from glaciers and float in the ocean. When water freezes, it forms tiny structures that look like crystals. You’ve seen this if you’ve ever looked closely at snowflakes. When these tiny crystal structures form, they form only out of water without salt in it. Therefore, when glaciers and icebergs form, they are almost pure water, and the salt gets left behind in the unfrozen water. The more ice that forms, the more salt that gets left behind, which makes the ocean water in Antarctica much saltier than in most other oceans around the world. The high concentration of salt in the water makes the water heavy (sort of like adding objects to a cardboard box makes the box heavier). Heavier liquids freeze at lower temperatures than lighter liquids (like water without salt in it). Therefore, the saltier the water is, the colder the temperature has to be to freeze it.
The following activity demonstrates how adding different amounts of salt to tap water affects the freezing temperature of the water. This activity should be completed over two days: