Each winter the surface of a large area of the Southern Ocean freezes, forming a sea ice cover that surrounds Antarctica. When sea ice forms, very salty cold water sinks to the ocean depths and influences the world's currents.
The National Snow and Ice Data Centre also has an excellent site on sea ice.
- Students can conduct a number of activities to investigate the difference between sea ice and freshwater ice. (The activities in Icebergs could be done in conjunction with these activities.)
- Students can make their own 'seawater' by completely dissolving 100 g of salt in 1 L of warm water and letting it cool to room temperature. Fill a polystyrene cup and retain the leftover salt water. Then fill another cup with freshwater, taking care that the cups are identical, and are filled with exactly the same amounts of fluid. Place the cups inside stubbie holders so that water freezes from the top and not the sides. Label the containers and place them inside a freezer. Examine the cups after half an hour, then every 10 minutes, until there is a thick crust of ice on the top. The class could then discuss:
- How long does it take for ice to form in each cup?
- Which container froze first?
- Did you notice anything unusual about the freshwater or saltwater ice?
- Remove the saltwater ice carefully from the cup and place it on a piece of frozen black card.
- Look at the ice with a magnifying glass. What shapes do you see?
- Carefully place both types of ice on a cement mat and hit them with a hammer to test their hardness.
- Which ice is the hardest? Why should this be the case?
- Students can use fresh and saltwater ice to see if the different types of ice melt at different rates.
- Taste the water from the melt. Compare it with the rest of the water they made up originally and with the water left in the cup after the ice formed. (The ice should be noticeably less salty than the saltwater left in the cup.)