Kobe Thomson and Bree Win, of Balclutha Primary School, asks :-

Why is ice sometimes clear and sometimes cloudy?

Sean Fitzsmons, a glaciologist in the Department of Geography at the University of Otago, responded.

When you are making ice in a household freezer from tap water the resulting ice can look cloudy because the tap water may contain chemicals in the water and because air bubbles become trapped in the ice as it freezes. Usually the chemicals dissolved in the water, and the air bubbles that form, are concentrated in the centre of an ice block because it freezes from the outside toward the centre. As the freezing progresses the chemicals and air are concentrated toward the centre. In addition when the ice freezes rapidly it usually has lots of small cracks that form because the ice expands by about 9 per cent as it freezes. The resulting cracks look cloudy or white because air is trapped along the cracks.

In the natural environment white or cloudy ice is very common but if you look closely to can observe that clear ice is also common. A very good example of white ice is glacier ice. Glaciers form when snow falls during winter and does not melt over the following summer. Subsequent snowfalls bury the snow and the light fluffy snow crystals slowly change into ice. During this process many of the air bubbles get trapped between the crystals of ice, which give the ice a white appearance. Icicles are an example of a type of ice that is very clear because the ice forms by slow freezing of layers of water that have flowed over the surface of the icicle as it has grown.

Why not try to make clear ice at home with these three experiments? Firstly, make the ice from distilled water, which a science teacher at school may be able to supply. This water will have very low concentrations of chemicals. Secondly, you can reduce the amount of dissolved gas in the water before you put it in the freezer. You can achieve this by ”flattening” the water by boiling the water and letting it cool twice before you put it in the freezer. Thirdly, you can slow down the freezing process to try to reduce the amount of gas bubbles that form and reduce cracking. Although this is difficult to do in a home freezer one method that you can try is to freeze the water in an insulated container like a vacuum flask.