Activity: Dancing Ice
Inspired by the weather we have had in Canberra recently, I thought we might try an activity using ice.
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Place the ice into a glass with some oil. I added some food colouring to the ice to make it easier to see. |
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As the ice melts, drops of water will fall from it, causing it to move up and down. |
You will need
- A tall glass or plastic container.
- Some vegetable oil. I used canola oil.
- Ice - you can add some food colouring to make it easier to see
What to do
- Fill the glass with oil.
- Drop a block of ice into the glass. I found my ice floated in the middle of the oil.
- Watch the ice as it melts.
- You will find drops form on the ice, then drop off to fall slowly through the water.
- As the drops form and fall, the ice will rock from side to side and move up and down.
What's happening?
This activity is all about density. The density of a material is how much a given volume of that material weighs. E.g. 1 cubic centimetre of liquid water weighs around 1 gram.
When you drop an object into a liquid, it feels two forces:
- Gravity is pulling it down.
- Buoyancy is pushing it up. When you drop something into a liquid, it displaces some of the liquid (pushes it out of the way). An object placed in a liquid feels an upward force equal to the weight of the liquid it is displacing. This force is called buoyancy.
If the weight of the object is less than the weight of the liquid it is displacing, it will float. If it is greater, it will sink. Another way to say this is that if the object is less dense than the liquid, it will float, but if it is denser it will sink. This is even true for liquids, liquid water is denser than oil, so the oil floats on the water.
Water is weird stuff. We don't normally notice it, because water is so common, but water does some things that almost no other chemical will do. One of the ways water is strange is that it is less dense as a solid than as a liquid.
Ice and vegetable oil have almost the same density, around 920 Kg per cubic metre, so a block of ice dropped into oil will barely move. However, as the water melts, it turns into denser, liquid water. The water tends to stick to the ice for a while before it drops off. If there is enough water on the ice, then the density of the ice and water together is greater than the oil, so they tend to sink. Once the drop of water falls off the ice, the ice floats up again.
It's a good thing that ice floats on water. In winter, some rivers and lakes freeze on the surface. If ice was denser than water, then when rivers and lakes grew cold, they would freeze from the bottom up, which would kill plants growing in them and starve most of the fish and other marine life living in the lakes and rivers.
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