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Pivotal Kids - Science Activity

 

Activity: Homemade ice-cream

[From Science by Email]

Warning: This experiment is edible. Be sure to use clean equipment. Never make food in your science lab. 

You will need

  • cup whole milk (low fat milk doesn't work as well)

  • 1 tablespoon sugar

  • flavour for the ice-cream (Milo was used in this experiment)

    Milo, milk and sugar in bag

    In the small bag combine milk, sugar and flavouring.

    Ice, salt and water in bag

    In the larger bag add ice, salt and water.

    Homemade ice-cream

    Place the small bag in the larger bag and gently squish the two together. The milk and flavour will form into ice cream.

     

  • 1 spoon

  • 1 small zip-lock bag

  • 1 larger zip-lock bag

  • 1 tray of ice cubes

  • 6 tablespoons of regular table salt

  • cup of water

What to do

  1. Combine the milk, sugar and flavouring together in the small zip lock bag.

  2. Seal the bag and shake it a few times so that everything is mixed.

  3. In the large zip-lock bag, add the ice cubes, salt and the water.

  4. Put the small bag (check the seal before doing this) into the big bag and seal the big bag.

  5. Gently squish the two bags together for about 5-10 minutes.

  6. As you do this the milk will start to freeze and the ice-cream will form. The longer you agitate the two bags the thicker your ice-cream will be.

What's happening?

The salt we add to the ice creates salt water which is actually colder than ordinary iced water. In other words, salt water has to get colder than 0 degrees Celsius (C) before it will freeze. In this case, milk is like regular water and freezes when surrounded by colder salt water. This allows the ingredients to mix together to make ice cream.

The salt works by lowering the melting or freezing point of water. The effect is termed 'freezing point depression'.

When you add salt to water, you introduce dissolved foreign particles into the water. The freezing point of water becomes lower as more particles are added until the point where the salt stops dissolving. For a solution of table salt (sodium chloride, NaCl) in water, this temperature is -21C under controlled lab conditions. In the real world, if you were to add salt to a frozen footpath, sodium chloride can melt ice only down to about -9C.

Since salt interferes with ice crystals forming in water, adding salt to water effectively lowers its freezing point. This is why the oceans don't freeze, even when the temperature is below 0C. The ocean, which is about 3.5% salt, freezes at approximately -2C .

For saltwater that's as saturated as it can possibly get (i.e. there's no way to dissolve any more salt in it no matter how hard you tried), the freezing point is -21.1 C . This is when the saltwater is 23.3% salt (by weight).

 

Applications

In colder parts of the world, salt is often used on footpaths and roads to melt ice and snow. Salt is used on roads and walkways because it is inexpensive and readily available.

When ice melts, it absorbs heat energy from its surroundings. When you add salt to ice, the rapid melting of the ice can cool the ice and water to below 0C. Before refrigeration was invented, ice and salt was used to produce the rapid cooling needed to make ice-cream.

There are a variety of ice-cream makers for home use. Some of the older style tried-and-true ice-cream makers use ice that is melted with rock salt to quick-freeze the cream mixture. These are either hand cranked or motorised. If you have the hand-cranked variety, have some friends over and make a party out of taking turns with the crank. It can be a lot of work.

More modern ice cream makers have a sealed metal canister base filled with an anti-freeze type of liquid that can maintain a lower temperature than ice.

You can make a more permanent ice cream maker by replacing the zip lock bags used in this experiment with sealable cans. Milo cans work well for this, but any food safe containers will do, you just need to make sure they are different sizes. Metal is best, but glass will work, too. Plastic does NOT work, as it doesn't conduct the cold very well. Be sure to tape or tie the lid down firmly on the smaller container. Place it in the larger can or other container and put in enough ice, alternating with layers of rock salt two or three times, to completely pack the cavity between the two cans. Remember to agitate the cans, rolling on your bench top works well for this.

 

 

 

 

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