Science Magic

Ice-cream in a Bag

Nuts & Bolts

Medium Ziploc bag

Small Ziploc bag

¼ cup of milk

¼ cup of whipping cream

3 tablespoons of sugar

¼ teaspoon of vanilla flavouring

¾ cup of sea salt

Enough ice cubes to ½ fill the medium bag

Spoon

Thermal gloves

Apprentice

Salsa music – optional!

Safety: The bag is not to be held by unprotected hands as the contents are cold enough to cause tissue damage! If you don’t have any thermal gloves you can have your apprentice just hold the top of the larger outer bag or you can replace this bag within a catering size coffee can which can be rolled back and forth on the floor. Gloves or tea towels are needed to protect the hands from the cold – more ice and salt will be required.

Secrets for Success

With the help of your apprentice put the following ingredients into the small Ziploc bag: milk, cream, sugar and flavouring. Securely seal the bag ensuring that as much air as possible has been removed from the bag at the same time. Shake the bag to mix the ingredients.

Half fill the larger bag with crushed ice cubes; put the sea salt and the smaller bag on top of the ice. Close this outer bag securely. Ask your apprentice to put on the thermal gloves; they are to shake the bag GENTLY from side to side for 3 minutes (or until the milk mixture in the small bag has solidified). This can be done to the rhythm of salsa music if you wish!

Remove the small bag and quickly rinse in cold water to remove any salt that maybe on the outside – we don’t want our delicious contents to be contaminated! Your apprentice (or apprentices as I am sure once the first batch has been made everyone else will want to make their own!) can eat the ice cream straight from the bag with a spoon or the contents could be squeezed into a cone and decorated with a chocolate flake. Yum! Bon Appetite!

Science in a Nutshell

Pure water freezes, or melts, at 0 ^oC and boils at 100 ^oC. So between 0 ^oC and 100 ^oC, water exists in the liquid state. Its molecules are provided with enough heat energy and hence kinetic (movement) energy to move around, but not enough energy to break the relatively loose, ‘sticky’, bonds between them. When you pour liquids into containers you will notice that they all flow and change shape to fit the dimensions of the base of any container you may put them in. The shape of a liquid can change but its volume, at specific temperatures and pressure, always remains the same; you can test this by pouring a liquid into lots of different shaped measuring jugs: tall, thin, wide and short; the volume is always the same.

However, when the temperature is lowered to below 0 ^oC, the molecules cease to move around – they vibrate only - and they form the crystalline structure of ice, in which the molecules are held together by stronger, ‘more sticky’, bonds.

When any substance freezes, the particles within it arrange themselves into an orderly pattern. This arrangement is called a crystal. When table or sea salt (sodium chloride NaCl) is added to water, a saline solution is formed and the forming of this solution interferes with the orderly arrangement of the particles in the crystal. The result of this is an increase in heat energy required to be removed from the solution before freezing can occur i.e. the solution freezes at a temperature lower than 0 oC; Salt acts as a freezing point depressant.

The movement energy of the molecules in a substance is related to the temperature. If the molecules initially have a lot of kinetic (movement) energy and we then remove heat from the substance, the molecules will then also lose kinetic energy; the less kinetic energy they have the lo