Science Ice-cream

Cold, creamy and scientifically intriguing!

Australian Curriculum links

  • Science > Chemical Sciences > Year 5 > ACSSU077

You'll need

  • small metal bowl or other metal container
  • 1/3 cup cream
  • 2 teaspoons of sugar
  • ½ teaspoon vanilla essence
  • small plastic spoon
  • large plastic bowl, half-filled with crushed ice
  • thermometer
  • 1½ cups table salt
  • large wooden spoon

Try this

  1. Position the end of the thermometer in the plastic bowl to measure the temperature of the ice. It should be between 0°C and 5°C.
  2. Add one cup of salt to the ice and stir well with the wooden spoon. After a few minutes measure the temperature of the salt and ice mixture. It should be below 0°C.
  3. Place the cream, sugar and vanilla essence into the small metal bowl and mix with the plastic spoon until the sugar dissolves.
  4. Place the metal bowl in the ice and salt mixture and push it down so it is surrounded by ice.
  5. Stir the cream mixture gently and continuously with the plastic spoon for about 8 minutes.
  6. Remove the metal bowl from the ice bath.
  7. Mix the remaining ½ cup of salt with the ice.
  8. Return the metal bowl to the ice and salt and continue stirring the cream mixture with the plastic spoon for a further five to ten minutes, or until the mixture freezes.
  9. Eat your ice cream!

What's happening?

To make ice cream, the ingredients need to be at a temperature less than -3°C and steadily mixed while the ice cream freezes. If the ingredients are simply mixed together and then placed in a freezer, the result would be a hard, icy lump. Stirring the ingredients constantly while the mixture is freezing prevents large, crunchy ice crystals forming and it whips air through the mixture to make it soft and light.

Mixing the ice cream is one thing, but how can we get it below 0°C without putting it in a freezer? Ice made from pure water has a freezing point of 0°C, but ice in a mixture of salty water has a lower melting point. The melting point of the salty ice depends on the concentration of salt but in this activity it should get well below -3°C.

To understand how salt can change the freezing point of ice, we first need to consider the fact that the bowl of melting ice contains water in two states: solid and liquid. Where the solid ice meets liquid water, water molecules are exchanged with some being released from the ice to join the liquid water and others being captured from the liquid to join the ice. The temperature of the mixture is 0°C which is the melting point of ice and we can say that the two states of water are in equilibrium, although it is an unbalanced equilibrium because the surrounding temperature is higher than 0°C so the amount of liquid increases as the ice melts.

Adding salt to the bowl causes more of the ice to melt and the temperature of the mixture to drop due to something called freezing point depression. Normal table salt is the chemical sodium chloride. When the salt is added to the bowl, it dissolves in the liquid water, forming sodium ions and chloride ions. Having these ions in the liquid water means that the water is no longer pure and each molecule in the liquid water is less likely to be captured from the liquid to join the ice. The liquid water needs to be at a lower temperature to freeze, hence the name ‘freezing point depression’. The salt isn’t dissolved in the solid water, so the rate of melting hasn’t changed. The unbalanced equilibrium is now even more unbalanced, with the liquid molecules more likely to stay in the liquid and ice molecules melting into the liquid and staying there. In the melting process, the ice molecules take up heat from their surroundings, thereby causing the mixture to cool to the freezing point of the salt water solution.

Freezing point depression doesn’t just happen with a solution of table salt and water. It happens with any non-volatile solute in solution, for example, other chloride salts and ethylene glycol which is used as anti-freeze in cars to prevent the water in the cooling system from freezing.

Real world links

In some towns and cities where the winters are very cold, salt is spread over icy roads and footpaths to melt the ice and prevent the water from re-freezing.