Teacher instructions

Work through this resource material in the following sequence:

20 minutes

Tune in: What will change?

20 minutes

Find out: Experiment time!

20 minutes

Reflection: What did we learn?

Tune in: What will change?

Step 1. Discuss:

what makes water change state? You might have to define solids, liquids, and gases. Students should be aware that heat can have a big impact on water's state. Warming up ice will make it melt from a solid into a liquid. Boiling liquid water will make it evaporate into a gaseous steam. However, going the other way and reducing the temperature of steam will turn it back into water, and further freezing water will turn it into ice.
what temperatures do these state changes occur at? Students might be able to tell you that water freezes at 0^OC and boils at 100^OC.

Step 2.

Explain the experiment to your students. Explain that you are going to test your theories on what impact salt will have on the freezing temperature of water.

Step 3.

Students write their predictions in the first section of the Student Worksheet.

Find out: Experiment time!

Step 1.

Fill three containers with water.

Step 2.

Add salt to one of the containers and mix well until the water is completely saturated with salt - that means the water is no longer absorbing the salt. You should be able to see solid salt piling up at the bottom of the container.

Step 3.

Leave these containers in the freezer for a few hours. Make sure your freezer is above -21^OC but below 0^OC.

Step 4.

Pull the containers out of the freezer. What can you notice? You should see that the containers with plain water a frozen into a solid, while the salt water container is still a liquid!

Step 5.

Take one of the frozen containers. Heap a spoonful of salt into the middle and leave for a few minutes. What happens? The ice should start melting where the salt meets the ice - and we know it's not just because of the temperature outside the freezer, because the third container is still frozen. You can continue to add in lines to the frozen water to create little ravines of water.

Reflection: What did we learn?

We first need students to understand that solids, liquids and gases are all made up of particles. Particles are the tiny units of matter that make up all of the substances around us, including solids, liquids, and gases. Students might think that everything, especially liquids and gases, are big blobs of 'stuff', but that's not quite the case.

In a solid, the particles are tightly packed together and arranged in a regular pattern, which gives solids a definite shape and volume. The particles in a solid are held together by strong attractive forces, which limit their movement to a small amount of vibration around their fixed positions.

In a liquid, the particles are still connected by attractive forces, though those bonds are not as strong. This allows the particles in a liquid are able to move past each other, which allows liquids to take on the shape of their container. However, liquids still have a definite volume because the particles are still in close proximity to each other. We are able to see that we can break the bonds of a liquid because, if we stretch a little pool of water too far, it splits into two smaller pools.

When we heat water, we are giving those little particles energy, making them move much faster. When they have so much energy they can bounce all over the place, the water goes from a liquid to a gas. Similarly, when we heat up ice, the tightly packed together particles begin to wiggle. They have too much energy to stay still! Eventually they start to move away from one another, and we can see this in the melting water. The ice at the centre remains frozen because it is the last to receive energy, being the furthest away from the heat on the outside.

When we put water in the freezer, we are cooling it down. The particles lose energy, and start to move closer together.

However, the salt particles in the salt water get in the way of the water particles, disrupting their ability to join together and form ice crystals.

The same thing happens when we put salt on the pure water ice. The salt isn't actually melting the ice, it is just disrupting the bonds of the solid.

Experiment - How Does Salt Affect Freezing Water?

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