Confession — these answers are by Pico technical staff none of whom will admit to having been in a physics or chemistry lesson for a few years. Please check these answers before use. We would be grateful to anyone who can email us with suggestions of better questions and answers!

Graph showing the results obtained for both freezing and melting

Graph showing the results we obtained for both freezing and melting.


Q1. Look at the trace showing pure water. By looking at the changes in gradient of the graph, see if you can identify the regions of the graph described below. For each region fill in the time and the temperature at the start of this region.
A1. The answers will obviously depend on the students graph.

Q2. Before the pure water begins to freeze, the temperature drops rapidly. Once it begins to freeze the temperature remains fairly constant. Explain why.
A2. The temperature remains constant as the water is absorbing the latent heat of freezing.

Q3. At what temperature did the salt water begin to freeze. Why is this lower than the pure water.
A3. The temperature again will depend on the students results. We are not sure why salt water freezes at a lower temperature — please let us know if you have a good explanation!

Q4. Identify the sections of the graph that show the freezing of the pure and salt waters. The pure water shows a fairly flat region whilst the salt water has a sharper gradient. Explain why.
A4. As the solution of salt water begins to freeze, the water freezes (as pure water) so the remaining solution becomes more concentrated (again we are not sure about this answer).

Q5. If you look carefully at the region where freezing starts you may notice a small ‘blip’ where the temperature actually increased for a short period. Why did this happen.
A5. As the liquids begin to freeze, there must be an exothermic reaction.

Q6. Think about what happens to the temperatures of the two samples as they warm back up to room temperature. Sketch a graph of temperature against time showing what you think will happen.
A6. Our graph above shows what happens during melting.

Teachers’ notes

The experiment does not take long to prepare, but will take a few hours to carry out. For this reason some planning is required. Either start the experiment first thing in the morning, or alternatively start in the afternoon and leave to run overnight (this will require the PC to be left running).

It is possible that the purity of the water may have an effect on the results, we had good results using bottled mineral water, tap water may not work as well.

Credits, Comments and Further Info

This experiment was written by Pico Technology. Please contact us if you have any comments about this experiment or suggestions for improvements.