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  Home   >   Heat & Energy   >   Suggested Teaching Activities    





Freezing of Pure and Salt Water

Introduction

This experiment looks at the temperature of two cups of water as they are frozen. One cup contains pure water whilst the other contains salt water. The experiment is ideal for demonstrating how salt depresses the freezing point of water. It is also a good introduction to the theories of latent heat.


Equipment

Datalogger interface connected to a PC
2 temperature sensors
2 containers for the water
Salt
Freezer


Experiment Setup



Fill both cups with enough water to cover the temperature probe. Obviously the more water used, the longer the experiment will take.
Add salt a little at a time to one of the cups of water and stir until it dissolves. Stop adding salt when the solution is saturated (i.e. the salt will not dissolve). Mark the cup that has salt water so that it can be identified later.
Place cardboard with two cut out holes on the top of the container. This stops the water spilling and also provides insulation.
Put one temperature sensor in each cup of water and secure in place with a rubber band around the cup. Ideally each probe should be in the middle of the cup and not touching the edges.
Plug the sensors to the datalogger interface and run the datalogger software. Name one channel "Pure Water" and the other "Salt Water". Check that the temperature readings look sensible.
Set-up the software to record one reading per minute.
Place both cups into the freezer and shut the door, being careful to feed the cables for the temperature sensor out the edge of the door without pulling the sensors out of the cups.
Start the datalogger software to collect data and display both channels on a graph. Check that the temperature from both channels begins to decrease.
Allow the experiment to continue until both temperatures have levelled off.

Discussion

1. Look at the trace showing pure water. By looking at the changes in the 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 each region.

Pure Water Start Time Start Temperature
Cooling from room temperature towards freezing 0 hours  
Freezing    
Cooling to the freezer's minimum temperature    
Stable region at minimum temperature    

2. Before the pure water begins to freeze, the temperature drops rapidly. Once it begins to freeze the temperature remains fairly constant. Explain why this is.

3. At what temperature did the salt water begin to freeze? Why is this lower than for the pure water?

4. 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 gentle slope. Explain why. (Hint - more salt can be dissolved in warm water).

5. If you look carefully at the region where freezing starts you may notice a small slip where the temperature actually increased for a short period. Why did this happen?

6. Think about what happens to the temperature 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.

7. How might the purity of the water effect the shape of the graph?

8. How would the concentration of salt effect the results?

9. What results would you expect if the samples had been boiled rather than frozen?

10. Use the results to estimate the latent heat of freezing (you will need to know the specific heat capacity of water).


Teacher's 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. Bottled water may have better results than tap water.

 
  Heat
  Observing Air Stratification
  How does heat travel through glass?
  Freezing of Pure and Salt Water



 

 

 

 

 

 

 

 

 

 

 

 

 



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