1. Salt & Freezing Point
D. Crowley, 2007

2. Salt & Freezing Point
Thursday, November 14, 2019Thursday, November 14, 2019
To understand how salt affects the freezing point of water

3. Experiment
Your task is to carry out an experiment to identify how pure water and salty water behave when you cool them
You will have two samples of water: one is pure, the other contains salt - your job is to carry out an experiment to identify which is which…

4. Prediction What is the freezing point of pure water?
How can knowing this help you decide which of the two samples will contain the pure water, and which one contains salty water?

5. Apparatus Sample A Sample B
2 boiling tubes, one half filled with water sample A and one half filled with water sample B
1 large beaker
Crushed ice
Salt
2 thermometers
Stop clock
Sample A
Sample B

6. Experiment
Place two boiling tubes into the large beaker, and pack crushed ice around them - pour salt onto this crushed ice
Half fill one boiling tube with water sample A; and half fill the other with water sample B
Start timing, measuring the temperature of each sample at regular intervals - decide how many measurements to take and record your results
Also record if the water in each tube is completely liquid; starting to freeze; or completely solid
Stop taking results when no further changes are occurring

7. Temperature (C) after time period (minutes)
Results
Temperature (C) after time period (minutes) 2 4 6 8 10 12
Sample A temp.
20C
Sample A
state
liquid
Sample B temp.
Sample B state

8. Graph
Graph your results (line), using different colours for each water sample
Describe what the graph shows, identifying which sample is pure and which is not - explain your answer
What was your freezing point of the pure water? How close is this to your predicted freezing point?
What other tests could you have done to show which was the pure water, and which was the salty water?
Explain why on a cold night ponds and lakes often freeze over, however rivers freeze over less often, and the sea hardly ever freezes over (use scientific knowledge)

9. Findings
Pure water should freeze at 0C - if the sample did not, then it wasn’t pure!
The ‘pure’ sample itself may not have frozen at 0C - this could be due to contaminants within the sample
We looked at the freezing (melting) point of the samples - we could have also looked at the boiling point: pure water should boil at 100C
Lakes and ponds are still, allowing sediment to settle to the bottom, and are likely to have similar properties to that of pure water - hence they freeze around 0C.
Rivers are generally fast moving, containing chemicals mixed within the water, causing their freezing points to differ
Sea water is extremely salty - salt lowers the melting point (freezing point), so it must get very cold for the sea to freeze!