Salt & Freezing Point D. Crowley, 2007
Salt & Freezing Point Thursday, November 14, 2019Thursday, November 14, 2019 To understand how salt affects the freezing point of water
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…
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?
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
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
Temperature (C) after time period (minutes) Results Temperature (C) after time period (minutes) 2 4 6 8 10 12 Sample A temp. 20C Sample A state liquid Sample B temp. Sample B state
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)
Findings Pure water should freeze at 0C - if the sample did not, then it wasn’t pure! The ‘pure’ sample itself may not have frozen at 0C - 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 100C 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 0C. 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!