DrDAQ is a versatile instrument that connects to the USB port of any PC. Using the supplied PicoScope software it can be used as an oscilloscope, spectrum analyzer and signal generator.
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.
It is suitable for ages 16+ and requires some knowledge of the processes involved in state changes of liquids.
If the freezer is more than 2 m away from the DrDAQ then you may also require one or more 5 m sensor extension cables (Part no EL032).
Start PicoLog collecting 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. In our freezer this happened at around –30 °C and took 5 hours. Note that, due to the action of the freezer thermostat, the temperature may fluctuate slightly.
Print out a graph of the results showing both channels. It will help if you select a display that shows time since start (use the button with the tick symbol on the graphs button bar)
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.
|Pure water||Start time||Start temp|
|Cooling from room temperature towards freezing||0 hours|
|Cooling to the freezer's minimum temperature|
|Stable region at minimum temperature|
Q2. Before the pure water begins to freeze, the temperature drops rapidly. Once it begins to freeze the temperature remains fairly constant. Explain why.
Q3. At what temperature did the salt water begin to freeze. Why is this lower than for the pure water.
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 gentle slope. Explain why. (Hint - more salt can be dissolved in warm water).
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.
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.