| When it is icy they put salt onto the roads to melt the ice. What the salt
does is to depress, or reduce, the freezing point of water. The result is that
the ice on the road melts more easily. If there's any truth in this, it means
that if ice freezes at zero degrees Celsius, then salty water must freeze at an
even lower temperature. |
What we did
We put temperature probes into each of three ice-lolly pots in the freezer.
One had 20 cm3 of water, one had 20 cm3 of salty water (brine) and one just had
air. We closed the freezer door, and got a data logger to take temperature
readings overnight.
The results
The graph shows the readings from three temperature probes: one placed in
water, another in brine and another in the freezer itself. After a whole night
in the freezer, the water had frozen solid but the salt water (the brine) was
still slightly slushy. Here's the graph...
The bumpy graphs are not due to a fault or sudden changes in temperature -
it's a common picture due to a mysterious thing called 'noise'. Appreciate that
these temperature probes are only supposed to work down to minus 10 degrees, so
they've worked quite well.
You will find some of the questions easier if you put these results into your
data logging software. Click here to get the results, then Open it in your data logging program.
Looking at the results
- Which graph trace is which? Remember that one temperature probe was placed
in water, another in brine and another in the freezer itself.
- What is the normal, steady temperature of the freezer?
- After starting the experiment and closing the door of the freezer how long
did the freezer take to get back to its normal temperature?
- What does the freezing point of water appear to be? Can you explain that
result?
- What can you say about the freezing point of brine?
- If you know a bit about depression of freezing point and latent heat you
might be able to say why the graphs are shaped as they are.
What you might find out
How is the freezing point of water affected by orange juice? What is the
freezing point of ice cream? Is there any truth in the idea that oily foods do
not freeze very easily?
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