A temperature probe is placed in a container of icewater, as seen in the photograph below. At the left is a digital timer, measuring elapsed time in seconds, in the center is the thermal probe, measuring the temperature of the icewater bath in degrees Celcius, and at the right is the 1000 ml beaker of icewater with the thermal probe.
Now 50 ml of salt is poured into the icewater, the timer started, and the thermal probe used to mix the salt into the icewater, as seen in an mpeg video by clicking your mouse on the photograph above.
The question is: What will happen to the temperature of the icewater as time goes on?
- (a) The temperature of the icewater will increase.
- (b) The temperature of the icewater will decrease.
- (c) The temperature of the icewater will remain about the same
To see the answer, click Read More on 12/12/14
The answer is (b): the temperature of the icewater will decrease, as seen in an mpeg video by clicking your mouse on the photograph below.
As time goes on and the salt is continually mixed into the icewater, the video has been edited to show short segments of the action such that you can see the temperature of the mixture as a function of time.
The freezing point of salt water is lower than that of pure water, due to the impurity. Therefore when the water is salted ice will melt, lowering the temperature of the icewater bath to the freezing point of salt water, as seen in the video. This technique is used on roads to lower the freezing point of the water on the roads and thus prevent the freeze-thaw cycle from further damaging the road surfaces. It is only good down to the ultimate freezing point of salt water, about 8o Celcius, or 12o Fahrenheit lower than the freezing point of fresh water. Thus salting roads is not an effective technique in far northern climates where the temperature often becomes considerably lower than the freezing temperature of salt water.