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Name: Laura
Status: educator
Grade: 4-5
Location: CA
Country: USA
Date: Fall 2011


Question:
We did an experiment with our classes. We had equal amounts of sand and water. After taking an initial temperature reading, we placed the containers in ice water and recorded the temperature of both containers every two minutes. Most students got the results that the sand cooled much more slowly than the water. That seems to contradict what we've learned; that land cools more quickly than water. Do you know why we would get these results?



Replies:
Heat transfer. The sand is a solid with air space. Heat transfers slowly through such a material. In the water, warm material quickly mixes with the cooler, allowing the heat energy to move through the whole sample more quickly.

The demonstration I use to show that sand heats up faster than water is to have two bottles, one containing 500 g water and the other 500 g sand. I pass them around the class, and instruct each student to give them ten good shakes. That way each material gets the same energy input. After the two bottles.have made the round of the class, the sand's temperature has increased more than the water's.

Richard E. Barrans Jr., Ph.D., M.Ed. Department of Physics and Astronomy University of Wyoming


There were probably equal volumes of sand and water, right? If so, there was far more mass of sand than the water, i.e. the container of sand probably weighed much more than the water. If you started out with sand and water at room temperature, this means that there was much more heat in the sand that needed to get into the ice water to reduce the sand's temperature.

Next, the cooling was through conduction, that is heat had to travel through all that sand to get to the cold outer surface next to the ice water. The air in between the sand grains acts as a good insulator slowing the flow of the heat through the sand.

Last, the thermometers were probably in the middle of the water and sand. When we say the land cools faster than the water we are measuring the surface temperature of each. Below a relatively shallow depth, the temperature of sea water and land remain very constant. This is why caves have constant temperatures year round regardless of winter's cold or summer's heat.

Hope this helps explain your contradictory results.

Bob Avakian Oklahoma State University Institute of Technology.


Laura,

While it is true that water has a higher heat capacity than sand - and this would mean that a lot more heat has to be removed from water before its temperature changes, other factors were affecting your experiment. I suspect that the amount of contact that the sand hand with the container walls limited the amount of heat extracted by the cooling bath. If this is true, then because of the slow heat extraction, there would be a slow drop in temperature.

You could try the experiment again with a range of mixtures of sand and water (totaling to the same mass), from pure sand all the way to pure water (with maybe 3 to 4 varying amounts of sand and water in between). The idea here is that the water mixed in with the sand would work to have as much contact as the pure water. You may find that there will be a direct relationship between amount of sand and cooling - except for the pure sand, which would indicate that something else (like what I described) is in effect with the pure sand.

Greg (Roberto Gregorius) Canisius College


You have to keep in mind "the relative amounts" of sand and water present in your lab container compared to a lake or stream. Also things become more complicated because you are only measuring the surface of the land, but the water at different temperatures has different densities and you cannot stir it up easily. In addition, water is most dense at 4 C. and less dense above and below that temperature, so the convection of the water is more complicated. Also it is not easy to keep the temperature of a lake or stream constant. The two things you are measuring are just very different in mixing and relative amounts.

Vince Calder


Laura,

The water forms a continuous volume over which cooling can take place and has a much greater thermal conductivity than sand. Furthermore, the sand has pores of air within it and air is an even poorer thermal conductor than sand.

Here are the thermal conductivities of sand, air, and water at 0 deg C (cal cm-1 s-1 deg C-1):

air 0.000058 sand (medium fine) 0.00063 water 0.00132

David R. Cook Meteorologist Climate Research Section Environmental Science Division Argonne National Laboratory



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