Science Buddies: "Ask an Expert"

If i am do a project on the effect of pure substances on the evaporation of water, I will be using units of milliliters probably 500ml. If i add a tablespoon if salt to that, it will be greater than 500ml. Should i keep my control at 500ml or adjust it to fit the others?

Good research topic. I would mix your solution to get a certain concentration (i.e. parts per million) and select your test fluid from that batch. As such, it doesn't matter what your starting volume is, as long as its the same for all samples. I might suggest having 2 identical experiments running concurrently to validate results (2 of all samples).

The thing to remember is that evaporation rate is afected by exposed surface area. For example, water in a glass will evaporate much slower than an equal amount of water in a pan with a much greater surface exposed to the air. Since you want to investigate the effects of impurities on evaporation make sure all solutions are in identical containers and held at the same environmental conditions (temp, humidity, etc..).

However you proceed, make sure you can measure what may turn out to be small changes in volume.

Hope this helps, let us know if you have additional questions.

theborg,
Expert

For future research, you may want to read up on the effect of pure substances on the freezing or melting process--similar to evaporation, but on the other side of the temperature scale. Find out if the effect is similar to what you find for the evaporation process.

Good luck in your project.

Raji Pingali