Rebecca Zahar
Ecole des Navigateurs
Floor Location : J 074 E

Have you ever wondered why when you swim in a lake, the water at the surface is warmer than when you dive down closer to the bottom? It seems logical that a heavier substance will sink and a lighter substance will rise, however, water in a lake is usually all the same substance. So, maybe it has something to do with temperature and the density of the water. In my experiment, I tested if I could replicate the action of the lake water. To do so, I mixed hot water and cold water and observed which one rose to the top. The results showed me which liquid was lighter or less dense. Because of my experience at the lake, my hypothesis was that I would have similar results, and the hot water would rise to the top.

First of all, I filled a glass with very cold water and added blue food coloring. Next, I filled a similar glass with very hot water and I added red food coloring. Then, I took a piece of hard plastic and placed it on the glass of cold water. Making sure that the water did not spill, I flipped the cold water glass and placed it on the hot water glass, rim to rim. When the glasses were stable, I took out the piece of plastic and observed what happened. The hot water rose to the top, through the cold water, therefore mixing with the blue and creating a purple liquid. For my next test, I did the same thing, except I put the hot water glass on top of the cold water glass. When I removed the plastic, the red and the blue mixed a bit, but quickly separated back into their two distinct colours. Since the hot water was already on top, it had nowhere to rise. So, the red stayed on top and the blue remained on the bottom. Eventually however, as the warm liquid cooled and the cold water warmed, the colours mixed together and all the water turned purple.

But why did this happen? We know that certain liquids float on others. For example, in unshaken salad dressing, oil floats on vinegar. This is because they have different densities. When you heat up water, the molecules get excited and start to separate. Because there is more space between the molecules, hot water becomes less dense than cold water and it floats to the top.

To conclude, the hot water either rose to, or stayed at, the top of the two mixtures, therefore proving that it is less dense than cold water. My results confirmed that my hypothesis was correct and explained why the water at the surface of the lake is warmer than the water at the bottom. It would be interesting to do this experiment with other liquids and compare the results. For example: I could try the same thing with oil and alcohol or salt water and fresh water. From the results of my experiment with warm and cold water, I could predict which of the above liquids was the less dense because it would float to the top. One more question arises however. If cold water is denser than warm water, and therefore sinks, why does ice float? Perhaps this is a question for another experiment!