Simple density experiments to do at home
The oceans are a complex system. Many are the processes and phenomena behind how they work. However, some of the “physics” after them are also present in our day to day live. We can actually represent many of these features in the lab, or even better, at our homes. In this blog post, as if in an online recipe, we provide with the instructions and ingredients to get hands on with some easy experiments!
Here we propose four experiments to see how temperature and salinity are very important determining the density of water. Water becomes denser as it cools down (before approaching its freezing level). Similarly, it becomes denser when it becomes saltier. By changing the density of water we will show how we can generate circulation, just like in the real oceans.
First of all, we need the following ingredients:
- 2 small transparent containers (like two glasses or jam jars)
- 1 medium transparent container (enough to fit ~3 pints of water)
- Water (both cold and warm)
- Salt
- Food colouring (2 different colours)
- An ice cube tray (to freeze coloured water)
- A small sponge
- A spoon or an eye dropper
Experiments 1 and 2. Cold water on top of fresh and salty water.
In the first two experiments, we will study what happens when very cold water arrives in the ocean’s surface. Beforehand, we will mix some food colouring with water. Just enough to fill an ice cube tray. We should have a couple of coloured ice cubes before starting the experiments. Once we are ready, we will fill the two smaller containers (or glasses) with water, so we can run the two experiments at once. In one of them we will disolve one or two table spoons of salt. This way, one glass will contain fresh water and the other one salty water. Then, at the same time, we will add one ice cube to each of the containers. Can you see any differences? What do you think it will happen to the cold water that melts off the ice on fresh water? And what happens when it melts on water that is much saltier? Feel free to make your guess and try out the experiment before seeing the solution at the end of the post!.
Experiment 3. Salty water on top of fresher water.
For the third experiment, we will look into what happens when salt water is formed on top of fresh, warm water. This time we need the larger container. We will fill half of it with warm water. On the side, we will prepare some salty and coloured water by adding food dye and a couple of table spoons of salt to a pint of water. Then, with a lot of care and with the help of a spoon handle or eye dropper, we will slowly pour in the coloured water in, letting it flow down the side of the container rather than pouring it directly. We want to avoid splashing and mixing. In this case, what do you think it will happen when adding saltier water onto fresh, warmer water? Can you think of any place in the world where you can find cold and salty water being added on top of fresher and (slightly) warmer water? Do try this experiment first, then see a demonstration at the end of the post.
Experiment 4. A layered ocean.
Finally, we propose a forth experiment. This time we want to represent the multi-layered ocean. We will see that in the real oceans, layers of different density are established all over the planet. In fact, by just measuring some of the water properties (like temperature and salinity), we are able to trace waters that were formed much further away in different places of the world! For this experiment we will use again the medium sized container and three different types of water. First a pint of transparent water, with 2 tablespoons of salt. Then another pint of fresh and warm water, with one of the food dyes. Finally, another pint, this time with a different dye colour and 4 tablespoons of salt. In the container, we will first add the transparent salty water. Then, with the help of a sponge floating on the top, we will very carefuly add the warm and fresh water on top of the sponge. This is to help adding the water without mixing, so that a distinct interface can be seen. Finally, with the help of a spoon handle or the eye dropper, we will pour the last salty and coloured water through the side. Now, how many layers do you expect to see (one mixed “soup”, two different layers or more?). If any layers are formed, in which order do you expect them to be?
And the demonstration…
We hope you could manage to reproduce these experiments at home. Now, let’s find out the solutions. In fact, you can “see” them shown by some of us. A couple of years ago we did these experiments and recorded a video. Watch it to see how we did and the results we got. We also discuss what is happening and where in the real world you can find these phenomena. And please, feel free to make any questions or comments below.