Watch the rise and fall of hot and cold fluids.
What you need
- Somewhere it’s OK to spill coloured water, like the bathtub or outside
- 4 identical small, wide-mouthed bottles. Something like baby food jars work well
- A flat, rigid piece of plastic that is larger than the opening of the bottles, like an old credit card. Some cardboard cut from a cereal packet or a postcard or business card might also work
- Cold and hot (not boiling!) water from the tap
- 2 different colours of food colouring
- Two plastic plates or trays (to hold any spilled water)
What to do
- Completely fill two bottle all the way to the top with hot water so a little bit is bulging over the top. Add a few drops of one of the food colours to these bottles.
- Completely fill the other two bottles all the way to the top with hot water so a little bit is bulging over the top. Add a few drops of the other food colour to these bottles.
- Place the plastic card or cardboard over the opening of the warm bottle. Making sure the card completely covers the opening. While holding the plastic card in place, turn the warm bottle upside down and stack it on top of the cold bottle. Make sure that the rims line up exactly. If you’re having trouble flipping the container without spilling lots of water, try using a stiffer plastic card.
- Repeat with the other two bottles, but this time, put the cold bottle on top.
- Carefully pull the plastic card out from between the bottles. Watch what happens and compare the results of the 2 experiments.
Did the waters mix or stay separated? If you didn’t see a difference, try using warmer and colder water (putting the water in the fridge might help).
Other ways to experiment
- Instead of using warm and cold water, try filling 2 bottles with water of the same temperature. Do you get the same result?
- Instead of warm and cold water, add some salt to one of the bottles. This will be like mixing fresh water and salt water. Repeat the experiment. What happens?
- What happens over time? After you’ve stacked your bottles and removed the plastic card, leave the bottles in a safe place where they won’t be knocked over. Wait until the water in both bottles reaches the same temperature and observe what happens.
What’s happening?
When you removed the cards from one set of bottles, the hot water stayed on top and the cold water stayed on the bottom, with the colours staying pretty much the same. In the other set, however, something very different happened. The hot water rose, and the cold water sank. As this motion occurred, the colours mixed. This happened because water with different temperature has different densities. You might have experienced this when getting into a pool or slow-moving river, you dip your toes in and it is nice and warm, but then all of a sudden it gets very cold the deeper down you go!
Density is how much ‘stuff’ is within a certain space. Think of a brick and a block of wood that are the same size. The brick is heavier than the wood, so it is denser. Things can be denser by having more atoms in the same amount of space, or heavier atoms, or both.
Temperature can affect the density of some objects. When the temperature of something goes up, the atoms inside it are moving around more and take up more space. This makes the volume go up, and the density go down. The opposite happens when something cools. The atoms slow down and take up less space. So the volume goes down, and the density goes up. Density can be worked out by dividing mass by volume.
Less dense objects float if they are placed in a liquid or gas that is more dense. This is why helium balloons and hot air balloons float upwards in the sky and why surfboards and ships float in the ocean. In this experiment, when the bottles are stacked, the less dense warm water floats on top of the cold water – so they do not mix. If the cold water is put on top, it sinks, causing the waters to mix.
The next time you go swimming in a pool, try noticing the temperature difference between the surface water and the deeper water!
This DIY Science activity was adapted from a combination of Questacon’s Hands on Activities and the Exploratorium’s Science Snacks. You can find the original experiments and more at: https://www.questacon.edu.au/learn-and-play/activities/water-density or https://www.exploratorium.edu/snacks/inverted-bottles
