Blue plus yellow equals green, right? Not always! In this experiment, you can pull off a colourful magic trick made possible by water density and temperature.
You will need
- Two small glasses of the same size
- Thin cardboard
- Scissors
- Cold water from the tap
- Warm water from the tap
- Blue and yellow food colouring
- Tray
Safety
This activity requires very warm water and glassware, adult supervision is recommended. Take care when flipping glasses, make sure you have a good grip and wear closed toe shoes in case it breaks. If glass does break, ask an adult to help you clean it up.
What to do
Flip one of your small glasses on top of your cardboard. Use scissors to cut out a square that is slightly larger than the glass.
Fill one of your small glasses with cold water, the colder the better. Fill it to the brim and place it on your tray.
Add a drop of blue food colouring and mix it into your cold water.
Run your tap on warm until it becomes very warm but not scalding to the touch. Fill your other small glass with this warm water and place it on your tray.
Add a drop of yellow food colouring and mix it into your warm water.
Here comes the first experiment: cover the cool blue glass with your cardboard square, press the cardboard onto the opening of the glass, grip the glass firmly and quickly flip it onto the top of the yellow glass. It’s okay if you spill a little.
What do you think will happen when the different coloured water touches? Hold onto the top glass and carefully slide the cardboard out. Make sure the openings of the two glasses line up as you do so.
Viola! What happened to the colour of the water? Is that what you expected?
Now set up the second experiment: repeat steps 1-5 to set up two glasses filled with coloured water.
Before you move on, test the temperature of the hot glass. You’ll need to hold onto it for a while, so if it’s too hot to touch you should wait for it to cool down.
This time cover the warm yellow glass with your cardboard square, press the cardboard into the lid of the glass, grip the glass firmly and flip it onto the top of the blue glass.
What do you think will happen when the water touches? Carefully slide the cardboard out from between the glasses. Make sure the openings of the two glasses line up as you do so.
Viola! What happened to the colour of the water? Is that what you expected?
What’s happening?
What changed between the two experiments? It seems to matter which liquid is on top. So, let’s start by tackling what makes something float.
Liquid water is made of lots and lots of tiny molecules that are all bouncing around. These bouncing molecules collectively push in all directions, including upwards. Objects that float on water weigh less than water’s upward push.
Floating actually depends on both weight and size. Light things like coins can sink if they’re small enough, and heavy things like inflatable boats can float, as long as they’re really big.
Weight and size are brought together in a concept called density, which you can calculate by dividing weight by size (more technically called mass and volume). If an object is less dense than water, then it will float. Likewise, if something is denser than water, it will sink.
But water can change its density! How? It all has to do with temperature.
Temperature is a measure of how fast the molecules in a substance are bouncing around. The higher the water temperature, the faster the water molecules go. And by bouncing more, the molecules get pushed apart, increasing the water’s size (or volume). Now we have the same amount of weight over a larger size. This means hot water has a lower density than cold water. And that means hot water floats on top of cold water.
This explains our second experiment where the colours didn’t mix. It also explains why the denser cold water splashed down into hot water in the first experiment. There you have it, a splashy, science-y “magic” trick!
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