Exploring Liquids: An Experiment

Here’s a fun experiment you can try using the contents of your kitchen cupboard. Explore the effects of different densities and learn about refraction, viscosity and the planet Jupiter. You’ll need five different liquids; I used golden syrup, dishwashing liquid, water, alcohol and vegetable oil. I also used some food colouring to make it easier to see what was going on (and because the alcohol I use is Tequila which looks just like water). If you have a chopstick around that will also be handy – but any stirring implement will do.

Instead of golden syrup you could use treacle or corn syrup: something gloopy and messy. For alcohol you can use almost anything from spirits to rubbing alcohol. If you’re a kid, please ask whoever is in charge before raiding the drinks cabinet (this is generally a good rule in life).

You will be putting all of these into one glass, so find a tall one that is reasonably straight-sided. The more angled the edges of the glass are, the more of each liquid you will need as you go along – this could get tricky. You’ll need about one sixth of a glass worth of each liquid, as shown above.

Start with the glass containing the syrup and carefully pour the dishwashing liquid into it. You want to pour down the sides of the glass if possible, this will help stop the two liquids mixing. For these two it should be easy enough as they are both fairly thick. Let the glass stand for a moment and you should see that the two liquids make layers in the glass.

Now we add the rest of the liquids, in the order shown in the image at the top. Pouring down the side of the glass may not be enough to prevent these remaining liquids from mixing. It helped me to use a teaspoon. Hold the teaspoon inside the glass just above the surface and pour gently into the spoon, allowing the liquid to pour over the sides. In this way, the liquid you are pouring is placed on top of the existing ones much more gently. Be careful and pour slowly and you should be fine. You can always practice your pouring in another glass if you want to.

Hopefully you will end up with a nice, layered glass of different coloured liquids. Why? Well it is all to do with density – the mass per volume – of the liquids. Liquids at the bottom have higher densities than those at the top. All liquids have different densities as well many other properties. We can explore some of them using our layered glass.

Take a chopstick, or similar stick-like object and insert it into the glass. Don’t disturb the glass too much though. See how the chopstick looks now. Each liquid bends the light coming through the glass in a different way. This is related to the density but is actually a property of all materials called the refractive index. In general the refractive index increases with density. It is a measure of how much light is bent as it passes through a material.

We can also explore two more properties of liquids: viscosity and miscibility. Take hold of the chopstick and stir the glass up. Stir well, but try not to spill the contents. Watch and feel how the liquids mix.

You’ll notice that it is very hard to mix the syrup and that you can also feel the washing up liquid dragging against your stirring. This is because heavy liquids tend to be viscous. Viscosity is the measure of a liquid’s resistance to you changing its shape. The syrup is highly viscous, i.e. it resists your chopstick very strongly. The water, alcohol and oil offer almost no noticeable resistance at all, by comparison. This is because they are all similarly viscous and we are all used to how water ‘feels’.

After you stop stirring the glass, wait a few moments for it to settle down again. Watch it during this time and you will see it appear to organise itself. Miscibility is the measure of how well two substances mix. Water and alcohol are miscible because they mix together just fine. You can see that they do not separate out again after you stop stirring.

Water and oil are immiscible. This means that they do not mix. It doesn’t take long after you stop stirring for the oil to float back to the top. Would you say that the washing up liquid is miscible with any of the others? I found it hard to tell because both it and the syrup were so viscous I could not stir them up very easily!

Liquids, and their many properties, shape all sorts of things in the world around you. Refraction, density, viscosity and miscibility are important in our everyday lives.

Anyone who has had to clean oil-based paints off a brush with water knows something about miscibility. If you have tried to hook something out of a pond or swimming pool who have experience the frustration of refraction. You also know that you float in the sea, this is because of the difference in density between the salt water in the sea and the water in your body. Have you ever had to wait for ketchup to slowly pour out of a bottle because of its high viscosity?

In astronomy you can see some of these properties in action. The planet Jupiter has a series of coloured bands running across its surface. The different fluids that make up Jupiter’s enormous mass do not mix well because of their make-up. The hydrogen- and helium-based fluids are thought not to be miscible and this is part of the reason that we see the striking bands or colour on the planet’s surface.

Refraction and density are a big problem when observing through a telescope. The air becomes unevenly heated and this creates patches of the sky with higher refractive indices than others. Pollution also changes the refractive index of the air. The result is a fuzziness that astronomers call the ‘seeing’. Because of this, astronomers prefer to put telescopes high-up and away from cities with their hot buildings and smog.

Viscosity is important in simulations of planet formation. Disks around young stars are made up of different materials and if they get dense enough they will become highly viscous and drag against the force of gravity as it tries to force them around the central star. The result is that parts of the disk could be dragged inward and this could greatly determine the formation and evolution of planets.