A great visual demonstration of density effects.
- pint glass
- lager or other fizzy drink
- vegetable oil
- food colouring
- Alka-Seltzer tablet
- Drink some of the lager (or other fizzy drink) so that the pint glass is roughly half full.
- Pour vegetable oil into the pint glass so that the glass is roughly 2/3 full in total. Point out to your audience that the oil and water (lager) don't mix.
- Add a few drops of food colouring and stir the mixture – again highlighting that the food colouring only affects the water-based liquid (although you should see some interesting effects of tiny food colouring drops as they move through the oil mixture). Wait until the two layers have clearly separated.
- Add approximately 2 tablespoons of salt to the liquid in one go – a great foam eruption occurs! Observe the liquids after the eruption has settled down – you should be able to see a lava-lamp style bubble effect.
- Add an Alka-Seltzer tablet to the liquid – you should see some interesting bubbling effects from the gas given off by the Alka-Seltzer, and the change in the speed of those bubbles as they travel through water versus oil. You can prolong the effect by breaking the Alka-Seltzer tablet into smaller pieces and getting members of the audience to add the pieces individually.
How Does it Work?
Water and oil do not mix; they are called immiscible liquids. Lager or other fizzy drinks are mainly made of water, so they will not mix with oil either. Water is more dense than oil therefore all the water will sink to the bottom of the glass, leaving a separate layer of oil on top. Food colouring is a water-based substance therefore it will only mix with the water layer, leaving the oil layer in the original colour. Sometimes small droplets of pure food colouring will get stuck in the oil layer, creating highly visible blobs, but they will not colour the oil layer.
Adding salt to a fizzy drink causes a release of large quantities of the carbon dioxide trapped within the drink, creating the highly visible eruption. After the initial reaction has slowed down you should be able to see salt crystals at the bottom of the glass – salt is more dense than water so sinks to the bottom. Pouring the salt into the liquid in one go also causes some oil (stuck around the salt crystals) to be dragged down into the water layer. As the salt dissolves in the water the oil is released, which rises back through the water layer, creating the lava lamp effect. Some air may also be trapped with the salt, which will also travel up through both layers of liquid.
When Alka-Seltzer is placed into water it starts fizzing, giving off bubbles of carbon dioxide. This effect is greatly magnified if you place the whole Alka-Seltzer tablet in a drink that is already fizzy, since the drink will also give off bubbles of carbon dioxide. If you use smaller pieces of Alka-Seltzer, it should be possible to see individual bubbles of carbon dioxide being given off by the Alka-Seltzer tablet. These bubbles are much less dense than either the water or the oil, so they travel upwards through the liquid layers. Since oil is much more viscous than water, the bubbles will travel through the layers at different rates. You may also be able to see small coloured bubbles passing through the oil layer – those occur when some of the coloured water has got trapped inside an air bubble. When those coloured bubbles get to the top and the air is released, the coloured water will sink back down again through the oil layer.
Tips for Success
This trick can keep working for quite a while – just add more Alka-Seltzer. You won't see the big fizzy eruption once the drink loses its fizz, but the bubble effect is still very clear. The salt will tend to supersaturate the drink solution after a while, making it go cloudy and reducing the effect of the trick. Make sure your fizzy drink isn't too dark in colour or you won't be able to see the bubbles passing through the liquid.
This is another great trick for pub environments, where most of the ingredients are readily to hand. However, it is easily transferable to other environments and age groups. For younger audiences, substitute the beer for any other fizzy drink – a light coloured one will absorb the food colouring best.
Did You Know?
The similarity between this trick and that classic of retro chic the lava lamp is obvious to anyone who sees it. However, while this trick relies on the movement of liquids of different density for its effect the lava lamp achieves this by heating wax which is less dense than water when warm and molten, but denser than water when cold.