Why are the bubbles in fizzy drink so small? The ones I blow are much bigger - Alison, aged seven, Aberdeen, UK
Thank you for your question, Alison! First of all, we have to know where the bubbles in the fizzy drink come from. This happens because they have a gas called carbon dioxide dissolved in them.
The gas and the liquid (and everything else) are made up of tiny bits of stuff called molecules. When the gas dissolves in the liquid, the molecules mix together really well so that the gas gets trapped without there being any bubbles.
The amount of gas you can dissolve into the liquid depends on how much pressure it is under. When the pressure is high, it is like there is a lot of weight pushing the gas into the liquid, so lots of it can dissolve. To make a fizzy drink, carbon dioxide is bubbled through liquid at a pressure that is five times greater than the normal pressure we live at.
We would feel the same amount of pressure if we swam to the bottom of a swimming pool that was 50 metres deep (50 metres is about eight giraffes standing on top of each other).
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This is how so much gas is trapped. A can of fizzy drink has enough gas dissolved in it to blow up a small balloon.
When you open a bottle or can of fizzy drink, the pressure on the liquid suddenly gets smaller. The drink can trap much less carbon dioxide at this pressure, so the extra gas stops being dissolved and forms bubbles.
Watch the bubbles
To learn more we will need to run some experiments. Pour some fizzy drink into a glass (ask a grown-up first). Look at how the bubbles start where the liquid touches the glass – at the bottom and sides, but not in the middle of the drink.
To make a new bubble, the molecules in the carbon dioxide and the drink move around and get organised to make a surface between them. Less new surface needs to be made if the bubble starts against the edge of the glass. This uses less energy and means new bubbles usually start here.
This is also why small bubbles tend to stick to the glass. More gas will join the small bubble, as it takes less energy to escape into a bubble to make it bigger than make a new one.
Eventually, the bubble gets big enough that it will float to the top of the drink. This happens when the bubble is still smaller than a grain of sand.
When you blow through a straw to make bubbles, the same forces are acting on the bubbles as before. But the bubble is stuck to the straw all round the edge of the hole. This means that you can’t make bubbles that are smaller than the hole in your straw.
This is why they are larger than the bubbles that form on their own in a drink. Try to find a thinner straw to see how small a bubble you can make.
You might be surprised to learn that now that you know how bubbles form in a fizzy drink, you also know why volcanoes erupt. If you dug down deep under a volcano, you’d find hot, liquid rock called magma. This has gases dissolved in it.
As the magma rises to the surface, the pressure goes down. It can hold less gas and so bubbles form in it, just like in your fizzy drink. The bubbles make the magma more buoyant, like a hot-air balloon. This pushes it upwards, causing an eruption.
We know that magma usually has lots of small crystals in it. Do they affect the bubbles?
Time for one final experiment: pour some salt into your fizzy drink and watch what happens. You should see lots of bubbles forming all at once because the salt crystals have lots of surfaces for the bubbles to form on. You’ve just made your own volcanic eruption!
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