Why do the bubbles in a drink go up?


I was enjoying a fizzy drink, when my 6-year-old pointed at my glass and asked “Why are the bubbles going up?” So, we talked about what these drink bubbles are made of and how different things float (or sink) in liquids.

The bubbles in a fizzy drink are made of a gas called carbon dioxide (which has the chemical formula CO2). Carbon dioxide is one of the gases in the Earth’s atmosphere. It has lots of uses in making foods, especially drinks.

When fizzy drinks like lemonade or cola are being made in a factory, a mixture of carbon dioxide and water is forced into the drink liquid at very high pressure before it is put into bottles or cans. Later, when you open the can or bottle at home, you hear a ‘pop’ or ‘fizz’ noise, as the pressure is released and bubbles of carbon dioxide rush through the liquid.

The reason the bubbles travel up from the bottom of the bottle, can or glass to the top is that carbon dioxide is less dense than the drink liquid. Density is a way of measuring how tightly the molecules or atoms of a chemical are packed together. In general, molecules in a gas are less tightly packed than those in a liquid.

So, the lower density of the carbon dioxide compared to water and many other liquids means that the bubbles in a fizzy drink will float upwards. And, when they reach the top of the drink, the bubbles burst and the carbon dioxide gas is released into the air.

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