Ruby Parker of Arthur St School, Dunedin, asks :-

Are bubbles liquids, solids or a gas?

Blair Blakie, a physicist researching ultra-cold gases at the University of Otago, responded.

A bubble is a gas pocket in some other substance which could be either a liquid or a solid. Usually when we think of bubbles what comes to mind is spherical film of liquid filled with air, as in a soap bubble. Some common examples of bubbles that occur in bulk liquids are carbon-dioxide bubbles in fizzy drinks, and the bubbles of steam that form in boiling water.

A well-known solid that contains bubbles is pumice rock, which is commonly used cosmeticically to remove rough skin. Pumice rock is formed from explosive volcanic eruptions in which the ejected liquid lava contains many bubbles that are frozen into the rock when it solidifies.

Bubbles feature in cutting edge scientific studies, such as the phenomenon of sonoluminescence (literally 'sound-light'), which is the emission of light by bubbles in a liquid excited by sound. The sound waves compress the bubbles and in some circumstances the bubbles may implode and give off energy in the form of light. Scientists believe that the temperature during collapse may be more than 10,000 degrees Celsius (some estimates suggest temperatures higher than a million degrees) and could provide conditions suitable for nuclear fusion to take place - the same process that occurs in the Sun. An experiment in 2002 has reported some evidence for fusion using sonoluminescence, but these results are still highly debated in the scientific community. Perhaps bubbles will provide an answer to our future energy needs?