Kirsten, Kim and Gemma of Paraparaumu Beach School asks :-

We added a teaspoon of bubble bath to two litres each of hot, warm and cold water then whisked each for 30 seconds. The cold one had more bubbles. Why?

Kathryn McGrath, a chemist at Otago University, responded.

A single bubble consists of air on both sides of a thin film of soap solution and a foam is where trapped air bubbles are surrounded by the soap solution.

Soap is required to form the bubble by decreasing what is known as the surface tension between the air and the water. You can think of surface tension as being a measure of the dislike two substances have for each other when they are forced into contact. This means for high surface tensions the two substances will want less contact with each other and so there will be fewer bubbles made and they will tend on average to be larger since this decreases the water/air contact overall.

In the case of water and air the surface tension can be decreased by the addition of soap, that is the soap decreases the amount that air and water dislike each other by interacting favourably with both, allowing bubbles to be formed; the more soap, the more bubbles.

Simple foams can be made by taking a solution of soap, such as bubble bath, and adding air eg by whisking. Changing the type of soap solution used can make different foams, for example more stable bubbles are produced by adding glycerol to the soap solution, or by changing the amount of air that gets added to the solution, e.g. by shaking more vigorously.

Another way of changing the final foam is to change the temperature of the soap solution. This is because surface tension changes with temperature, and as you change the surface tension you change the number of bubbles formed, their size and how long they last.

One other thing has to be considered with soap solutions. As you increase the temperature the soap becomes more soluble in the water and hence less sits at the boundary between the air and the water so for a soap solution you may actually reverse the surface tension effect and in fact more bubbles can be formed at lower temperatures. Hence the number of bubbles is controlled by both the surface tension and the solubility of the soap.