Ultrasound in liquid can generate and then collapse gas bubbles. The compressed gas inside the collapsing bubbles becomes hot, in much the same way as the gas within a bicycle pump gets hot way new pump up the bicycle tyre. In fact, it  can reach temperatures similar to those found on the surface of the sun (6000K). This can cause chemical reactions to take place within the gas and the liquid, one result of which is the emission of light, called sonoluminescence. The ability of ultrasound to cause chemical reactions in this way is called sonochemistry.

In this exercise we are going to illustrate these effects by looking at one example problem in the field. Chemists often examine the efficiency of ultrasound to produce chemical reactions, by comparing the sonochemical yield as the frequency of the sound field changes.  In this way they hope to find the optimal frequency to use.  We will here show that the usual approach, of looking at a number of discrete frequencies (say, 100 kHz, 250 kHz, 500 kHz, 1000 kHz) is misleading, because the effectiveness of the ultrasound field in a typical vessel can vary hugely over changes are frequency of as little as 1 kHz.  This is demonstrated by monitoring the sonoluminescence given out by the bubbles.