what happens

• when you heat a pan of cold water on the hob, the heat energy passes through the metal pan by conduction and is then transferred to the water molecules in the pan.
• as the amount of heat energy going into the water increases, the water molecules start to move upwards where the water is cooler, and collide with other molecules as they do so, thus passing on the heat energy into the food cooking in the water, such as vegetables, rice or pasta
• the more heat energy that passes into the water, the faster the water molecules move the more they collide with other molecules
• when they reach the surface of the water, the molecules start to slow down and sink back down again
• as they reach the bottom of the pan, they receive more heat energy and start to move upwards again
• this sets up convection currents, and eventually when there is enough heat energy in the water, it will boil, which means that the water molecules are moving at their fastest and you can see the water rapidly moving - this is sometimes called 'a rolling boil'