What contains more heat, a cup of coffee or a glass of iced tea? In chemistry class, that would be a trick question (sorry!). In thermodynamics, heat has a very specific meaning that is different from how we might use the word in everyday speech. Scientists define heat as thermal energy transferred between two systems at different temperatures that come in contact. Heat is written with the symbol q or Q, and it has units of Joules (\text JJstart text, J, end text).
Three melting ice cubes in a puddle of water on a mirrored surface.Heat is transferred from the surroundings to the ice, causing the phase change from ice to water. Photo of ice cubes from flickr, CC BY 2.0.Heat is sometimes called a process quantity, because it is defined in the context of a process by which energy can be transferred. We don’t talk about a cup of coffee containing heat, but we can talk about the heat transferred from the cup of hot coffee to your hand. Heat is also an extensive property, so the change in temperature resulting from heat transferred to a system depends on how many molecules are in the system.