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# Relationship between heat and temperature

Heat and temperature are two different but closely related concepts. Note that they have different units: temperature typically has units of degrees Celsius (^\circ\text C∘Cdegrees, start text, C, end text) or Kelvin (\text KKstart text, K, end text), and heat has units of energy, Joules (\text JJstart text, J, end text).

Temperature is a measure of the average kinetic energy of the atoms or molecules in the system. The water molecules in a cup of hot coffee have a higher average kinetic energy than the water molecules in a cup of iced tea, which also means they are moving at a higher velocity. Temperature is also an intensive property, which means that the temperature doesn’t change no matter how much of a substance you have (as long as it is all at the same temperature!).

This is why chemists can use the melting point to help identify a pure substance-−minusthe temperature at which it melts is a property of the substance with no dependence on the mass of a sample.On an atomic level, the molecules in each object are constantly in motion and colliding with each other. Every time molecules collide, kinetic energy can be transferred. When the two systems are in contact, heat will be transferred through molecular collisions from the hotter system to the cooler system. The thermal energy will flow in that direction until the two objects are at the same temperature. When the two systems in contact are at the same temperature, we say they are in thermal equilibrium.