Heat, Temperature, and the Particle Theory

What actually is the difference between water at 20ºC and water at 50ºC? What is the difference between heat and temperature? How are these questions related? Can any one hypothesis answer both questions?
The Particle Theory
Scientists over the years came up with many ideas, or hypotheses, to try to explain the difference between heat and temperature. One such hypothesis was thought of by Lavoisier. He suggested that heat might be a substance with mass, which he called caloric. But Lavoisier's idea was not supported by experimental observations, and scientists looked for other ways to explain heat.

Scientists now use the kinetic molecular theory, or particle theory, to explain heat and temperature and the difference between, say, 20ºC and 50ºC. The particle theory is based on a model that suggests that all matter is made up of tiny particles too small to be seen. According to this model, these particles are always moving- they have energy. The more energy they have, the faster they move. So far, all the evidence is made up of moving particles. That is why we call the particle model for matter a theory.

Heat and Temperature
So what is the difference between heat and temperature? According to the particle theory, heat is energy, and it is transferred from hotter substances to colder ones. Temperature is a measure of the average energy level of the particles in a substance. 

Both hot and cold water are made up of moving particles, some moving quickly, and some moving slowly. But on average, the particles move faster in hot water than in cold water. 

Expanding and Contracting 

The particle theory is a useful model to explain why substances expand when they are heated and contract when they are cooled. At high temperatures, particles have more energy, move more quickly, and have more collisions. As a result, they take up more space, and the substance expands. At lower temperatures, particles have less energy, move more slowly, and have fewer collisions. They take up less space, and the substance contracts. 
Self Check 
  1. In your own words, describe the difference between heat and temperature. 
  2. Two cups contain the same amount of water.

  3. a) What device would be best for comparing the average energy of the particles of each sample of water? 
    b) Explain how this device would help you compare the energies of the particles.
  1. Use the particle theory to explain how an outdoor thermometer works. 


' From Heat and Temperature, ITP Nelson. © Copyright 1997 
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