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Kinetic Molecular Model of Matter and Temperature

States of Matter


  • They have a fixed shape and volume.
  • They have strong forces of attraction between their particles.
  • They have a fixed pattern (lattice).
  • Their atoms vibrate but cannot change position.


  • They have a fixed volume and change shape depending on their container.
  • They have weaker attractive forces than solids.
  • They do not have a fixed pattern; they take the shape of their container.
  • Particles slide past each other.


  • They do not have a fixed shape or volume; they fill up containers.
  • They have almost no inter-molecular forces.
  • Their particles are far apart, and move quickly.
  • They collide with each other and bounce in all  directions.

Pressure in Gases

  • The pressure gases exert on a container is due to the particles colliding on the container walls.
  • If the volume is constant, then increasing the temperature will increase the pressure.
Brownian Motion

Photo Credit: ResearchGate.net

  • Gas molecules move randomly.
  • This is because of repeated collisions with other gas molecules.
  • Small molecules move much faster and have higher energy than larger molecules.
  • The small particles can help move the larger particles.
  • Brownian motion can be see visually in smoke.
  • It is the escape of more energetic particles and occurs constantly on the surface of liquids.
  • If more energetic particles escape, liquid contains few high energy particles and more low energy particles, so the average temperature decreases.
  • Evaporation can be accelerated by:
    • Increasing the Temperature: More particles have energy to escape.
    • Increasing the Surface Area: More particles are close to the surface.
    • Reducing the Humidity Level in Air: If the air is less humid, fewer particles are condensing.
    • Blow Air across the Surface: This removes molecules before they can return to the liquid.
  • Pressure Change: P1V1 = P2V2
  • Temperature does not measure the amount of thermal energy in a body:
    • Two objects of different masses made of the same material at the same temperature would have different amounts of heat.
    • When a substance melts or boils, heat is added but there is no temperature energy.
Thermal Equilibrium
  • Thermal energy is transferred from a region of higher temperature to a region of lower temperature.
  • Thermal equilibrium is a condition when two or more objects in contact have the same temperature so there is no net flow of energy between them.
    NOTE: Not equal internal energy because the number of molecules are unknown.
  • Regions of equal temperature are in thermal equilibrium.
Measuring Temperature
  • A physical property that varies with temperature may be used for the measurement of temperature. For example:
    • Change in volume of a liquid or gas
    • Change in pressure of a gas
    • Change in electrical resistance
    • Change in E.M.F. of a thermo-couple.
  • The physical property should have the following qualities:
    • The change in property with temperature should be large enough to be measured accurately.
    • The value of temperature recorded should be reproducible, i.e. the measured property should be the same when recorded a second time
    • The property being used must be suitable over the temperature range being measured.
    • It should be able to be calibrated easily, hence the property should change uniformly with temperature.
Thermodynamic Scale
  • Thermodynamic (Kelvin) Scale is a theoretical scale that is independent of properties of any particular substance.
  • Based on the idea that the average K.E. of particles of a substance increase with temperature, and the average K.E. is the same for all substances at a particular temperature. K = °C + 273.15
  • Absolute zero is the temperature at which a system has minimal internal energy (not zero) – it is impossible to remove any more energy – at 0 Kelvin.
  • Triple point of pure water is the temperature at which water exists as a vapor, a liquid, and a solid – at 273.16 K (0.01 °C)
Practical Thermometers

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