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Kinetic Theory of Matter

  • Kinetic energy is the energy a body has by virtue of its motion.
  • The kinetic theory of matter assumes that the particles are in constant motion.
  • Basic assumptions of the kinetic theory of matter are:
    • Matter is made up of tiny invisible particles that are in constant motion and therefore posses kinetic energy.
    • Increase in the heat supply and increase in temperature causes an increase in the motion of the particles.
    • Increase in the kinetic energy due to an increase in heat supply brings about a change in state, i.e., solid, liquid, and gas.
States of Matter

States of Matter (Photo Credit: Pinterest.com)

  • Solids
    • The particles are closely packed together by strong attraction forces of cohesion.
    • The cohesive forces are strong enough to resist the movement particles.
    • These particles however possess kinetic energy due to vibration and are able to rotate about their fixed position but are not able to move from place to place.
    • As a result, solids have a definite shape and definite volume and are difficult to compress.
  • Liquids
    • The particles are slightly farther apart than in solids.
    • Therefore, the forces of attraction between them are less and the particles are slightly free from each other.
    • The particles move or flow and have definite or fixed volume but no definite shape.
  • Gases
    • The particles are farther apart than they are in solids and liquids.
    • Thus, the cohesive forces between the particles are negligible.
    • The particles have more kinetic energy than those in liquids and solids and therefore move about in all directions at the greatest speed.
    • As a result, gases do not have definite shape nor volume but occupy the volume of their containers.
Change of State (Phase Change)
  • With adequate amount of energy, matter can be changed from one state to another, e.g. solid can be changed to liquid, solid can be changed to gas, etc.

Phase Change (Photo Credit: LumenLearning.com)

  • Melting
    • This is the change of a substance from the solid state to the liquid state.
    • This change occurs as a result of the addition of heat.
    • The temperature at which this change occurs is called the melting point of the substance.
    • Melting points differ by substance, e.g. the melting point of iron is 1538 °C while the melting point of butter is 35 °C, the melting point of water is 0 °C.
  • Freezing
    • This is the change of a substance from the liquid state to the solid state.
    • This change occurs as a result of the removal of heat.
    • The temperature at which this change occurs is called the freezing point of the substance.
    • Freezing points differ by substance, e.g. the freezing point of water is 0 °C while the freezing point of milk is around 32 °C, and the freezing point of water is 0 °C.
  • Evaporation
    • This is the change of a substance from the liquid state to the gaseous state.
    • This change occurs as a result of the addition of heat.
    • Liquids tend to evaporate at their boiling point; water begins to evaporate at 100 °C, alcohol begins to evaporate at 78.5 °C.
  • Condensation
    • This is the change of a substance from the gaseous state to the liquid state.
    • This change occurs as a result of the removal of heat.
    • The temperature at which this change occurs is called the condensation point of the liquid, which is the same as the boiling point of the liquid.
    • Examples: Water condenses at 100 °C, alcohol condenses at 78.5 °C, etc.
  • Sublimation
    • This is the change of a substance from the solid state directly into the gaseous state.
    • This change occurs as a result of the addition of heat.
    • Examples of substances that can undergo sublimation naturally are: dry ice (solid CO2), metal iodine, sulfur, etc.
Evaporation and Boiling
  • Evaporation is the escape of particles in gaseous form, from a liquid body while it is being heated.
  • Factors which affect evaporation are:
    • Temperature: The rate of evaporation increases as temperature increases and vice versa.
    • Pressure: Liquids evaporate more rapidly at lower pressures.
    • Area of Liquid Surface Exposed: The larger the area of the surface exposed, the more rapidly the evaporation will be, and vice versa.
    • The Nature of the Liquid: The lower the boiling point of the liquid, the greater the rate of evaporation, and vice versa.
    • Wind and Dryness of Air: Wind blows the water vapor around the material causing more evaporation to take place.
  • Boiling is a state in which all the particles of a liquid have acquired heat energy and bubbles of vapor in the liquid rise to the surface.
  • Boiling points vary from liquid to liquid. E.g. The boiling point of water is 100°C while that of ethanol is 78°C.
Differences between Evaporation and Boiling
EvaporationBoiling
Occurs at the surface of the liquidOccurs throughout the mass of the liquid
Occurs at all temperaturesOccurs at a particular temperature called boiling point.
Transformation; liquid to vaporTransformation; freezing to boiling
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