KINETIC MOLECULAR MODEL OF MATTER

States of Matter

Solids

• 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.

Liquids

• 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.

Gases

• 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.

Evaporation

• 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

• 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