Molecules and matter

• Density = Mass / volume
• To measure the density of a solid object or a liquid, measures its mass and its volume, then use the density equation p=m/v
• Objects that have a lower density than water (<1000kg/m^3) float in water.
• The particles of a solid are held next to each other in fixed positions. They are the least energetic of the states of matter.
• The particles of a liquid move about at random and are in contact with each other. They are more energetic than particles in a solid.
• The particles of a gas move about randomly and are far apart (so gases are much less dense than solids and liquids). They are the most energetic of ther states of matter.
• When a substance changes state, its mass stays the same because the number of particles stays the same.
• For a pure substance:

  • its melting point is the temperature at which it melts (which is the same temperature at which it solidifies)
  • its boiling point is the temperature at which it boils (which is the same temperature at which it condenses)
• Energy is needed to melt a solid or to boil a liquid.
• Boiling occurs throughout a liquid at its boiling point. Evaporation occurs from the surface of a liquid when its temperature is below its boiling point.
• The flat section of a temperature-time graph gives the melting point or the boiling point of a substance.
• Increasing the temperature of a substance increases its internal energy.
• The strength of the forces of attraction between the particles of a substance explains why it is a solid, a liquid or a gas.
• When a subsyance is heated:

  • if its temperature rises, the kinetic energy of its particles increases
  • if it melts or it boils, the potential energy of its particles increases
• The pressure of a gas on a surface is caused by the particles of the gas repeatedly hitting the surface.
• Latent heat is the energy needed for a substance to change its stae without changing its temperature.
• Specific latent heat of fusion (or of vaporisation) is the energy need to melt (or to boil) 1kg of a substance without changing its temperature.
• In latent heat calculations, use the equation E=mL
• The specific latent heat of ice (or of water) can be measured using a low-voltage heater to melt the ice (or to boil the water).
• The pressire of a gas is caused by the random impacts of gas molecules on surfaces that are in contact with the gas.
• If the temperature of a gas in a sealed container is increased, the pressire of the gas increases because:

  • the molecules move faster so they hit the surfaces with more force
  • the number of impacts per second of gas molecules on the surfaces of a sealed container increases, so the total force of the impacts increases
• The unpredictable motion of smoke particles is evidence of the random motion of gas molecules.