P6 - Molecules and matter

• Density


  Mass per unit volume
• Measuring the density of a solid object


  • Measure the mass of the object using an electronic balance
  • Measure the dimensions of a regular solid using a ruler, vernier callipers or micrometer to calculate the volume
  • For an irregular solid, lower it on a thread into a measuring cylinder partly filled with water and measure the rise in water level to find the volume

  
• Measuring the density of a liquid

  • Use a measuring cylinder to measure the volume of the liquid
  • Measure the mass of an empty beaker, pour the liquid into the beaker, and measure the total mass to calculate the mass of the liquid

  
• The density of pure water is 1000 kg/m³. Objects that float in water have a density less than 1000 kg/m³.
• Rearranging the density equation ρ = m/V gives m = ρV or V = m/ρ
• The volume of a cuboid is calculated as V = a x b x c
• Concrete has a much higher density than wood due to its higher mass per unit volume
• Standard form is useful when working with very large numbers, particularly when converting to SI units
• 1000 kg/m³ = 1000000 g/1000000 cm³ = 1 g/cm³
• particles of a liquid move about at random and are in contact with each other
• particles in a liquid are more energetic than particles in a solid
• the particles of a gas move about randomly
• Changes that take place in the movement and arrangement of particles

  1. When ice cube melts

  2. When water vapour condenses on a cold surface

  
• Liquids and solids are much denser than gases
• Melting point


  Temperature at which a solid changes to a liquid

  
• Boiling point


  Temperature at which a liquid turns to a gas
• Freezing point


  Temperature at which a liquid changes to a solid (same as melting point of solid)

  
• Melting point and boiling point are affected by impurities in the substance
• Adding salt to water lowers the melting point
• Salt is added to grit used for gritting roads in freezing weather to prevent roads getting icy
• Measuring the melting point of a substance


  1. Place substance in solid state in test tube in beaker of water

  2. Heat water and measure temperature when substance melts

  3. Plot measurements on graph

  4. Melting point is temperature of flat section of graph

  
• Same arrangement without beaker can be used to find boiling point of liquid
• Energy transferred to a substance when it changes state is called latent heat

  
• Temperature does not change at melting point or boiling point as latent heat is absorbed

  
• Differences between evaporation and boiling

  • Evaporation happens at any temperature, boiling happens only at substance's boiling point
  • Boiling occurs throughout liquid at boiling point, evaporation occurs from surface when below boiling point

  
• Plotting temperature-time graph for pure solid substance X

  1. Measure temperature every 30 seconds as substance is heated

  2. Use graph to find melting point of X

  
• Salt water has lower freezing point than pure water
• Purpose of salt and grit mixture used for gritting roads

  Salt lowers freezing point, grit provides traction
• Melting point is temperature at which solid melts, boiling point is temperature at which liquid boils

  
• Energy is needed to melt a solid or boil a liquid
• Boiling occurs throughout liquid at boiling point, evaporation occurs from surface when below boiling point

  
• Flat section of temperature-time graph gives melting or boiling point

  
• Internal energy of a substance is the total energy in kinetic and potential energy stores of its particles
• Heating a substance increases the total kinetic energy of its particles
• When physical state changes, the total potential energy of particles changes
• Particles in a solid

  • Arranged in 3D structure
  • Strong forces of attraction between particles
  • Particles vibrate about fixed positions
• Particles in a liquid

  • Weaker forces of attraction between particles
  • Particles not held in rigid structure
  • Some particles gain enough energy to escape liquid surface
• Particles in a gas

  • Forces of attraction between particles are insignificant
  • Particles move about at high speed in random directions
  • Collide with each other and container surface
• Heating a gas


  • Particles gain kinetic energy and move faster
  • Causes pressure of gas to increase
• Particles in a box

  • Move about at high speed in random directions
  • Collide with each other and with the internal surface of their container