Changing State

• Substances can change from one state to another
• Physical changes don't change the particles - just their arrangement or their energy
• When a solid is heated, its particles gain more energy. This makes the particles vibrate more, which weakens the forces that hold the solid together

  
• At a certain temperature, called the melting point the particles have enough energy to break free from their positions. This is called melting and the solid turns into a liquid

  
• When a liquid is heated, again the particles get even more energy. This energy makes the particles move faster, which weakens and breaks the bonds holding the liquid together

  
• At a certain temperature, called the bioling point, the particles have enough energy to break their bonds. This is boiling (or evaporating). The liquid becomes a gas

  
• As a gas cools, the particles no longer have enough energy to overcome the forces of attraction between them. Bonds form between the particles

  
• At the boiling point, so many bonds have formed between the gas particles that the gas becomes a liquid. This is called condensing

  
• When a liquid cools, the particles have less energy, so move around less. There's not enough energy to overcome the attraction between the particles, so more bonds form between them

  
• At the melting point, so many bonds have formed between the particles that they're held in place. The liquid becomes a solid. This is freezing

  
• The amount of energy needed for a substance to change state depends on how strong the forces between the particles are
• The stronger the forces, the more energy is needed to break them, and so the higher the melting and boiling points of the substances
• You have to be able to predict the state of a substance
• The bulk properties such as the melting point of a material depend on lots of atoms interact together. An atom on its own doesn't have these properties
• If the temperature's below the melting point of a substance, it'll be a solid. If it's above the boiling point, it'll be gas. If it's in between the 2 points, then it's a liquid
• Example

  Which of the molecular substances in the table is a liquid at room temperature (25°C)?


  melting point boiling point

  Oxygen -219°C -183°C

  Nitrogen -210°C -196°C

  Bromine -7°C 59°C

  > Oxygen and nitrogen have boiling points below 25°C, so will both be gases at room temperature. So the answer is bromine. It melts at -7°C and boils at 59°C. So it'll be a liquid at room temperature