Changing States

• When a substance changes state (e.g. from solid to liquid)

  • The particles themselves stay the same  
  • The way the particles are arranged changes  
  • The way the particles move changes 
• We can change the state of a substance by putting in or taking out energy
• If we heat a solid, it can change state to a liquid - this is called melting
• If we heat a liquid it can change state to a gas - this is called boiling
• If we cool a liquid , then we can convert it back into a solid - this is called freezing
• If we cool a gas , it can change state into a liquid - this is called condensing
• The melting point of a pure substance is the specific temperature where that substance will melt and freeze at
• The boiling point of a pure substance is the specific temperature where that substance will boil and condense at
• Explain why we have to put in energy to change a solid to a liquid

  • The particles in the liquid are moving  
  • So they have more kinetic energy than the particles in the solid  
  • This means that we have to put energy in to convert the solid into a liquid  
  • This energy is needed to break the forces of attraction in the solid  
  • Once these are broken, the particles in the solid can move around , changing the solid into liquid 

  
• The amount of energy required for a substance to change state depends on the amount of energy required to overcome the forces of attraction between particles
• The stronger the forces of attraction

  • The greater the amount of energy needed to overcome them  
  • The higher the melting and boiling point 
• Describe the forces of attraction between the particles in candle wax

  • Candle wax is a solid with a relatively low melting point  
  • In candle wax , the forces of attraction are relatively weak  
  • So they are fairly easy to break  
• Describe the forces of attraction between the particles in sodium chloride

  • Sodium chloride is a solid  
  • The forces of attraction between particles in sodium chloride are strong  
  • So sodium chloride has a very high melting point