4. Particle Model of Matter

• Density

  The mass per unit volume of a material
• Objects made from low density materials typically have a low mass

  Similar sized objects from high density materials have a high mass

  Example : bag of feathers is lighter compared to a bag of metal
• Density Calculation

  $m=$$pv$ (m=mass, v=volume & p=density)

  
• Gases are less dense than solids because the molecules are more spread out

  
• Volumes of common 3D shapes

  
• The Particle Model 


  A model that describes the arrangement and movement of particles in a substance

  It can be used to explain : the different states of matter, eg. solids, liquids and gases & physical properties eg. differences in density
• Solid

  • Particles are closely packed together
  • Particles vibrate in fixed positions
  • Physical properties : definite shape, rigid
  • Definite volume
  • High density, low energy
• Liquid

  • Particles are closely packed together
  • Particles can flow over one another
  • Physical properties : no definite shape, able to flow and can take the shape of a container
  • A definite volume
  • Medium density, greater energy
• Gas

  • Particles are far apart
  • Particles move randomly
  • Physical properties : no definite shape, they will take the shape of their container, no fixed volume - will expand to fill the container
  • Highly compressible -> large gaps between the particles, easier to push the particles closer together than solids or liquids
  • Low density, highest energy
• 3 States of Matter - Shape & Volume

  
• Differences in Density

  Solids & Liquids -> molecules are tightly packed together (liquids have the energy to push past each other, solids don't) -> densities are roughly the same -> high density

  Gas -> molecules are widely separated -> lower densities than solids or liquids (at room temperature the distance between the molecules is 10 times the distance between solids or liquids)
• Densities in Solids & Liquids

  
• Density in Gases

  
• Changes of State

  When a substance changes state, the number of molecules in that substance does not change, therefore its mass does not change

  Physical changes like changes of states are reversible
• 6 Changes of State

  Melting -> solid into liquid, when energy is transferred to the system

  Boiling -> liquid into gas (evaporating), when energy is transferred to the system

  Condensing -> gas into liquid, when energy is transferred away from the system

  Freezing -> liquid into solid, when energy is transferred away from the system

  Subliming -> solid into gas, when energy is transferred to the system
• Changes of State - Diagram

  
• Internal energy


  The total energy stored inside a system by the particles that make up the system due to their motion and positions
• Molecules within a substance have energy in their :

  Kinetic store -> due to their random motion/vibration

  Potential store -> due to their position relative to each other

  Together these stores form the total energy that makes up the internal energy of the system
• Heating

  Heating a system changes a substance's internal energy by increasing the kinetic energy of its particles

  The temperature of the material, therefore is related to the average kinetic energy of the molecules

  The higher the temperature, the higher the kinetic energy of the molecules (vice versa) -> they move faster
• The increase in kinetic energy (therefore internal energy) can :

  Cause the temperature of the system to increase

  Or, to produce a change of state (solid to liquid or liquid to gas)

  
• Change of State

  When a substance reaches a certain temperature, energy will stop being transferred to the kinetic store of the molecules and will be transferred to their potential store instead

  This energy goes into overcoming the intermolecular forces of attraction between the molecules, causing them to move further apart from one another leading to a change of state eg. liquid to gas
• When a substance changes its state :

  The potential energy of the molecules increases, allowing them to overcome the intermolecular forces of attraction

  The kinetic energy remains the same, meaning the temperature will remain the same, even though the substance is still being heated

  
• If the temperature of the system increases :

  The increase in temperature depends on the mass of the substance heated, the type of material and the energy input to the system
• Specific Heat Capacity

  Equation : change in thermal energy = mass × specific heat capacity × temperature change

  The specific heat capacity of a substance is the amount of energy required to raise the temperature of 1kg of the substance by 1°C
• Latent Heat

  The energy needed for a substance to change state
• When a change of state occurs, the energy transferred to/away from a substance changes the internal energy of the substance but not its temperature

  This means, while the substance changes state, the temperature remains constant despite the energy still being transferred to the substance

  This is because the energy is used to overcome intermolecular forces of attraction between the molecules instead of increasing the kinetic energy of the molecules
• Molecules in a solid are tightly bound together, whereas in a liquid they are free to flow over one another

  Therefore, to change the state from solid to liquid, the molecules need to gain enough energy to overcome the intermolecular forces of attraction holding them in their rigid, solid structure
• Molecules in a liquid are less tightly bound together whereas in a gas, they are free to move completely apart from one another

  Therefore, to change the state from a liquid to a gas, the molecules need to gain enough energy to overcome the intermolecular forces of attraction holding them close together in their liquid structure
• Done by latent heat

  Temperature remains constant when melting and boiling, despite energy transferred to the substance

  
• 2 types of Latent Heat depending on the change of state

  1. Latent Heat of Fusion
  2. Latent Heat of Vaporisation
• Specific Latent Heat


  The amount of energy required to change the state of 1kg of a substance with no change in temperature
• Specific Latent Heat of Fusion

  Changing the state between a solid and liquid

  Solid -> Liquid , Liquid -> Solid
• Specific Latent Heat of Vaporisation

  Changing state between a liquid and gas

  Liquid -> Gas , Gas -> Liquid
• Latent Heat

  Represented by the symbol $L$

  Units : Joules per Kilogram, J/kg
• Changes of State with Heat supplied against Temperature

  
• Specific Latent Heat of Fusion

  The energy required to convert 1kg of a substance between a solid and a liquid state with no change in temperature
• Specific Latent Heat of Fusion :

  • Applies when melting a solid or freezing a liquid
  • When a solid is melted, its temperature stays constant until all of the substance has melted
  • This latent heat is the amount of energy needed per kg for all the particles in the substance to overcome the intermolecular forces of attraction holding them together in their solid state
  • If a substance in its liquid state is frozen, the substance will solidify at the same temperature as its melting point
• For example, if a substance in its liquid state is frozen -> the latent heat of fusion is the amount of energy per kg transferred away from the substance until all the particles in the substance have succumbed to the intermolecular forces of attraction that hold them together in their solid structure
• Specific Latent Heat of Vaporisation

  The energy required to convert 1kg between a liquid and a gaseous state with no change in temperature
• Specific Latent Heat of Vaporisation:

  • Applies when vaporising a liquid or condensing a gas
  • When a liquid substance is vaporised, its temperature will stay constant until all of the substance has vaporised
  • This latent heat is the amount of energy per kg needed for all the particles in the substance to overcome the intermolecular forces of attraction holding them together in their liquid state