density of materials (topic 3)

• density equation

  $density (kg/m^3) =$$mass (kg) / volume (m^3)$
• Density

  • density deends on what the material is made of and how its particles are arrranged
  • a dense material has its particles densly packed together
  • the particles in a less dense material are spread out - if you compressed the material the particles would move closer together and it would become more dense (you are not increasing its mass but rather decreasing its volume)
• Solids

  • strong forces of attraction hold the particles close together in a fixed regular arrangement
  • the particles dont have much energy so vibrate about a fixed point
  • the density is generally highest in this state as particles are closest together
• Liquids

  • weaker forces of attraction between particles
  • particles are close together, but can move past each other and form irregular arrangements
  • more energy than particles in a solid - they move in random directions at low speeds
  • generally less dense than solids
• Gases

  • almost no forces of attraction between particles
  • particles have more energy than liquid and solids- free to move and travel in random directions at high speeds
  • less dense than liquids (generally)
• Internal energy

  • is the energy stored by the particles making up a system
  • the particles in a system vibrate or move around - they have kinetic energy stores
  • the energy stored by a system is stored by its particles
  • the internal energy of a system is the total energy that its particles have in their kinetic and potential stores
• Changing the state of a system

  • heating the system transfers energy to its particles (they gain energy in their kinetic stores and move faster )- increasing the internal energy
  • this leads to a change in temperature or a change in state
  • a change in state occurs when the substance is heated enough - the particles will have enough energy in their kinetic stores to break the bonds holding them together
• What effects the size of temperature change ?

  • the mass of the substance
  • what the substance is made of (its specific heat capacity)
  • energy input
• what happens when a substance changes state due to cooling ?


  the particles loose energy and form bonds
• a change of state is a physical change (rather than a chemical change), this means you dont end up with a new substance- its the same one you started with just in a different form
• what happens if you reverse a chnage of state?

  the substance returns to its orginal form and get back its original properties
• when a substance changes state the number of particles doesnt change- they're just arranged differently. this means mass is conserved - none of it is lost when the substance chnages state
• why is energy required to melt or boil a substance?

  • you need to increase the internal energy which is then used to break bonds between particles
  • there are flat spots on heating graphs where enrgy is being transferred by heating but not used to change the temperature
• what happens to internal energy when a substance is frozen or condensed?

  • bonds are forming between particles which releases energy - meaning internal energy decreases
  • however the temperature doesnt go down until all the substance has turned to a liquid (condensing) or a solid (freezing) hence the flat parts on the temperature graphs
• What is Specific latent heat ?

  • the amount of energy needed to change 1kg of a substance to one state to another without changing its temperature
• what is the SLH when a substance cools ?

  the energy released by the change in state
• SLH is different for different materials and for changing between different states
• SLH for changing between a solid and a liduid (melting or freezing) is called the specific latent heat of fusion
• SLH for changing between a liquid and a gas (evapourtating, boiling or condensing) is called the Specific latent heat of vapourisation
• equation for SLH

  $SLH (j/kg) =$$energy (J) / mass (kg)$
• how is the temperature of a gas related to the average energy in the kinetic energy stores of the particles?

  the higher the temperature, the higher the average energy
• what happens to the speed of particles as you increase the temperature of a gas

  the speed increases
• what happens when a gas particle collides with something ?

  • they exert a force (and so a pressure) on it
  • in a sealed container the outwards gas pressure is the total force exerted by sall of the gas particles on a unit area of the container walls
• what increases net force (and so gas pressure )

  • faster particles and more frequent collisions will both lead to an increase in net force
  • increasing temperature will increase speed and so the pressure will increase too (if volume is kept constant )
• if temperature is kept constant, increasing the volume of a gas means the particles will get more spread out and hit the walls of the containter less often (gas pressure decreases)
• Change in pressure can cause a change in volume :

  • the pressure of a gas causes a net outward force to the surface of its container
  • there is also a force on the outside of the container due to the pressure of the gas around it
  • if a container can easily change its size (e.g a balloon) then any change in these pressures will cause the container to compress or expand due to the overall force
• at a fixed temperature, if the pressure of a gas doubles then its volume halves
• Boyle's Law states that at a fixed temperature, the pressure of a given mass of gas varies inversely with its volume.
• doing work on a gas to change its temperature

  • if you transfer energy by applying force then you do work
  • doing work on a gas increases its internal energy which can increase its temperature
  • you can do work mechanically (e.g. a bike pump) :
  • the gas applies pressure to the plunger of the pump and so exerts a force on it. work has to be done against this force to push down the plunger
  • this transferrs energy to the kinetic stores of the gas particles increasing the temperature. if the pump is connected to a tyre, you should feel it getting warmer