Chapter 6: Molecules and matter

Question 1

Density

Answer 1

• Density, p = mass, m / volume, V (kg/m3)
• To measure the density of a solid object or liquid, measure its mass and volume

• Rearranging the equation gives
m = p V or V = m/p

• Objects that have a lower density than water will float in water

Question 2

States of matter

Answer 2

• Solid particles: held nest to each other in fixed positions, least energetic
• Liquid particles: move at random but are still in contact with each other, more energetic than solid
• Gas particles: move randomly and are far apart (so gases are less dense), most energetic particles out of states of matter
• When a substances changes state, its mass stays the same because the number of particles stays the same

Question 3

Changes of state

Answer 3

• For a substance:

  o	Its melting point is the temperature at which it melts (same as freeze)

  o	Its boiling point is the temp it boils (same as condense)

• Energy is needed melt a solid or to boil a liquid
• Boiling occurs throughout a liquid at its boiling point. Evaporation occurs from the surface of a liquid when its temperature is below its boiling point
• The flat section of a temperature-time graph gives the melting or boiling point of a substance

Question 4

Internal energy

Answer 4

• Increasing the temperature of a substance increases its internal energy
• The strength of the forces of attraction between the particles of a substances explains why it is either a solid, liquid or gas.
• When a substance is heated:

  o	If its temperature rises, the kinetic energy of its particles increases
  
  o	If it melts or it boils the potential energy of its particles increases

• The pressure of a gas on a surface is cause by the particles of the gas repeatedly hitting the surface

Question 5

Specific latent heat

Answer 5

• Latent heat is the energy needed for a substance to change its state without changing its temperature
• Specific latent heat of fusion (or of vaporisation) is the energy needed to melt (or to boil) 1kg of a substance without changing its temperature
• In latent heat calculations, use the equation E = m L
• The specific latent heat of ice (or of water) can be measured using a low-voltage heater to melt the ices (or to boil the water)

Question 6

Gas pressure and temperature

Answer 6

• The pressure of a gas is caused by the random impacts of gas molecules on surfaces that are in contact with the gas
• If the temp of a gas in a sealed container increases, the pressure increases because:

  o	The molecules move faster so they hit the surfaces with more force
  
  o	The number of impacts per second of gas molecules on the surfaces of a sealed container increases, so the total force of the impacts increases

• The unpredictable motion of smoke particles is evidence of the random motion of gas molecules

Question 7

Gas pressure and volume

Answer 7

• For a fixed mass of gas at a constant temp:

  o	Its pressure is increased if its volume  is decreased

  o	Reducing the volume of a gas increases the number of molecular collisions per second on the surfaces that are in contact with the gas

• Use the equation pressure, p x volume, V = constant if the mass and the temperature of the gas do not change
• The temp of a gas can increase if it is compressed rapidly because energy isn’t transferred quickly enough to its surroundings.