Solids liquids and gases

Units

degree Celsius (°C) → unit of temperature, used to measure how hot or cold something is; based on fixed points where water freezes at 0°C and boils at 100°C

kelvin (K) → unit of temperature used in physics, starts at absolute zero (0 K), the lowest possible temperature; temperature in K = temperature in °C + 273

joule (J) → unit of energy, measuring the amount of work done or energy transferred in processes such as heating or motion

kilogram (kg) → unit of mass, measuring the quantity of matter in an object

kilogram per metre cubed (kg/m³) → unit of density, measuring how much mass is contained in a given volume

metre (m) → unit of length or distance between two points

metre squared (m²) → unit of area, measuring the size of a surface

metre cubed (m³) → unit of volume, measuring the amount of space an object or substance occupies

metre per second (m/s) → unit of speed or velocity, describing how fast an object moves and in which direction (for velocity)

metre per second squared (m/s²) → unit of acceleration, describing how quickly velocity changes over time

newton (N) → unit of force, defined as the force required to accelerate a 1 kg mass by 1 m/s²

pascal (Pa) → unit of pressure, equal to 1 newton per metre squared; measures how concentrated a force is over an area

joules per kilogram degree Celsius (J/kg °C) → unit of specific heat capacity, measuring the energy needed to raise the temperature of 1 kg of a substance by 1°C

Density and Pressure

Density

• Density describes how closely packed the particles in a substance are.

• High density means particles are closely packed; low density means particles are more spread out.

• Solids usually have the highest density, followed by liquids, then gases.

• Density determines whether an object will float or sink in a fluid.

density = mass / volume
ρ = m / V

• Increasing mass (same volume) increases density.

• Increasing volume (same mass) decreases density.

Density Practical

• Measure mass using a balance (ensure zero error is corrected).

• Measure volume:

  • Regular objects: calculate using dimensions (e.g. length × width × height).

  • Irregular objects: use water displacement method in a measuring cylinder or Eureka can.

• Submerge object fully and measure change in water level to find volume.

• Calculate density using the formula.

• Repeat measurements and take averages to improve reliability.

• Measuring cylinder:

  • Put a measuring cylinder on a flat table to take a reading

  • Put some water in your measuring cylinder about 1/3 full

  • Then put your object in the measuring cylinder and read the new water level

  • You must read the measuring cylinder at eye level to avoid parallax error

  • Read the water level to the bottom of the meniscus

• Eureka can

  • Put the eureka can on a horizontal surface so that you can catch the water that comes out of this sprout

  • Drop the object in and measure how much water comes out of the can

Pressure

• Pressure is the force acting per unit area.

• It explains why sharp objects cut more easily (small area → high pressure).

pressure = force / area
p = F / A

• Increasing force increases pressure.

• Increasing area decreases pressure.

• The indentation is caused by when pressure is exerted by the blocks when a mass is placed on them

• If the force is pread over a larger area then the pressure is smaller so the indentation is shallower

• To get a high pressure, you can either have a large force, or you can concentrate the force on a small contact area

• This is how sharp knives work

• Units: pascal (Pa), where 1 Pa = 1 N/m².

Pressure in Fluids (Liquids and Gases)

• Pressure at a point in a fluid acts equally in all directions.

• This is due to random motion of particles colliding with container walls.

• In gases, particles move freely and rapidly, causing pressure in all directions.

• In liquids, particles are close together, so pressure is transmitted throughout the fluid.

• Explains why liquids can exert pressure on the sides and base of containers.

• E.g. when you swim underwater the water exerts a pressure on you

• The water molecules are pushing each other and on surfaces and this liquid pressure acts in all directions

• Furthermore it is liquid pressure that produced upthrust, the force that keeps things afloat

• If you push a balloon into a bucket of water, you can feel the water pushing back

Pressure Difference in Liquids

• Pressure increases with depth because of the weight of liquid above.

• The deeper you go, the greater the force from the liquid above.

pressure difference = height × density × gravitational field strength
p = h × ρ × g

• h = height or depth (m)

• ρ = density (kg/m³)

• g = gravitational field strength (N/kg)

• Greater depth → greater pressure

• Denser liquid → greater pressure at the same depth

• This explains why dams are thicker at the bottom and why deep-sea pressure is high

Atmospheric pressure

• The Earth’s atmosphere is its outermost layer

• The atmosphere is a mixture of gases and extends about 700km above the Earths surface

• Atmospheric pressure at sea level is about 100000Pa or 100 kPa

• However this decreases as the altitude increase

• As the height above the surface (altitude) increase:

  • The number of gas molecules in a given volume of air decreases

  • The frequency of collisions with objects in the air decreases

  • The pressure decreases

Change of State

Energy and Heating

• Heating increases the internal (thermal) energy of a system.

• Internal energy consists of:

  • Kinetic energy (movement of particles)

  • Potential energy (energy stored in bonds between particles)

• Energy supplied can:

  • Increase temperature (increase kinetic energy)

  • Cause a change of state (increase potential energy by breaking bonds)

Changes of State

• Melting: solid → liquid

• Freezing: liquid → solid

• Boiling: liquid → gas (throughout the liquid at a fixed temperature)

• Evaporation: liquid → gas (at the surface, can occur below boiling point or at any temperature)

• Condensation: gas → liquid

• During melting and boiling, temperature remains constant because energy is used to overcome intermolecular forces.

• What happens in evaporation:

  • The surface particles with the most KE leave the liquid

  • This process cools the liquid because the most energetic particles leave

  • The particles with less energy are left behind

  • The average KE decreases

  • The temperature in Kelvin is directly proportional to average KE of particles

  • This process is why sweating cools you down

Stearic acid experiment:

• 1= liquid - cooling down

• 2= freezing - turning from liquid to solid, temperature is barely changing as bonds are forming, heat energy is still being given off

• 3= Solid - cooling down

• Ideal graph:

Particle Model

Explanation of Changes of State

• Solid to liquid (melting): particles gain energy, vibrate more, and break out of fixed positions.

• Liquid to gas (boiling/evaporation): particles gain enough energy to overcome attractions and escape.

• Gas to liquid (condensation): particles lose energy and come closer together.

• Liquid to solid (freezing): particles lose energy and become fixed in position.

Temperature–Time Graph (Practical)

• When heating:

  • Temperature increases steadily as particles gain kinetic energy.

  • Flat (horizontal) sections occur during melting and boiling.

• Flat sections show energy being used to change state rather than increase temperature.

• The length of the flat section depends on the amount of substance and energy supplied.

Specific Heat Capacity

• Specific heat capacity is the energy required to raise the temperature of 1 kg of a substance by 1°C.

• Substances with high specific heat capacity (e.g. water) require more energy to heat up.

• Substances with low specific heat capacity heat up quickly.

• Important in applications like heating systems and climate (oceans store heat).

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

ΔQ = m × c × ΔT

• ΔQ = energy transferred (J)

• m = mass (kg)

• c = specific heat capacity (J/kg °C)

• ΔT = temperature change (°C)

• Increasing mass or temperature change increases energy required.

Specific Heat Capacity Practical

• Measure mass of the substance using a balance.

• Use an էլectrical heater to supply energy.

• Measure voltage, current, and time to calculate energy supplied.

• Record temperature change with a thermometer.

• Calculate specific heat capacity using the equation.

• Reduce errors by:

  • Insulating the container to reduce heat loss

  • Stirring to ensure even temperature

  • Taking repeated measurements and averaging results

Ideal Gas Molecules

Solids liquids and gases - kinetic theory

• Solids:

  • Vibrating in fixed positions

  • Not compressible

• Liquids:

  • Particles can slide over each other

  • Not compressible

• Gases:

  • Rapid random motion

  • Easily compressible

• Evidence for particle model:

  • Diffusion of ink happens faster in hot water than cold water

  • Brownian motion - smoke particles can be seen to jiggle around when hit by air particles

  • Perfume diffusion in air

Motion and Pressure of Gas Molecules

• Gas molecules are in constant random motion in all directions.

• They move in straight lines between collisions with other molecules or the walls of the container.

• Collisions are elastic, meaning no kinetic energy is lost overall during collisions.

• When molecules collide with the container walls, they exert a force on the walls.

• This is because they change momentum and force = change in momentum/time

• This exerts a small force on the container

• This force acting over the surface area produces pressure.

• Pressure is therefore caused by the continuous bombardment of particles on the container walls.

• Increasing the number of collisions per second increases pressure.

• Increasing the force of each collision also increases pressure.

Absolute Zero

• Absolute zero is the lowest possible temperature: –273°C, which is 0 K.

• At this temperature:

  • Particles have the minimum possible kinetic energy.

  • Particle motion effectively stops (no thermal motion).

• It is a theoretical limit and cannot be reached in practice.

• All temperature measurements in gas laws are based on this reference point.

Kelvin Scale

• The Kelvin scale is an absolute temperature scale used in physics.

• It starts at absolute zero (0 K).

• There are no negative values on the Kelvin scale.

• One Kelvin represents the same temperature change as one degree Celsius.

temperature (K) = temperature (°C) + 273

• Kelvin is used in gas equations because it is directly proportional to particle energy and motion.

Temperature and Speed of Molecules

• Increasing temperature increases the average kinetic energy of gas molecules.

• This results in an increase in their average speed.

• Faster-moving molecules:

  • Collide more frequently with the container walls

  • Collide with greater force

• This leads to an increase in pressure if volume is kept constant.

• Decreasing temperature has the opposite effect, reducing speed and pressure.

Temperature and Kinetic Energy

• The Kelvin temperature of a gas is directly proportional to the average kinetic energy of its molecules.

• This means:

  • If temperature doubles (in Kelvin), average kinetic energy also doubles.

• This relationship explains why temperature must be measured in Kelvin for gas calculations.

• It links microscopic particle behaviour to macroscopic properties like pressure.

Pressure and Volume (Constant Temperature)

• Boyle’s law:

  • The gas in the tube is trapped, so the number of gas particles doesn’t change

  • When air is pumped in on the left, the gas is compressed

  • The pressure gauge tells us the pressure

• Graph and analysis:

  • If pressure increases, volume decrease

  • It is not linear

  • Pressure is indirectly proportional to volume

• For a fixed mass of gas at constant temperature:

  • Pressure is inversely proportional to volume.

  • Increasing volume decreases pressure.

  • Decreasing volume increases pressure.

• Explanation:

  • Larger volume → particles have more space → fewer collisions with walls → lower pressure

  • Smaller volume → particles are closer together → more frequent collisions → higher pressure

  • p₁V₁ = p₂V₂

  • How to use this equation:

    • pV is constant for a particular setup (it is not always the same value)

Pressure and Temperature (Constant Volume)

• Pressure law:

• Graph and analysis:

  • The graph is a straight line

  • It does not go though the origin if the temperature is in celcius

  • But at 0 Kelvin, the pressure is zero as the gas particles are not moving so they don’t collide with anything

• For a fixed mass of gas at constant volume:

  • Pressure is directly proportional to temperature (in Kelvin).

  • Increasing temperature increases pressure.

  • Decreasing temperature decreases pressure.

• Explanation:

  • Higher temperature → particles move faster → collisions are more frequent and forceful → pressure increases

• If temperature decreases towards absolute zero, pressure also decreases.

• p₁ / T₁ = p₂ / T₂

• For this equation to work you need:

  • Temperature in K

  • Volume of gas must be constant

  • Number of gas particles must be constant

Combined Understanding of Gas Behaviour

• Gas pressure depends on:

  • Speed of particles (linked to temperature)

  • Frequency of collisions (linked to volume and number of particles)

  • Force of collisions (linked to kinetic energy)

• These relationships assume:

  • A fixed mass (constant number of particles)

  • No loss of gas from the container

• Real gases behave approximately like ideal gases under many conditions, especially at low pressure and high temperature.

Graphical Relationships

• Pressure–volume graph (constant temperature):

  • Curve that slopes downwards (inverse relationship).

• Pressure–temperature graph (constant volume):

  • Straight line through the origin when plotted in Kelvin.

• These graphs help visualise how gas variables are related.