Particle ModelCambridge IGCSE Physics: Revision notes

The particle model describes all matter as being made up of tiny particles (atoms or molecules) that are constantly in random motion. The key differences between the three states of matter relate to how particles are arranged, how far apart they are, and how fast they move.

These differences in arrangement and motion explain the observable properties of each state:

• Solids have fixed shape and volume because particles are held rigidly in place
• Liquids have fixed volume but take the shape of their container because particles can move but are held closely together
• Gases have no fixed shape or volume because particles move freely with large separations

Temperature is a measure of the average kinetic energy of the particles in a substance. When temperature increases, particles move faster and have more kinetic energy; when temperature decreases, particles move slower and have less kinetic energy.

Absolute zero (−273°C or 0 K) is the theoretical lowest possible temperature. At absolute zero, particles would have the least kinetic energy and would theoretically stop moving completely (although in practice, quantum effects prevent complete stillness).

This relationship holds across all three states:

• In a solid, heating causes particles to vibrate more vigorously around their fixed positions
• In a liquid, heating causes particles to move faster and more randomly
• In a gas, heating causes particles to move at much higher speeds and collide more forcefully

The kinetic energy of particles is given by the relationship: Kinetic energy ∝ absolute temperature (measured in Kelvin)

Conversion between temperature scales:

• Temperature in Kelvin = Temperature in °C + 273
• Example: 27°C = 27 + 273 = 300 K

Pressure in a gas arises from the collisions of rapidly moving gas particles with the walls of their container. Each collision exerts a small force on the wall, and with billions of collisions per second, these add up to a measurable pressure.

Pressure is defined as:

Pressure = Force / Area (P = F/A, measured in Pa or N/m²)

The collisions create a force perpendicular (at right angles) to the surface. The pressure depends on:

1. Number of particles – more particles means more collisions and higher pressure
2. Temperature (particle speed) – faster particles collide with greater force and higher frequency, increasing pressure
3. Volume – if volume decreases, particles are closer together and collide with the walls more frequently, increasing pressure
4. Surface area – the same total force distributed over a smaller area creates higher pressure

Changes in pressure explained by particle motion:

• Heating a gas → particles move faster → more forceful and frequent collisions → pressure increases
• Compressing a gas (reducing volume) → particles closer together → more frequent collisions with walls → pressure increases
• Increasing the number of particles → more collisions per unit time → pressure increases
• Cooling a gas → particles move slower → less forceful and less frequent collisions → pressure decreases

Brownian motion is the random, erratic movement of microscopic particles (such as pollen grains or smoke particles) suspended in a fluid (gas or liquid). This motion is visible under a microscope and provides direct evidence for the kinetic particle model.

Why does Brownian motion occur?

Microscopic particles are bombarded continuously by the fast-moving atoms or molecules of the surrounding fluid. These collisions are:

• Random in direction (from all angles)
• Unbalanced in force – at any instant, more collisions occur on one side than the other
• Constant in nature

Because microscopic particles are much larger than individual atoms/molecules but still small enough to be affected by the cumulative momentum from many collisions, they experience a net random force that causes their visible random motion.

Key observations about Brownian motion:

• The motion is more vigorous at higher temperatures (more energetic collisions)
• The motion is more vigorous for smaller particles (lighter particles are more easily pushed)
• The motion never stops while the fluid contains moving particles
• The motion is direct evidence that particles in fluids are in constant random motion

Important terminology distinction:

• Microscopic particles (like pollen or smoke particles) are visible objects being moved by collisions
• Atoms and molecules (like air molecules) are the light, fast-moving particles causing the collisions

The physical properties of solids, liquids and gases arise directly from:

1. The strength and range of forces between particles
2. The average kinetic energy and motion of particles

These two factors interact to determine observable properties:

How forces and motion work together:

• Solids: Strong intermolecular forces overcome the low kinetic energy of particles → particles stay fixed in rigid arrangement → solid properties (hardness, fixed shape)

• Liquids: Moderate intermolecular forces are strong enough to keep particles close but not strong enough to prevent sliding past each other; sufficient kinetic energy allows movement → particles cluster but move freely → liquid properties (fixed volume, fluidity)

• Gases: Weak intermolecular forces cannot hold particles together; high kinetic energy particles overcome any attractive forces → particles spread far apart and move independently → gas properties (no fixed shape or volume, high compressibility)

Effects of changing conditions:

• Increasing temperature → increases kinetic energy → particles vibrate/move faster → can overcome intermolecular forces more easily
• Decreasing temperature → decreases kinetic energy → particles vibrate/move slower → intermolecular forces dominate more strongly
• Applying pressure → decreases separation → intermolecular forces strengthen (closer together) and collisions increase → resistance to further compression increases