Kinetic Particle Model Of Matter

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Notes

States of Matter

**Solids** have a definite shape and volume; cannot flow; not compressible.
**Liquids** have no definite shape but a definite volume; can flow; not compressible.
**Gases** have no definite shape or volume; can flow; highly compressible.
Changes of state are **physical changes** (reversible); mass and number of molecules remain constant.
**Melting**: solid → liquid; **freezing**: liquid → solid.
**Boiling/evaporating**: liquid → gas; **condensing**: gas → liquid.

Molecular Matter – Particle Arrangement & Motion

In a **solid**, particles are closely packed in a regular pattern and **vibrate about fixed positions**.
In a **liquid**, particles are close but randomly arranged; they can **slide past each other**.
In a **gas**, particles are widely spaced (~10× apart in each direction) and move **rapidly and randomly**.
**Density**: solid (high), liquid (medium), gas (low).
**Intermolecular forces** are strongest in solids, weaker in liquids, and negligible in gases.
Particles in a gas have the **highest kinetic energy**; in a solid the lowest.

Temperature & Pressure

Increasing temperature increases the **average speed** and **kinetic energy** of particles.
Gas pressure is defined as **force per unit area**: $p = \frac{F}{A}$ .
Pressure arises from **collisions** of gas particles with container walls, producing a force perpendicular to the wall.
At constant volume, higher temperature → more frequent and forceful collisions → **higher pressure**.

Brownian Motion

**Brownian motion** is the random, erratic movement of visible particles (e.g., pollen, smoke) suspended in a fluid.
It is caused by **collisions** with invisible, fast-moving atoms or molecules of the fluid.
Each collision gives the visible particle a small nudge, changing its speed and direction randomly.
This provides evidence for the **kinetic theory of matter** – that all matter is made of moving particles.

The Gas Laws (Extended Tier Only)

**Boyle’s Law**: at constant temperature, pressure $p$ is inversely proportional to volume $V$ ( $pV =$ constant).
Compressing a gas (decrease volume) increases pressure because particles hit walls more often.
**Pressure–Temperature Law**: at constant volume, pressure is directly proportional to absolute temperature ( $p \propto T$ ).
Heating a gas at constant volume increases particle speed, leading to more frequent and forceful collisions → pressure rises.

Particle arrangement in solids, liquids, and gases.

Practice questions

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1.What causes the motion of smoke particles when viewed through a microscope?
Easy
- AThe smoke particles are attracted to each other.
- BThe smoke particles vibrate because they are warm.
- CSmaller air particles are colliding with the smoke particles.
- DThe microscope is vibrating slightly, making the smoke particles appear to move.
2.A sealed piston is used to expand a gas at constant temperature. What happens to the density and pressure of the gas?
Easy
- ADensity stays the same, pressure stays the same
- BDensity increases, pressure decreases
- CDensity decreases, pressure stays the same
- DDensity decreases, pressure decreases
3.Which state of matter has molecules that are very close together in fixed positions, vibrating about these positions?
Easy
- ASolid
- BLiquid
- CGas
- DPlasma
4.Air is trapped inside a cylinder of fixed volume. The temperature of the air is increased. Which graph shows the correct relationship between pressure and temperature?
Medium
- AA straight line through the origin (pressure proportional to temperature)
- BA curve increasing with decreasing gradient
- CA horizontal line (pressure constant)
- DA curve decreasing with increasing gradient
5.A beaker of water at its boiling point (100 °C) is heated so that liquid becomes gas at the same temperature. Which row correctly compares the liquid and gas phases?
Medium
- AAverage distance between particles: same; Average speed: same
- BAverage distance: greater in liquid; Average speed: greater in gas
- CAverage distance: greater in gas; Average speed: greater in liquid
- DAverage distance: greater in gas; Average speed: same
6.A sealed plastic drinks bottle is left in sunshine on a warm day. What happens to the average speed of air molecules and the pressure inside?
Medium
- ASpeed increases, pressure increases
- BSpeed increases, pressure decreases
- CSpeed decreases, pressure increases
- DSpeed decreases, pressure decreases
7.In an experiment, tiny pollen grains are suspended in water and observed under a microscope. They appear as small bright dots moving randomly. What is the correct explanation?
Hard
- AThe bright dots are water molecules being hit by other water molecules.
- BThe bright dots are water molecules vibrating due to thermal kinetic energy.
- CThe bright dots are pollen grains being hit many times per second by water molecules.
- DThe bright dots are pollen grains vibrating due to their thermal kinetic energy.
8.A syringe half filled with air is sealed and the plunger pulled out slowly at constant temperature. What happens to the pressure and the speed of air molecules?
Hard
- APressure stays the same, speed stays the same
- BPressure increases, speed decreases
- CPressure decreases, speed stays the same
- DPressure decreases, speed decreases

Slides

States of Matter

Solids have a definite shape and volume, cannot flow, and are not compressible.
Liquids have no definite shape but a definite volume, can flow, and are not compressible.
Gases have no definite shape or volume, can flow, and are highly compressible.

Which state of matter has particles that are very close together in fixed positions and vibrate about these positions?

Solid
Liquid
Gas
Plasma

Changes of State

Melting: solid → liquid (e.g., ice to water).
Freezing: liquid → solid.
Boiling/Evaporating: liquid → gas.
Condensing: gas → liquid.
Changes of state are physical and reversible; mass remains constant.

When a liquid turns into a gas, the process is called ____ .

Molecular Matter: Particle Arrangement & Motion

Solid: particles close together in regular pattern, vibrate about fixed positions.
Liquid: particles close together but randomly arranged, can slide past each other.
Gas: particles far apart (≈10× in each direction), move randomly at high speeds.

Which of these are true about the particle model?

In a solid, particles are arranged in a regular pattern.
In a liquid, particles are far apart and move randomly.
In a gas, particles are widely separated and move rapidly.
In a solid, particles can slide past each other.

Forces and Distances Between Particles (Extended)

In solids, strong intermolecular forces hold particles in fixed positions with small distances.
In liquids, particles have enough energy to overcome some forces, allowing sliding; distances similar to solids.
In gases, particles have high energy, overcome forces, and are far apart, allowing compression and expansion.

A substance has particles that are quite closely packed, move about at random, and do not have fixed positions. What state is it?

Solid
Liquid
Gas
Cannot tell

Temperature & Pressure

As temperature increases, the average speed and kinetic energy of particles increase.
Gas pressure is caused by particles colliding with container walls, exerting force per unit area $(p = \frac{F}{A})$ .
Higher temperature → more frequent and forceful collisions → higher pressure.

Pressure is defined as force per unit ____ .

Brownian Motion

Brownian motion: random, erratic movement of visible particles (e.g., pollen, smoke) suspended in a fluid.
Caused by collisions with smaller, fast-moving atoms or molecules (e.g., air or water molecules).
The smaller particles are invisible under a microscope, but their existence is inferred from the motion.

What causes the random motion of smoke particles observed under a microscope?

The smoke particles are attracted to each other.
The smoke particles vibrate because they are warm.
Smaller air particles are colliding with the smoke particles.
The microscope is vibrating slightly.

The Gas Laws: Pressure & Volume (Extended)

At constant temperature, pressure and volume are inversely proportional (Boyle's law).
Compressing a gas (decreasing volume) increases pressure because particles hit walls more often.
Expanding a gas (increasing volume) decreases pressure.

A sealed piston expands a gas at constant temperature. What happens to the density and pressure?

Density stays the same, pressure stays the same
Density increases, pressure decreases
Density decreases, pressure stays the same
Density decreases, pressure decreases

The Gas Laws: Pressure & Temperature (Extended)

At constant volume, pressure is directly proportional to temperature (in Kelvin).
Increasing temperature increases average speed of particles → more frequent and forceful collisions → higher pressure.
Graph of pressure vs. temperature (in K) is a straight line through the origin.

At constant volume, increasing the temperature of a gas increases $its ____$ .

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