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Chemistry - The Particle Model of Matter

Grade 8IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Kinetic Theory of Matter states that all matter is composed of tiny particles (atoms, molecules, or ions) that are in constant, random motion.

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In a solid, particles are packed closely together in a fixed, regular lattice. They vibrate about fixed positions and have the lowest kinetic energy among the three states.

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In a liquid, particles are close together but arranged randomly. They can move past each other, which allows liquids to flow and take the shape of the bottom of a container.

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In a gas, particles are far apart and move rapidly in all directions. There are negligible forces of attraction between particles, allowing them to fill any available volume.

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Changes of state involve energy transfers: Melting and Boiling require an input of energy (endothermic) to overcome attractive forces, while Freezing and Condensing release energy (exothermic).

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Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration until they are evenly spread. The rate of diffusion is higher in gases than in liquids and increases with temperature.

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Gas pressure is the result of gas particles colliding with the walls of their container. Each collision exerts a tiny force over an area, defined as P=FAP = \frac{F}{A}.

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Brownian motion refers to the random, jerky movement of larger visible particles (like smoke or pollen) caused by collisions with invisible, fast-moving air or water molecules.

📐Formulae

Density(ρ)=Mass(m)Volume(V)\text{Density} (\rho) = \frac{\text{Mass} (m)}{\text{Volume} (V)}

P=FAP = \frac{F}{A}

T(K)=T(∘C)+273.15T(K) = T(^\circ C) + 273.15

Kinetic Energy(Ek)∝Temperature(T)\text{Kinetic Energy} (E_k) \propto \text{Temperature} (T)

💡Examples

Problem 1:

A sample of an unknown liquid has a mass of 120 g120\text{ g} and occupies a volume of 150 cm3150\text{ cm}^3. Calculate the density of the liquid in g/cm3\text{g/cm}^3.

Solution:

ρ=120 g150 cm3=0.8 g/cm3\rho = \frac{120\text{ g}}{150\text{ cm}^3} = 0.8\text{ g/cm}^3

Explanation:

Using the density formula ρ=mV\rho = \frac{m}{V}, we divide the mass by the volume to find the mass per unit volume.

Problem 2:

Explain why the rate of diffusion of NH3NH_3 (ammonia) gas is faster than the rate of diffusion of HClHCl (hydrogen chloride) gas at the same temperature.

Solution:

Rate∝1MrelativeRate \propto \frac{1}{\sqrt{M_{relative}}}

Explanation:

At a constant temperature, particles have the same average kinetic energy. Since NH3NH_3 has a lower relative molecular mass (Mr≈17M_r \approx 17) compared to HClHCl (Mr≈36.5M_r \approx 36.5), the ammonia particles must move at a higher average velocity to maintain the same kinetic energy, leading to a faster rate of diffusion.

Problem 3:

A sealed container of gas is heated from 20∘C20^\circ C to 80∘C80^\circ C. Describe what happens to the gas pressure and explain why using the particle model.

Solution:

P∝T (at constant volume)P \propto T \text{ (at constant volume)}

Explanation:

As temperature increases, the particles gain more kinetic energy and move faster. This leads to more frequent and more forceful collisions with the walls of the container, which results in an increase in gas pressure.

The Particle Model of Matter - Revision Notes & Key Formulas | IGCSE Grade 8 Science