Forces Between Particles

Introduction

Why does a block of iron hold together, while the particles of air drift apart? Particles are not simply heaped up by chance. They attract one another, and these attractions act as an invisible glue. How strong the glue is, and how much energy the particles have to pull against it, decides whether a substance is a solid, a liquid or a gas at a given temperature.

Core explanation

Particles attract each other. All particles exert attractive forces on neighbouring particles. These forces are electrical in origin, arising from the charged parts inside atoms. You do not need the details yet; the key facts are:

- The attractions are strongest when particles are close and become very weak as they move apart. - Different substances have attractions of different strengths . - If particles are pushed too close, they repel strongly, which is one reason solids and liquids resist compression.

Attractions in each state.

- Solid: the attractions are strong enough to hold each particle in a fixed position. Particles vibrate but cannot break free. - Liquid: particles are still close, so attractions still act, keeping the liquid together at a fixed volume. But the particles have enough energy to keep breaking and re-forming attractions, so they slide around. - Gas: particles are far apart and moving fast. The attractions are negligible, so nothing holds the particles together.

A tug of war: attraction versus energy. The state of a substance depends on a balance between two things:

1. The attractions , which pull particles together. 2. The energy of movement of the particles, which tends to pull them apart.

If attractions win easily, the substance is a solid. If they win only partly, it is a liquid. If the particles' energy wins completely, it is a gas. Heating raises the particles' energy; when it becomes enough to overcome the attractions holding them in place, the solid melts; when it is enough to separate particles completely, the liquid boils.

Strength of attractions and melting and boiling points. Substances with strong attractions between their particles need a lot of energy, and therefore a high temperature, to melt or boil. Substances with weak attractions melt and boil at low temperatures.

Substance Melting point (°C) Boiling point (°C) Attractions --- --- --- --- Oxygen −218 −183 Very weak Water 0 100 Moderate Sodium chloride 801 1413 Very strong Iron 1538 2862 Very strong

So oxygen is a gas at room temperature, water is a liquid and salt and iron are solids. The particles in all of them move; what differs is how strongly they hold on to one another.

Evidence for attractions. Water forms rounded drops, and some insects can stand on a pond surface. This is surface tension : particles at the surface are pulled inwards and sideways by their neighbours, making the surface behave like a stretched skin.

Step-by-step reasoning

To predict which of two substances has the higher boiling point:

1. Compare the strength of attractions between their particles. 2. Stronger attractions need more energy to overcome. 3. More energy means a higher temperature. 4. So the substance with stronger attractions has the higher boiling point.

Visual explanation

Picture particles joined by springs. In the solid, short stiff springs connect every neighbour. In the liquid, the springs are constantly snapping and re-attaching as particles move. In the gas, there are no springs at all between the widely separated particles. In the simulation, raising the temperature makes particles shake until these links give way.

Real-world analogy

Think of friends holding hands in a playground game. Standing still, they hold on firmly (solid). Running around, they keep letting go and grabbing new hands (liquid). Sprinting at full speed, they cannot hold on at all and scatter (gas).

Real-world example

A water strider (pond skater) walks on water. Its light, spread-out legs press on the surface, but the attractions between water molecules at the surface are strong enough to support it without it breaking through.

Why?

Why is carbon dioxide a gas at room temperature while water is a liquid, although their molecules are similar in size? The attractions between water molecules are much stronger than those between carbon dioxide molecules, so more energy is needed to separate them.

Common misconception

"When a substance boils, the particles themselves break apart." Boiling separates whole particles from each other by overcoming the attractions between them. The molecules themselves stay intact; water vapour is still H₂O.

Worked example

Question: Substance A boils at −33 °C and substance B at 78 °C. Which has stronger attractions between its particles?

Reasoning: A higher boiling point means more energy is needed to separate particles, which means stronger attractions.

Answer: Substance B has the stronger attractions.

Quick check

1. What must happen to the attractions between particles when a liquid boils? Answer: They must be overcome, so the particles separate completely.

Exam focus

Use the words "attractions between particles" and "overcome" precisely. When comparing melting or boiling points, always link: stronger attractions → more energy needed → higher temperature. Do not say that bonds inside molecules break during boiling.

Advanced insight

The forces between separate molecules are called intermolecular forces and are much weaker than the chemical bonds inside a molecule. This is why simple molecular substances such as oxygen and water have low melting and boiling points, while giant structures such as salt, diamond and metals, held entirely by strong bonds, have very high ones.

Summary

Particles attract one another, most strongly when close together. In solids the attractions hold particles in fixed positions, in liquids they keep particles close, and in gases they are negligible. The state depends on a balance between attractions and particle energy. Stronger attractions need more energy to overcome, giving higher melting and boiling points.

Practice questions

1. How does the strength of attraction between two particles change as they move further apart? Answer: It becomes weaker. 2. Explain why iron is a solid at room temperature but oxygen is a gas. Answer: Iron particles have very strong attractions that room-temperature energy cannot overcome; oxygen molecules have very weak attractions that are easily overcome. 3. Describe the attractions between particles in a liquid. Answer: They are strong enough to keep particles close together, but particles have enough energy to keep breaking and re-forming them, so they can move past each other. 4. Water boils at 100 °C and ethanol at 78 °C. Which has stronger attractions between its particles? Explain. Answer: Water, because a higher boiling point shows more energy is needed to separate its particles.