Interparticle attractions

Interparticle attraction is the attractive force that holds the constituent particles of a substance together, pulling neighbouring particles towards each other. It is not the same thing as interparticle space — space is the gap between particles, attraction is the force acting across that gap — but the two are closely linked, because the strength of the force depends directly on how big that gap is. Two factors decide how strong the interparticle attraction is in any given substance:

• The nature of the substance. Different substances are simply built differently at the particle level, so the attractive pull between their particles differs in strength from one substance to another, even before distance is considered.
• The interparticle distance. This is the more dramatic factor. Interparticle attraction is extremely sensitive to spacing — even a small increase in the distance between particles causes the attractive force to fall off sharply, not gradually. Particles that are pushed only slightly farther apart already feel noticeably weaker pull.

Because of this second factor, the same substance can show very different strengths of interparticle attraction depending on how tightly its particles happen to be packed at a given moment — which is exactly what changes when a substance moves between the solid, liquid, and gaseous states.

where: $F_{\text{attraction}}$ is the strength of the pull between neighbouring particles, and $d_{\text{interparticle}}$ is the average distance separating them. This is a qualitative relationship, not a numerical formula — at the Class 8 level there is no equation to calculate interparticle attraction, only the comparative ranking "strong," "weaker," and "negligible." Reading this relationship across the three states of matter gives the complete picture:

• Solids — particles are packed very close together, so interparticle attraction is strong. It locks each particle to a fixed position, allowing only small vibrations, never a full escape past neighbours. Result: definite shape, definite volume.
• Liquids — particles are somewhat farther apart than in a solid, so interparticle attraction is weaker. Particles can slip and move around within a limited region near their neighbours, but they cannot break completely free. Result: definite volume, but no fixed shape — it takes the shape of its container.
• Gases — particles are very far apart, so interparticle attraction is negligible (very small, though not stated to be exactly zero at this level). Particles move freely in every direction with essentially nothing holding them back. Result: no definite shape and no definite volume.

There is no numerical formula for interparticle attraction at this level, so the reasoning has to be built qualitatively — but it is worth walking through carefully, because it explains every physical property of the three states in one go. Think of interparticle attraction as a short-range pull that particles exert on their immediate neighbours. In a solid, particles are squeezed into the tightest possible arrangement, so each particle sits right next to several neighbours with almost no gap. At such tiny separations, the attractive pull is at its strongest, strong enough that particles cannot break away from their neighbours at all — they can only jiggle, or vibrate, about a fixed spot. That fixed arrangement of positions is exactly what gives a solid its definite shape and definite volume: nothing is free to wander off. Now stretch that same arrangement out slightly, as happens when a solid melts into a liquid. The particles are still fairly close, but the small increase in separation is enough — because attraction is so sensitive to distance — to weaken the pull considerably. Particles are no longer locked to one spot; they can slide around and change neighbours, though they still stay close enough together that the total volume they occupy barely changes. This is why a liquid keeps a definite volume (it still resists being squeezed) but readily takes the shape of whatever container it is poured into. Push the separation out much further still, as happens when a liquid evaporates into a gas, and the attractive pull drops to something negligible. With almost nothing holding neighbouring particles back, each particle moves independently in whatever direction it likes, spreading out to occupy all the space available to it. That is why a gas has neither a fixed shape nor a fixed volume — it expands to fill any container, however large. So the single idea "attraction weakens sharply as distance increases" is doing all the work: it is the same underlying force, just evaluated at three very different spacings, and those three spacings are what we experience as solid, liquid, and gas.

Question: A thin sheet of aluminium foil can be bent into a new shape and it stays that way — it does not flow like water when tilted. Explain this using interparticle attraction, and then explain why a gas jar of orange bromine vapour placed mouth-to-mouth with an empty jar spreads evenly into the empty jar within minutes.

Step 1. Aluminium foil is a solid, so its constituent particles sit very close together and experience strong interparticle attraction, which locks each particle to a fixed position relative to its neighbours, allowing only small vibrations.

Step 2. When the foil is bent, enough force is applied to shift whole groups of particles into new fixed positions, but the strong attraction between them never lets go — the particles stay bound together as one connected solid piece rather than separating or flowing.

Step 3. Because the particles cannot slide freely past one another (unlike in a liquid), tilting the bent foil does not make it flow or lose its new shape — the strong interparticle attraction of the solid state is exactly what allows it to hold a definite, even a bent, shape.

Step 4. Bromine vapour, in contrast, is a gas, so its particles are far apart and the interparticle attraction between them is negligible — there is almost nothing pulling neighbouring particles back together.

Step 5. With negligible attraction and constant particle motion, the bromine particles move independently in every available direction, including through the open mouths of the jars into the previously empty one, until the orange colour is spread evenly across both.

The strong interparticle attraction of the solid foil keeps it in a fixed, bent shape, while the negligible interparticle attraction of the gaseous bromine lets its particles spread freely to fill both jars — the same underlying force, at two very different particle spacings, producing two opposite behaviours.

Competitive stretch

If a liquid like water is heated gently, its particles gain energy and drift slightly farther apart on average even before boiling — this is consistent with the same rule: greater average spacing means weaker interparticle attraction, which is part of why a liquid becomes easier to stir and pour as it warms, well before it turns to vapour.

• Confusing interparticle attraction with interparticle space — attraction is a force between particles; space is the gap between them. They are linked (bigger gap, weaker force) but are not the same quantity.
• Assuming interparticle attraction is exactly zero in a gas — the correct Class 8 description is "negligible," meaning very small, not stated to be precisely zero.
• Treating the weakening of attraction with distance as gradual or steady — the chapter stresses that even a small increase in interparticle distance causes a sharp drop in attractive force, not a smooth, even decline.
• Reaching for terms like ionic bond or covalent bond to explain interparticle attraction — at this level the idea stays at "attractive force between particles," with named bond types left for later grades.

Ask the tutor: Curious how this same idea of interparticle attraction eventually connects to chemical bonds like ionic and covalent bonds in later grades — ask your AI tutor to preview that link.