Solids, Liquids, and Gases: Study Guide (Chemistry 3.3)

One substance, three phases

Water can be ice, liquid water or steam. In all three phases it is made of the same H₂O molecules. Each molecule has the same mass, the same shape and the same covalent bonds. What changes is how the molecules are arranged and how they move.

This topic is about drawing and explaining those differences at the particle level. Two questions sit behind everything:

• How close are the particles? Touching (solids and liquids) or far apart (gases)?
• How do they move? Vibrating in place (solids), moving and sliding past each other (liquids), or flying freely between collisions (gases)?

The answers depend on a balance. Interparticle forces (Topic 3.1) pull particles together. Thermal motion, which increases with temperature, pushes them apart.

Solids: fixed positions, crystalline or amorphous

In any solid, each particle stays near one position. It vibrates about that position, but it does not travel through the sample or swap places with its neighbours. Chemists say there is no overall translation of the particles relative to one another. That is why a solid keeps its own shape.

Solids come in two kinds.

• Crystalline solids. The particles sit in a regular pattern that repeats in all three dimensions. Table salt, ice, diamond, copper and quartz are crystalline.
• Amorphous solids. The particles touch their neighbours but have no regular, repeating pattern. Glass is the standard example, and many plastics are mostly amorphous too.

What decides the arrangement? Two things:

1. The interparticle forces. Ions arrange so that opposite charges are close (Topic 2.3). Water molecules in ice line up so that each hydrogen points at an oxygen on a neighbouring molecule, which builds an open hexagonal pattern.
2. How well the particles can pack. Spheres of one size pack neatly. Long, tangled or irregular particles often cannot reach an ordered pattern before they lose the energy to move, so they get stuck in a disordered, amorphous arrangement.

Liquids: touching but moving

In a liquid, the particles are still in close contact. But they have enough energy to move past one another all the time. They keep colliding, swapping neighbours and changing direction. That is why a liquid flows and takes the shape of its container, but keeps its own volume.

The arrangement in a liquid is not completely random. Forces between neighbours still matter:

• In liquid water, hydrogen bonds keep forming and breaking. At any instant most molecules are oriented so that an H on one molecule points at the O of a neighbour.
• In a nonpolar liquid such as hexane, only dispersion forces act. Molecules have no strong preferred orientation towards their neighbours.
• Temperature matters too. Heating a liquid makes the particles move faster and spread slightly further apart, so short-lived structures (such as hydrogen-bonded groups) break up more often.

Gases: far apart and always moving

In a gas, the particles are far apart compared with their own size. They move in straight lines until they collide with another particle or with the container wall. Forces between gas particles have only a small effect, because the particles spend almost all their time far from one another.

So a gas has neither a definite shape nor a definite volume: it spreads out to fill any container. The average spacing between gas particles, and how often they collide, depend on the conditions:

Change (other conditions fixed)	Average spacing	How often particles collide
Compress the gas into a smaller volume	Smaller	More often
Raise the pressure at constant temperature	Smaller	More often
Raise the temperature at constant pressure	Larger (the gas expands)	Slightly less often (more space to cross), but each collision is harder
Raise the temperature at constant volume	Unchanged	More often, and harder

You will turn these trends into equations in Topic 3.4 (the ideal gas law) and explain them with particle speeds in Topic 3.5.

Particulate models of the phases

A good particulate model follows a few rules. They are the ones you will be judged on.

1. Keep the particles identical in every phase: same size, same shape, same number of atoms.
2. Solids and liquids: particles touch. Leave only small gaps. Show order for a crystalline solid and no pattern for an amorphous solid or liquid.
3. Gases: particles far apart, spread through the whole container, not collected at the bottom.
4. Use a key if you use symbols. Arrows for motion are optional, but if you draw them, label what they mean.
5. Molecules stay whole. In a model of Br₂ in any phase, every particle is two bonded Br atoms.

Why solids and liquids have similar molar volumes

The molar volume is the volume that one mole of a substance occupies: molar volume = M / density.

In a solid and in a liquid, the particles touch at all times. Melting only loosens the arrangement a little. So the molar volumes of the solid and liquid phases of one substance are close, and the difference is small compared with what happens on boiling. Boiling is different: the particles separate completely, and the molar volume becomes hundreds or thousands of times larger.

Worked example 1: drawing chlorine in three phases

Question. Chlorine, Cl₂, is a gas at room temperature. It can be cooled to a liquid and then to a crystalline solid. Describe the particulate models you would draw for solid, liquid and gaseous chlorine in three boxes of the same size, and explain one feature of each.

1. Choose the particle. Each particle is a Cl₂ molecule: two touching circles of equal size, bonded together. Use this same symbol in all three boxes.
2. Solid. Draw about 12 to 16 molecules in contact, lined up in a regular repeating pattern (for example, rows with every molecule pointing the same way). Feature: the order shows a crystalline solid, and the molecules touch because dispersion forces hold them in fixed positions.
3. Liquid. Draw roughly the same number of molecules, still touching, but tilted at random angles with no pattern. They fill the lower part of the box. Feature: the molecules touch, so the liquid takes up only slightly more space than the solid, but the lack of order shows they can move past one another.
4. Gas. Draw only a few molecules (say 4 to 6), spread across the whole box with plenty of empty space. Feature: the large spacing shows weak forces between molecules and constant motion that fills the container.

Check. Are any molecules split into single Cl atoms? They should not be. Boiling and melting overcome the forces between Cl₂ molecules; the Cl–Cl covalent bond inside each molecule is unchanged.

Worked example 2: molar volumes of ice, water and steam

Question. Use these approximate densities to find the molar volume of water in each phase: ice 0.92 g cm⁻³ at 0 °C; liquid water 1.00 g cm⁻³ at 0 °C; steam 0.60 g L⁻¹ at 100 °C and 1 atm. Then estimate how many times farther apart the molecules are in steam than in liquid water.

1. Molar mass: M(H₂O) = 2(1.008) + 16.00 = 18.016 g mol⁻¹.
2. Ice: 18.016 g mol⁻¹ ÷ 0.92 g cm⁻³ = 19.6 cm³ mol⁻¹.
3. Liquid: 18.016 ÷ 1.00 = 18.0 cm³ mol⁻¹.
4. Steam: 18.016 g mol⁻¹ ÷ 0.60 g L⁻¹ = 30.0 L mol⁻¹ = 30 000 cm³ mol⁻¹ (2 significant figures).
5. Compare: ice ÷ liquid = 19.6 ÷ 18.0 = 1.09, so the solid and liquid differ by under 10 per cent. Steam ÷ liquid ≈ 30 000 ÷ 18.0 ≈ 1700.
6. Spacing. Volume scales with the cube of length, so the average distance between molecules grows by the cube root of the volume ratio: ∛1700 ≈ 12. As a check, the volume per molecule in liquid water is 18.0 cm³ ÷ 6.022 × 10²³ = 2.99 × 10⁻²³ cm³, a cube about 0.31 nm across. In steam it is a cube about 3.7 nm across.

Interpretation. Ice and liquid water have similar molar volumes because the molecules touch in both. (Ice is the slightly larger one because its hydrogen-bonded pattern is open; for most substances the solid is the smaller.) In steam, each molecule is roughly 12 molecule-widths from its neighbours on average, so the gas is mostly empty space.

Common misconceptions

• “Particles in a solid do not move.” They vibrate all the time. They just do not travel past one another.
• “Every solid is crystalline.” Amorphous solids, such as glass, are solids with no long-range order. Their particles still stay in fixed positions.
• “Particles get bigger when a substance melts or boils.” The particles are the same size in every phase. The spaces between them change.
• “Boiling breaks the bonds inside molecules.” Boiling water gives H₂O molecules, not H and O atoms. Phase changes overcome interparticle forces, not covalent bonds within molecules.
• “A liquid is halfway between a solid and a gas in spacing.” It is not. Liquid particles touch, just as solid particles do. The big jump in spacing is between liquid and gas.
• “Gas particles sink to the bottom of the container.” Gas particles spread through the whole volume. Drawing them in a heap at the bottom is a common lost mark.
• “Particles in a liquid are arranged randomly no matter what.” Forces such as hydrogen bonding give liquids short-range order, and temperature changes how much of it survives.

Where this leads

Phase diagrams are not part of this topic’s assessment, so focus on particle models and the reasons behind them. Next, Topic 3.4, Ideal Gas Law, turns the gas behaviour in the table above into the equation PV = nRT. Topic 3.5 then explains it with particle speeds and kinetic energy. Try the practice questions now, then use the revision notes and the checklist to consolidate.