# Liquids and Solids

> Chemistry · CIE A-Level 9701
> Source: https://www.owlsprep.com/study/cie-9701-u4-liquids-and-solids/

This sub-topic explores the particle arrangement, motion and physical properties of the liquid and solid states of matter, connecting intermolecular force strength to observable properties like melting and boiling points.

**Prerequisites:** [Intermolecular forces](https://www.owlsprep.com/study/cie-9701-u3-intermolecular-forces/); [Gaseous state](https://www.owlsprep.com/study/cie-9701-u4-gases/)

## Learning objectives

- Describe properties of liquids and solids using kinetic particle theory
- Explain vapor pressure and how intermolecular forces affect it
- Distinguish between crystalline and amorphous solids
- Relate structure of solids to melting point and other physical properties

## Particle Model of Liquids and Solids

**Liquid and Solid States** — Two condensed states of matter where particles are close together and intermolecular forces are significant, unlike the gaseous state. Liquids have fixed volume but no fixed shape; solids have fixed shape and fixed volume.

*Example:* Liquid water (liquid) and ice (solid) at 0°C and 1 atm

Kinetic particle theory describes each state based on particle ordering and kinetic energy, which is directly related to the strength of intermolecular interactions holding particles together.

- **Liquids**: Random arrangement of particles, enough kinetic energy to slide past one another, intermolecular forces weaker than solids but stronger than gases
- **Solids**: Fixed ordered arrangement of particles, low kinetic energy so particles only vibrate around fixed positions, strong intermolecular forces hold particles in place

**Worked example:** Explain why solids have fixed shape and volume, while liquids have fixed volume but take the shape of their container.

1. In solids, very strong intermolecular forces hold particles in a fixed regular lattice structure. Particles can only vibrate around their fixed positions, they cannot move to new locations.
2. This fixed arrangement means the overall shape and volume of the solid cannot change, hence solids have fixed shape and volume.
3. In liquids, intermolecular forces are weaker than in solids. They are strong enough to keep particles close together (so volume is fixed) but too weak to hold particles in fixed positions.
4. Particles can slide past each other and move throughout the liquid, so the liquid can flow to fit the shape of its container, resulting in no fixed shape.

> **Exam tip:** Marks are awarded for both intermolecular force strength and particle movement in explanations — always include both.

## Vapor Pressure and Changes of State

**Vapor Pressure** — The pressure exerted by a vapor when it is in dynamic equilibrium with its liquid phase in a closed container at a given temperature.

When a liquid is placed in a closed container, high-energy molecules at the surface escape into the gas phase. Over time, the rate of molecules leaving the liquid equals the rate of vapor molecules condensing back, creating dynamic equilibrium. Vapor pressure depends only on temperature and the identity of the liquid.

> **info**
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> Vapor pressure always increases with increasing temperature, as more molecules gain enough kinetic energy to escape the liquid surface.

**Worked example:** Propanone has a boiling point of 56°C, and water has a boiling point of 100°C, both at 1 atm pressure. Which has higher vapor pressure at 25°C? Explain your answer.

1. Boiling occurs when the vapor pressure of the liquid equals the external atmospheric pressure. A lower boiling point means the liquid reaches equal vapor pressure to the atmosphere at a lower temperature.
2. Propanone has a lower boiling point than water, so it has weaker intermolecular forces between its molecules than water.
3. Weaker intermolecular forces mean more molecules can escape the liquid surface at 25°C, leading to higher vapor pressure.
4. Final answer: Propanone has higher vapor pressure at 25°C.

**Check your understanding**

Check your understanding:

1. Which statement about vapor pressure is correct?

   - A: Vapor pressure decreases with increasing temperature
   - B: Vapor pressure depends on the volume of the container
   - C: A liquid with weaker intermolecular forces has higher vapor pressure
   - D: Vapor pressure only exists when the liquid is boiling

   *Why:* Correct. Weaker intermolecular forces allow more molecules to escape the liquid at a given temperature, increasing vapor pressure. A is wrong: vapor pressure increases with temperature. B is wrong: vapor pressure depends only on temperature and liquid identity. D is wrong: vapor pressure exists at all temperatures.

## Crystalline Solids

**Crystalline Solids** — Solids with a regular, repeating three-dimensional arrangement of particles called a crystal lattice, with long-range order.

*Example:* Sodium chloride, diamond, copper, ice

All bonds or interactions between particles in a crystalline lattice are of equal strength, so crystalline solids have a sharp, specific melting point. Crystalline solids are classified into four types based on their particles and bonding:

1. Ionic solids: Ions held together by ionic bonds
2. Metallic solids: Metal cations held together by delocalized electrons
3. Covalent network solids: Atoms held together by a continuous network of covalent bonds
4. Molecular solids: Molecules held together by intermolecular forces

**Worked example:** Explain why crystalline sodium chloride has a sharp melting point of 801°C.

1. Sodium chloride is a crystalline ionic solid, with a regular repeating lattice of $Na^+$ and $Cl^-$ ions.
2. All ionic bonds between the ions are of equal strength throughout the entire lattice.
3. At 801°C, all ionic bonds gain enough thermal energy to break at the same time, so the solid melts completely at this specific temperature, giving a sharp melting point.

## Amorphous Solids

**Amorphous Solids** — Solids with no long-range regular arrangement of particles, with a disordered structure similar to a frozen liquid.

*Example:* Glass, rubber, amber

The irregular structure of amorphous solids means bond strengths vary across different regions of the solid. As a result, they do not have a sharp melting point, and instead soften gradually over a range of temperatures as weaker regions break first, followed by stronger regions.

**Worked example:** Explain why glass does not have a sharp melting point.

1. Glass is an amorphous solid, with an irregular, disordered arrangement of silicon dioxide molecules, with no repeating crystal lattice.
2. Intermolecular bond strengths vary across different regions of the disordered structure, so different regions require different amounts of thermal energy to break.
3. Weaker bonds break at lower temperatures, and stronger bonds break at higher temperatures, leading to gradual softening over a temperature range instead of a single sharp melting point.

## Common pitfalls

- **Wrong:** Claiming that particles in solids do not move at all
  - Why it fails: Solid particles are not stationary, they do move
  - Correct: State that particles in solids vibrate around fixed positions, and cannot flow to new locations
- **Wrong:** Confusing evaporation and boiling, claiming evaporation only occurs below boiling point
  - Why it fails: Evaporation occurs at any temperature, including at boiling point
  - Correct: Clarify evaporation is a surface process at any temperature, boiling is a bulk process only at boiling point when vapor pressure equals external pressure
- **Wrong:** Claiming vapor pressure depends on the amount of liquid or volume of the container
  - Why it fails: Vapor pressure is an equilibrium property that only depends on temperature and liquid identity
  - Correct: State that as long as some liquid is present at equilibrium, vapor pressure does not change with volume or amount of liquid at constant temperature
- **Wrong:** Claiming all solids have a sharp melting point
  - Why it fails: Only crystalline solids have sharp melting points, amorphous solids do not
  - Correct: Specify that only crystalline solids with regular lattices have sharp melting points
- **Wrong:** Stating higher vapor pressure means higher boiling point
  - Why it fails: Boiling occurs when vapor pressure equals external pressure, so higher vapor pressure reaches this point at a lower temperature
  - Correct: Higher vapor pressure at a given temperature corresponds to a lower boiling point at the same external pressure

## Cheatsheet

| Property | Liquid | Crystalline Solid | Amorphous Solid |
| --- | --- | --- | --- |
| Particle arrangement | Random, close-packed | Regular repeating lattice | Irregular, disordered |
| Shape | Fits container, fixed volume | Fixed shape and volume | Fixed shape |
| Melting point | Fixed | Sharp, fixed value | Range of temperatures |

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