Density of Solids, Liquids and Gases

Introduction

Solids, liquids and gases differ in many ways, and density is one of the clearest. A typical solid or liquid is roughly a thousand times denser than a gas. Comparing densities across the three states gives strong evidence for the particle model of matter and helps explain everyday observations, from bubbles rising in a drink to steam filling a kitchen.

Core explanation

Typical ranges. At room temperature and pressure:

- Most solids have densities between about 0.5 and 20 g/cm³ (wood around 0.5, glass 2.5, iron 7.9, gold 19.3). - Most liquids lie between about 0.7 and 1.6 g/cm³ (petrol about 0.74, water 1.00, glycerol 1.26), with liquid metals such as mercury (13.5 g/cm³) as exceptions. - Gases are around 0.0001–0.005 g/cm³ (hydrogen 0.00008, air 0.0012, carbon dioxide 0.0018).

Why gases are so much less dense. In solids and liquids the particles are close together, almost touching. In a gas at room conditions, the particles are on average about ten times further apart than their own size, so a given volume holds roughly a thousand times fewer particles. Fewer particles in the same volume means much less mass per cm³.

The same substance in different states. When a substance melts, its particles usually move slightly further apart, so the liquid is a little less dense than the solid — solid wax sinks in molten wax, for example. When a liquid boils, the particles move very far apart and the density drops dramatically. Liquid water has a density of about 1.0 g/cm³; steam at 100 °C and normal pressure has a density of about 0.0006 g/cm³ — about 1600 times less.

An important exception. Water is unusual: ice (0.92 g/cm³) is less dense than liquid water, which is why ice floats. Page 48 explains the reason.

Solids and liquids are hard to compress. Because their particles are already close together, pushing harder barely changes their volume, so their densities hardly change with pressure. Gases are easily squeezed, so their density rises as pressure increases.

Step-by-step reasoning

To explain why 1 cm³ of steam has far less mass than 1 cm³ of water:

1. Both contain the same kind of particle (water molecules). 2. In liquid water the molecules are packed closely. 3. In steam the molecules are very far apart. 4. So 1 cm³ of steam contains far fewer molecules. 5. Fewer molecules means less mass in the same volume, so a lower density.

Visual explanation

Picture three equal boxes representing 1 cm³ each. The solid box is full of particles in neat rows; the liquid box is almost as full, but the particles are jumbled; the gas box contains only a few scattered particles. The simulation shows the same pattern, and lets you watch the spacing change as a substance is heated from solid to liquid to gas.

Real-world analogy

A packed football stadium, the same crowd leaving through the car park, and those people spread across a whole city represent solid, liquid and gas. The number of people is the same, but the space they occupy grows enormously, so the "density of people" falls dramatically.

Real-world example

Liquefied petroleum gas (LPG), used in cooking cylinders, is stored as a liquid under pressure. As a liquid it is roughly 250 times denser than as a gas, which is why a small cylinder can hold enough fuel for weeks of cooking. When the valve is opened, the liquid turns to gas and expands enormously.

Why?

Why do gas bubbles rise through a liquid? A bubble of gas is far less dense than the liquid around it. The liquid pushes up on the bubble with a force (upthrust) larger than the bubble's tiny weight, so the bubble rises. This is the same reason a cork floats — lower density than the surrounding fluid.

Common misconception

Students sometimes think that the particles themselves get bigger when a substance changes from solid to gas. The particles stay the same size; it is the spaces between them that increase. Density falls because there are fewer particles in each cm³, not because each particle becomes lighter or larger.

Worked example

Question: 1.0 g of water has a volume of 1.0 cm³ as a liquid. As steam at 100 °C it occupies about 1700 cm³. Calculate the density of the steam.

Reasoning: Density = mass ÷ volume = 1.0 ÷ 1700.

Answer: About 0.00059 g/cm³, roughly 1700 times less dense than liquid water.

Quick check

1. Which state of matter usually has the lowest density, and why? Answer: Gas, because its particles are much further apart, so each cm³ contains far fewer particles.

Exam focus

When explaining density differences between states, always refer to particle spacing and the number of particles per unit volume. Mention that the particles themselves do not change size. Remember water's exception: ice is less dense than liquid water.

Advanced insight

The ratio of gas to liquid volume explains how much a liquid expands when it boils: for water about 1700 times at 100 °C. This expansion drives steam engines and pressure cookers, and it is why sealed containers of liquid must never be heated — the pressure can rise dangerously.

Summary

Solids and liquids have similar, relatively high densities because their particles are close together; gases have densities about a thousand times lower because their particles are far apart. The same substance usually becomes slightly less dense on melting and far less dense on boiling, with water's solid–liquid change as a key exception.

Practice questions

1. Put these in order of increasing density: water, air, iron. Answer: Air, water, iron. 2. Explain why solids are difficult to compress but gases are easy to compress. Answer: Particles in solids are already touching, so they cannot be pushed much closer; gas particles are far apart with large spaces that can be reduced. 3. A substance has a density of 0.0018 g/cm³ at room temperature. Which state is it most likely to be in? Answer: A gas (its density is typical of gases such as carbon dioxide). 4. Why does a bubble of air rise in water? Answer: Air is much less dense than water, so the upward push of the water on the bubble exceeds the bubble's weight.