Ch 1 · Solids, Liquids and Gases · O Level Chemistry 5070
Topic 1 of 2 in States of Matter — O Level Chemistry 5070 — 48 lessons.
Introduction
An ice cube melts into water, and the water can be boiled away as steam: the same substance, water, exists as a solid, a liquid or a gas. Every substance can exist as a solid, a liquid or a gas, and the differences between the three states come down to how far apart the particles are, how they are arranged and how they move. This kinetic particle theory underpins diffusion, rates of reaction and gas volumes later in the course.
Key ideas — Distinguishing properties of the three states
🧑🏫 Teacher's explanation
Imagine a crowded dance floor at a music festival. In a solid, everyone is packed tightly together, standing shoulder-to-shoulder in a strict grid pattern. You can move your arms slightly, but you can't move from your spot. This is like a solid: particles are packed tightly in a regular arrangement and only vibrate about fixed positions.
📖 Definitions
🔍 Worked example — Easy
A sample of oxygen gas occupies a volume of 20.0 cm^3 at a certain temperature. If the pressure is doubled while temperature remains constant, what happens to the volume?
🔍 Worked example — Standard
Calculate the mass of 4.0 mol of liquid water (H_2O).
🔍 Worked example — Exam-style
A gas is held in a 500 cm^3 container. If the temperature is increased from 300 K to 600 K at constant pressure, what is the new volume?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the arrangement and motion of the sulfur particles in the solid state. [2] (b) Explain, in terms of kinetic particle theory, why the volume of the sulfur increases when it melts into a liquid. [2] [4 marks]
Key ideas — Structure in terms of particles
The key to explaining these properties is the separation, arrangement and motion of the particles.
🧑🏫 Teacher's explanation
Imagine a crowded concert venue. In a solid, it is like a standing-room-only crowd where everyone is packed shoulder-to-shoulder in neat rows; you can't move from your spot, you can only wiggle in place. In a liquid, it is like a slow-moving mosh pit; you are still touching the people around you, but you are sliding and shuffling past them to get to the stage. In a gas, it is like a few people running wildly across a massive football stadium; there is huge empty space between you, and you're sprinting in straight lines until you hit a wall.
📖 Definitions
🔍 Worked example — Easy
A substance has a molar mass of M_r = 44 g/mol. If you have 2.2 g of this gas, how many moles are present?
🔍 Worked example — Standard
Compare the compressibility of a solid block of iron and a container of oxygen gas. Explain the difference using particle theory.
🔍 Worked example — Exam-style
A heating curve shows a horizontal plateau at 100circC while heat is being added. What is happening to the particles and the temperature during this plateau?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Describe the arrangement and motion of the water particles when the temperature is 25circC. [2] (b) Explain why the temperature of the water remains constant at 0circC while it is melting. [2] [4 marks]
Key ideas — Changes of state and heating curves
A heating curve is a graph of temperature against time as a substance is heated steadily. It has two flat (horizontal) sections: one at the melting point and one at the boiling point. During these sections the temperature does not rise even though energy is still being supplied, because the energy is used to overcome the forces of attraction between particles rather than to make them move faster. A cooling curve is the mirror image: temperature falls, with flat sections at the boiling point (condensing) and the melting point (freezing).
🧑🏫 Teacher's explanation
Imagine a crowded dance floor at a music festival. In a 'solid' state, everyone is packed tightly together, standing in neat rows, only able to wiggle slightly in place. In a 'liquid' state, the crowd is still dense, but people are now moving around each other, sliding past one another while staying close. In a 'gas' state, everyone has broken free and is sprinting wildly across a massive open field, barely ever bumping into anyone else.
📖 Definitions
🔍 Worked example — Easy
A sample of pure water is heated. At what temperature (in ^circC) does the temperature stop rising while the substance is transitioning from a liquid to a gas throughout the bulk of the liquid?
🔍 Worked example — Standard
A substance is heated at a constant rate. A heating curve shows a horizontal plateau at T = 50 ^circC. What is happening to the particles at this temperature?
🔍 Worked example — Exam-style
A 50 g sample of ice is heated from -10 ^circC to 20 ^circC. Calculate the total energy required if the specific heat capacity of ice is 2.1 J/g^circC and the heat of fusion is 334 J/g, and the specific heat capacity of water is 4.2 J/g^circC. (Assume melting occurs at 0 ^circC)
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Explaining changes of state with kinetic particle theory
When a solid is heated its particles gain kinetic energy and vibrate more vigorously. At the melting point the vibrations are strong enough to overcome some of the forces holding the particles in the lattice, so the regular arrangement breaks down and the particles can move past one another: the solid melts. Further heating makes the liquid particles move faster. At the boiling point they have enough energy to overcome the remaining forces of attraction completely, so they escape from the liquid as a gas.
🧑🏫 Teacher's explanation
Imagine a crowded dance floor. In a solid, everyone is standing in a strict grid, shoulder-to-shoulder, just swaying slightly in place. As the music (heat) gets faster, people start bumping into each other more wildly. Eventually, they can't stay in their spots and start moving around the room—this is a liquid. If the music gets even more intense, people start sprinting wildly, breaking away from the crowd entirely to fly across the room; this is a gas.
📖 Definitions
🔍 Worked example — Easy
A sample of water is heated from 20^°C to 100^°C. Describe the change in particle motion during this process.
🔍 Worked example — Standard
A gas is held at a constant temperature in a cylinder. If the volume is halved, what happens to the pressure?
🔍 Worked example — Exam-style
Explain why increasing the temperature of a gas in a fixed volume container increases the pressure.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain, in terms of kinetic particle theory, why the pressure increases. [3] (b) If the volume were allowed to increase instead, what would happen to the pressure? [1] [4 marks]
Worked example
Question. A pure solid substance was heated at a steady rate from 20 °C to 250 °C. Its temperature stayed constant at 80 °C for two minutes and again at 218 °C for four minutes. State what is happening at each flat section and explain, using kinetic particle theory, why the temperature does not rise during these times. [5]
🔍 Worked example — Easy
A sample of pure substance X melts at 45 °C. If a student heats 10.0 g of substance X from 20 °C to 50 °C, describe the temperature change and identify the state of the substance at 45 °C.
🔍 Worked example — Standard
A heating curve shows a plateau (flat section) at 100 °C. Explain why the temperature does not increase during this plateau despite continuous heating.
🔍 Worked example — Exam-style
An unknown substance is heated. The temperature graph shows a flat section at 80 °C and another at 150 °C. If the substance was a solid at 0 °C, what are the melting and boiling points?
⚠️ Common errors
Common mistakes
Practice
Answers Any three: a gas has no fixed shape / fills its container [B1]; has no fixed volume [B1]; can be compressed [B1]; flows [B1]; has a much lower density [B1] (max 3). Particles are arranged randomly / irregularly [B1]; close together / touching [B1]; and move past (slide over) one another [B1]. Gas particles are far apart with large spaces between them that can be reduced [B1]; water particles are already close together / touching so cannot be pushed closer [B1]. Particles gain kinetic energy and move faster [B1]; they collide with the walls (and piston) more frequently and with greater force [B1]; the piston moves out so the volume increases to keep the pressure constant [B1]. Temperature falls steadily, then stays flat at the freezing point while the liquid solidifies, then falls steadily again [B1]; during the flat section particles are losing kinetic energy slowly enough for forces of attraction to lock them into a regular lattice, so their average speed (and temperature) does not change [B1].