What are States of Matter?
Everything around us – the desk, the air you breathe, the water in your bottle – is made of tiny particles called atoms or molecules. When these particles group together, they can arrange themselves in three main ways we call states of matter: solid, liquid and gas.
💡 In Simple Words: Matter can be a solid, a liquid or a gas depending on how its tiny particles move. When they jiggle a lot, it’s a gas; when they slide past each other, it’s a liquid; when they stay almost locked in place, it’s a solid.
Key features of each state
- Solid: Particles are tightly packed in a fixed pattern, only vibrate in place. Shape and volume stay the same.
- Liquid: Particles are close but can move past one another. Takes the shape of its container but keeps its own volume.
- Gas: Particles are far apart and move freely in all directions. Neither shape nor volume is fixed.
Quick comparison
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Fixed lattice | Close but not fixed | Widely spaced |
| Particle movement | Vibration | Sliding past each other | Rapid, random motion |
| Shape | Definite | Takes container’s shape | Fills container |
| Volume | Definite | Definite | Indefinite |
| Compressibility | Very low | Low | High |
Change of State: How Matter Switches Between Forms
When we add or remove heat, particles speed up or slow down, and the material can jump from one state to another. This is called a change of state (or phase transition). The main types you’ll see in class are:
- Melting – solid to liquid (e.g., ice turning into water).
- Freezing – liquid to solid (water becoming ice).
- Vaporisation – liquid to gas. It includes boiling (occurs throughout the liquid at its boiling point) and evaporation (surface‑only, at any temperature).
- Condensation – gas to liquid (steam turning back into water droplets).
- Sublimation – solid directly to gas (dry ice disappearing).
- Deposition – gas directly to solid (frost forming on a cold window).
Why does heat matter?
Think of particles like cars on a highway. When traffic moves slowly (low heat), cars stay close – like a solid. Turn up the heat, the cars speed up and can change lanes, resembling a liquid. Crank the heat more, and they zip all over the road, just like gas particles spreading everywhere.
Important temperatures
Each pure substance has a specific melting point (temperature where solid becomes liquid) and boiling point (temperature where liquid becomes gas). For water, these are 0 °C and 100 °C at 1 atm pressure.
Factors that shift the temperature
- Pressure: Raising pressure usually raises the boiling point (think of a pressure cooker).
- Impurities: Adding salt lowers the freezing point of water (why roads are salted).
- Surface area: A larger surface speeds up evaporation.
Worked Example: Calculating Energy for Melting Ice
Question: How much heat is required to melt 50 g of ice at 0 °C into water at 0 °C? (Latent heat of fusion of ice = 334 J g⁻¹)
Solution:
- Identify the formula: Q = m × L_f, where Q = heat energy, m = mass, L_f = latent heat of fusion.
- Plug in the numbers: Q = 50 g × 334 J g⁻¹ = 16 700 J.
- Answer: 16.7 kJ of heat is needed.
Quick Revision Checklist
- Remember the three states and how particle arrangement changes.
- Know the six common changes of state and their everyday examples.
- Be able to define melting point, boiling point, latent heat of fusion and vaporisation.
- Understand how pressure and impurities affect the temperatures.
- Practice a numeric problem on latent heat.
📝 Likely Exam Questions
- Define ‘state of matter’ and list its three common forms.
Answer: A state of matter is the distinct form that different phases of matter take on. The three common forms are solid, liquid and gas. - What happens to the particles of a substance during evaporation?
Answer: Particles gain kinetic energy, move faster, and some escape from the liquid surface into the air as gas. - Explain why ice melts faster on a metal plate than on a wooden table.
Answer: Metal conducts heat better, supplying more thermal energy to the ice, so its particles vibrate faster and break the solid lattice sooner. - Calculate the heat required to convert 20 g of water at 100 °C to steam at 100 °C. (Latent heat of vaporisation of water = 2260 J g⁻¹)
Answer: Q = m × L_v = 20 g × 2260 J g⁻¹ = 45 200 J = 45.2 kJ. - State two factors that lower the freezing point of water and give a practical example of each.
Answer: (i) Adding solutes like salt – used to melt ice on roads. (ii) Increasing pressure – water under high pressure freezes at lower temperatures (used in high‑pressure ice‑cream makers).