Ice, water and steam are the same substance in three different states. The particle model explains why, and how matter changes from one state to another.
A puddle freezes, a pan of water boils dry, a bottle of perfume smells across the room. All of these are about the same idea: matter is made of tiny moving particles, and what they do decides whether a substance is a solid, a liquid or a gas. This article explains the particle model, what happens during changes of state, and how gases behave when you squeeze or heat them.
Everything around us is matter: it has mass and takes up space. Matter usually exists in three states: solid, liquid and gas. The particle model says that all matter is made of very small particles (atoms, molecules or ions) that are always moving, with forces of attraction between them. The state depends on how strongly the particles are held together compared with how much energy they have.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Particle arrangement | Regular, tightly packed | Close together, random | Far apart, random |
| Particle movement | Vibrate in fixed positions | Move around and slide past each other | Move quickly in all directions |
| Forces between particles | Strong | Weaker | Almost none |
| Shape | Fixed | Takes the shape of its container | Fills its container |
| Volume | Fixed | Fixed | Changes to fill the container |
| Compressible? | No | Almost not | Yes, easily |
| Density | Usually high | Usually slightly less than the solid | Very low |
Water is an important exception: ice floats on liquid water because its particles are held in an open pattern, so ice is less dense than liquid water. This is why lakes freeze from the top down and fish can survive underneath.
A change of state is a physical change: the particles are the same, only their arrangement and energy change. No new substance is formed, and the change can be reversed.
| Change | From → To | Energy | Example |
|---|---|---|---|
| Melting | Solid → liquid | Taken in | Ice melting in a cup |
| Freezing | Liquid → solid | Given out | Water becoming ice |
| Boiling | Liquid → gas (throughout the liquid) | Taken in | Water boiling in a kettle |
| Evaporation | Liquid → gas (at the surface only) | Taken in | Puddles drying |
| Condensation | Gas → liquid | Given out | Mist on a cold mirror |
| Sublimation | Solid → gas directly | Taken in | Dry ice (solid CO₂), iodine |
| Deposition | Gas → solid directly | Given out | Frost forming on a cold window |
When a solid is heated, its particles vibrate more strongly. At the melting point they have enough energy to break free from their fixed positions and the solid becomes a liquid. At the boiling point the particles have enough energy to overcome the forces between them completely and form bubbles of gas inside the liquid.
Dry ice is extremely cold (about −78 °C) and can cause burns, so handle it only with insulated gloves. Never seal it in a closed container or bottle, because as it turns to gas it can build up pressure and burst the container. Use it in a well-ventilated room.
A heating curve shows temperature against the energy supplied. Look at the flat parts (plateaus): while a substance is melting or boiling, the temperature stays constant even though heat is still being supplied. The energy goes into breaking the forces between particles, not into making them move faster, so the temperature does not rise until the change is complete.
The boiling plateau is longer than the melting plateau because far more energy is needed to separate particles completely into a gas than to loosen them into a liquid. The same curve run backwards, as a substance cools, shows plateaus at the freezing and condensing points.
Question: How much energy is needed to warm 200 g of water from 20 °C to 70 °C? (Specific heat capacity of water ≈ 4.2 J/(g·°C).)
Answer: 42 kJ. This formula applies only while the temperature is changing, not during a change of state.
Melting and boiling points tell you the state of a substance at a given temperature. The values below are rounded and refer to normal atmospheric pressure.
| Substance | Melting point (°C) | Boiling point (°C) | State at 25 °C |
|---|---|---|---|
| Nitrogen | −210 | −196 | Gas |
| Oxygen | −218 | −183 | Gas |
| Ethanol | −114 | 78 | Liquid |
| Mercury | −39 | 357 | Liquid |
| Water | 0 | 100 | Liquid |
| Sodium chloride | 801 | 1413 | Solid |
| Iron | 1538 | 2861 | Solid |
Question: What is the state of ethanol at 90 °C, and of mercury at −50 °C?
Rule: below the melting point is solid; between melting and boiling points is liquid; above the boiling point is gas.
Pure substances have sharp melting and boiling points. If a substance melts over a range of temperatures, it is probably impure. Substances with strong forces between particles, such as ionic compounds and metals, have high melting points, while those held together by weak forces between small molecules, such as oxygen, have very low ones.
| Evaporation | Boiling | |
|---|---|---|
| Where it happens | At the surface only | Throughout the liquid (bubbles) |
| Temperature | At any temperature | Only at the boiling point |
| Speed | Slow | Fast |
| Energy | Fastest particles escape, so the remaining liquid cools | Heat is supplied continuously |
Evaporation is faster when the liquid is warmer, has a larger surface area, is in moving air (wind carries vapour away) and when the air is dry. Evaporation causes cooling because the particles with the most energy escape first; this is how sweating cools your body.
The boiling point depends on pressure. At high altitudes the air pressure is lower, so water boils at a lower temperature and food takes longer to cook. In a pressure cooker the pressure is higher, so water boils at a higher temperature and food cooks faster.
Diffusion is the movement of particles from a region of higher concentration to a region of lower concentration, caused by random particle motion, until they are spread evenly. It explains why a smell spreads through a room, and why a drop of ink slowly colours a glass of water. Diffusion is fastest in gases, slower in liquids and very slow in solids.
The reaction is NH₃ + HCl → NH₄Cl. Because ammonia (relative molecular mass 17) is lighter than hydrogen chloride (36.5), its particles move faster on average, so they travel farther in the same time. In general, lighter particles diffuse faster at the same temperature, and higher temperature makes diffusion faster for all particles.
Ammonia and hydrogen chloride fumes irritate the eyes, nose and lungs. This experiment is a teacher demonstration in a well-ventilated room or fume cupboard, with eye protection.
Gas pressure is caused by gas particles colliding with the walls of their container. More frequent or harder collisions mean higher pressure.
For a fixed mass of gas at constant temperature, pressure × volume stays constant:
Question: A gas has a volume of 120 cm³ at 100 kPa. It is compressed to 40 cm³ at the same temperature. What is the new pressure?
For a fixed mass of gas at constant pressure, volume is directly proportional to the temperature in kelvin. To convert, add 273: K = °C + 273 (absolute zero is −273 °C, or 0 K).
Question: A balloon has volume 600 cm³ at 27 °C. What is its volume at 57 °C at constant pressure?
Always use kelvin, never °C, in gas-law calculations.
Plasma is often called the fourth state of matter. It is a very hot gas whose atoms have lost electrons, so it contains charged particles. Plasma is found in lightning, the Sun and other stars, neon signs and fluorescent lamps. Some other special states exist at extreme temperatures in laboratories, but they are beyond school level.
Try each question first, then tap it to reveal the answer.
Answer: They move quickly and randomly in all directions, with almost no forces between them.
Answer: The energy supplied is used to break the forces holding the particles in place, not to increase their speed.
Answer: Sublimation.
Answer: The most energetic particles evaporate from the skin, taking energy with them so the remaining liquid cools.
Answer: Hydrogen, because its particles are lighter.
Answer: p₂ = 50 × 200 ÷ 100 = 100 kPa.
Answer: 25 + 273 = 298 K.
Suggested use: ask students to act out the three states in the classroom (tightly linked, sliding, moving freely), then record a cooling curve for water or stearic acid with a thermometer and sketch the plateau. Heating and any experiments with hot water or chemicals should be supervised. Parents can use kitchen examples such as steam, ice cubes and perfume. Please follow your own syllabus for required notation and gas-law content.
These reputable sources were consulted to verify the facts in this article:
Copyright note: This article is original EDUSAMBAM educational writing. The diagrams are original inline SVG schematics created for this page and are not copied from any source. Links open external websites; the sources were used to check facts and are suggested for further study.
Last updated: 21 October 2026. Written for students up to age 18 and checked against standard school-level chemistry references. This is general education, not a substitute for your teacher, textbook or syllabus.
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