Sciences · Chemistry
2.2 States of Matter
Matter is everything around us that has mass and takes up space. It exists in three main states — solid, liquid, and gas — and the behaviour of tiny particles explains why.
The Big Idea
Everything you can see, touch, or breathe is made of incredibly tiny particles (atoms and molecules). The way these particles are arranged and how they move determines whether something is a solid, a liquid, or a gas.
What You Will Learn
- The kinetic particle theory and how particles behave
- Properties of solids, liquids, and gases
- How matter changes between states (melting, boiling, freezing)
- The difference between physical and chemical changes
- The difference between mixtures and compounds
- How to separate mixtures using different techniques
Sciences · Chemistry
The Particle Model
The kinetic particle theory states that all matter is made of tiny particles that are in constant motion. The amount of energy the particles have determines the state of matter.
Key Ideas
- All matter is made of tiny particles (atoms or molecules).
- Particles are always moving (they have kinetic energy).
- Particles have spaces between them.
- Particles have forces of attraction between them (stronger in solids, weaker in gases).
- Adding heat energy makes particles move faster.
Common Mistake: Particles do NOT expand when heated. The particles themselves stay the same size — they just move faster and spread further apart, which is why materials expand.
Sciences · Chemistry
The Three States of Matter
Solids, liquids, and gases have different properties because their particles are arranged and move differently.
Comparing the Three States
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed shape | Takes the shape of the container | Fills the entire container |
| Volume | Fixed | Fixed | Not fixed (expands to fill space) |
| Particle arrangement | Close together, regular pattern | Close together, irregular | Far apart, random |
| Particle movement | Vibrate in fixed positions | Slide past each other | Move quickly in all directions |
| Forces between particles | Strong | Moderate | Very weak |
| Energy of particles | Low | Medium | High |
| Can be compressed? | No | No | Yes |
Why Can Gases Be Compressed?
Gas particles have large spaces between them. When you push on a gas (like pushing a syringe), the particles can be forced closer together. In solids and liquids, particles are already close together, so there is no space to squeeze into.
Sciences · Chemistry
Changes of State
When you add or remove heat energy, matter can change from one state to another. These are physical changes — no new substance is created.
The Changes of State
| Change | From → To | Energy |
|---|---|---|
| Melting | Solid → Liquid | Heat energy added |
| Freezing | Liquid → Solid | Heat energy removed |
| Boiling / Evaporation | Liquid → Gas | Heat energy added |
| Condensation | Gas → Liquid | Heat energy removed |
| Sublimation | Solid → Gas (directly) | Heat energy added |
Key Temperatures
- Melting point of water: 0 °C — ice turns to liquid water
- Boiling point of water: 100 °C — liquid water turns to steam
- At these points, the temperature stays constant while the state changes — all the energy goes into breaking or forming bonds between particles.
Physical Change vs Chemical Change
| Physical Change | Chemical Change |
|---|---|
| No new substance is created | A new substance IS created |
| Usually reversible | Usually irreversible |
| Examples: melting ice, dissolving sugar | Examples: burning wood, rusting iron |
Critical Rule: Dissolving sugar in water is a physical change (the sugar can be recovered by evaporation). Burning sugar is a chemical change (a new substance is formed and cannot be reversed).
Sciences · Chemistry
Elements, Compounds & Mixtures
All matter is made of atoms. The way atoms are combined determines whether something is an element, a compound, or a mixture.
Key Definitions
| Term | Definition | Example |
|---|---|---|
| Element | A pure substance made of only one type of atom | Oxygen (O), Iron (Fe), Gold (Au) |
| Compound | A substance made of two or more different elements chemically bonded | Water (H₂O), Carbon dioxide (CO₂) |
| Mixture | Two or more substances combined but NOT chemically bonded | Salt water, air, sand and iron filings |
Comparing Compounds and Mixtures
| Feature | Compound | Mixture |
|---|---|---|
| Chemical bonding? | Yes — atoms are chemically bonded | No — substances are only mixed together |
| Easy to separate? | No — needs a chemical reaction | Yes — can be separated by physical methods |
| Fixed ratio? | Yes — always the same proportions | No — any proportions |
| Properties | Different from its elements | Same as its individual substances |
Real-World Example
Air is a mixture of gases (78% nitrogen, 21% oxygen, 1% other gases). The gases are not bonded together and can be separated by cooling and distilling. Water (H₂O) is a compound — hydrogen and oxygen are chemically bonded, and the water has completely different properties from either gas on its own.
Sciences · Chemistry
Separation Techniques
Because the substances in a mixture are not chemically bonded, they can be separated using physical methods. The technique you choose depends on the type of mixture.
Common Techniques
| Technique | How It Works | Used For |
|---|---|---|
| Filtration | Pour mixture through filter paper — solid stays behind, liquid passes through | Separating an insoluble solid from a liquid (e.g., sand from water) |
| Evaporation | Heat the solution so the liquid evaporates, leaving the solid behind | Getting a dissolved solid from a solution (e.g., salt from salt water) |
| Distillation | Heat to boil one liquid, then cool the vapour to collect it as a liquid | Separating liquids with different boiling points (e.g., water from ink) |
| Chromatography | Substances travel at different speeds through paper or another medium | Separating colours in ink or dyes |
| Magnetic separation | Use a magnet to attract magnetic materials | Separating iron filings from sand |
Choosing the Right Technique
- Solid that does NOT dissolve in liquid? → Filtration
- Solid that IS dissolved in liquid? → Evaporation
- Two liquids with different boiling points? → Distillation
- Identifying different substances in a mixture? → Chromatography
Sciences · Chemistry
Worked Examples
These examples show structured answers using proper scientific vocabulary and reasoning.
EXAMPLE 1A student heats ice from −10 °C to 110 °C. Describe what happens at each stage, referring to particle behaviour.
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Full Solution
−10 °C to 0 °C (solid): Particles vibrate faster as they gain energy. The temperature rises but the ice remains solid.
At 0 °C (melting): The temperature stays constant. All the energy goes into breaking the bonds between particles. Particles move from fixed positions to sliding past each other. The ice melts into water.
0 °C to 100 °C (liquid): Particles slide past each other faster. Temperature rises as kinetic energy increases.
At 100 °C (boiling): Temperature stays constant again. Energy breaks the remaining forces between particles. Particles escape as gas and move freely in all directions.
100 °C to 110 °C (gas): Steam particles move even faster. Temperature rises again.
At 0 °C (melting): The temperature stays constant. All the energy goes into breaking the bonds between particles. Particles move from fixed positions to sliding past each other. The ice melts into water.
0 °C to 100 °C (liquid): Particles slide past each other faster. Temperature rises as kinetic energy increases.
At 100 °C (boiling): Temperature stays constant again. Energy breaks the remaining forces between particles. Particles escape as gas and move freely in all directions.
100 °C to 110 °C (gas): Steam particles move even faster. Temperature rises again.
EXAMPLE 2A student has a mixture of sand, salt, and iron filings. Describe how to separate all three substances.
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Full Solution
Step 1 — Magnetic separation: Pass a magnet over the mixture. The iron filings are magnetic and will stick to the magnet. Sand and salt are not magnetic and will remain.
Step 2 — Dissolving: Add water to the remaining sand and salt. The salt dissolves in the water (it is soluble), but the sand does not (it is insoluble).
Step 3 — Filtration: Pour the mixture through filter paper. The sand is trapped by the filter (residue), and the salt water passes through (filtrate).
Step 4 — Evaporation: Heat the salt water. The water evaporates, leaving salt crystals behind.
Result: All three substances are now separated.
Step 2 — Dissolving: Add water to the remaining sand and salt. The salt dissolves in the water (it is soluble), but the sand does not (it is insoluble).
Step 3 — Filtration: Pour the mixture through filter paper. The sand is trapped by the filter (residue), and the salt water passes through (filtrate).
Step 4 — Evaporation: Heat the salt water. The water evaporates, leaving salt crystals behind.
Result: All three substances are now separated.
EXAMPLE 3Classify each of the following as an element, compound, or mixture: (a) oxygen gas, (b) carbon dioxide, (c) air, (d) pure water.
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Full Solution
(a) Oxygen gas (O₂) — Element. It contains only one type of atom (oxygen). Even though O₂ has two atoms, they are both the same element.
(b) Carbon dioxide (CO₂) — Compound. It is made of two different elements (carbon and oxygen) that are chemically bonded together in a fixed ratio (1 carbon : 2 oxygen).
(c) Air — Mixture. Air contains several gases (nitrogen, oxygen, carbon dioxide, argon, etc.) that are not chemically bonded together. They can be separated by fractional distillation.
(d) Pure water (H₂O) — Compound. It is made of hydrogen and oxygen atoms chemically bonded in a fixed 2:1 ratio. Its properties are completely different from hydrogen or oxygen gas.
(b) Carbon dioxide (CO₂) — Compound. It is made of two different elements (carbon and oxygen) that are chemically bonded together in a fixed ratio (1 carbon : 2 oxygen).
(c) Air — Mixture. Air contains several gases (nitrogen, oxygen, carbon dioxide, argon, etc.) that are not chemically bonded together. They can be separated by fractional distillation.
(d) Pure water (H₂O) — Compound. It is made of hydrogen and oxygen atoms chemically bonded in a fixed 2:1 ratio. Its properties are completely different from hydrogen or oxygen gas.
EXAMPLE 4A student says: “When water boils, it undergoes a chemical change because it turns into a different substance (steam).” Evaluate this claim.
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Full Solution
The student is incorrect. Boiling is a physical change, not a chemical change.
Reasoning: When water boils, it changes from liquid water to gaseous water (steam). However, the chemical formula remains H₂O — no new substance is created. The only change is the arrangement and movement of the particles. The process is also reversible: cooling the steam (condensation) gives liquid water again.
A chemical change would involve forming a completely new substance with different properties, such as the electrolysis of water into hydrogen and oxygen gases.
Reasoning: When water boils, it changes from liquid water to gaseous water (steam). However, the chemical formula remains H₂O — no new substance is created. The only change is the arrangement and movement of the particles. The process is also reversible: cooling the steam (condensation) gives liquid water again.
A chemical change would involve forming a completely new substance with different properties, such as the electrolysis of water into hydrogen and oxygen gases.
EXAMPLE 5Explain why perfume can be smelled from across a room, using the particle model.
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Full Solution
When perfume is sprayed, its particles enter the air as a gas. According to the kinetic particle theory, gas particles move rapidly in all directions with large spaces between them.
The perfume particles spread out and mix with the air particles through a process called diffusion — the movement of particles from an area of higher concentration to an area of lower concentration. Over time, the perfume particles reach the other side of the room.
This is faster in a warm room because higher temperature means particles have more kinetic energy and move faster.
The perfume particles spread out and mix with the air particles through a process called diffusion — the movement of particles from an area of higher concentration to an area of lower concentration. Over time, the perfume particles reach the other side of the room.
This is faster in a warm room because higher temperature means particles have more kinetic energy and move faster.
EXAMPLE 6A student needs to find out how many different dyes are in a sample of food colouring. Which technique should they use and why?
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Full Solution
The student should use chromatography.
Method: Place a spot of the food colouring on a piece of chromatography paper near the bottom. Stand the paper in a shallow layer of water (solvent). As the water rises up the paper, it carries the dyes with it.
Why it works: Different dyes have different solubilities and different attractions to the paper, so they travel at different speeds. Each dye separates into a distinct spot at a different height on the paper.
Result: The number of separate spots visible on the paper tells you how many different dyes are in the food colouring.
Method: Place a spot of the food colouring on a piece of chromatography paper near the bottom. Stand the paper in a shallow layer of water (solvent). As the water rises up the paper, it carries the dyes with it.
Why it works: Different dyes have different solubilities and different attractions to the paper, so they travel at different speeds. Each dye separates into a distinct spot at a different height on the paper.
Result: The number of separate spots visible on the paper tells you how many different dyes are in the food colouring.
Sciences · Chemistry
Practice Q&A
Attempt each question before revealing the model answer.
EXPLAINExplain why a gas fills the entire container it is placed in.
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Model Answer
According to the kinetic particle theory, the particles in a gas move rapidly in all directions with a lot of space between them. There are no significant forces of attraction holding the particles together. Therefore, the particles spread out and fill any available space, taking the shape and volume of the entire container.
DESCRIBEDescribe what happens to the particles when a solid is heated until it melts.
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Model Answer
When a solid is heated, its particles gain kinetic energy and begin to vibrate more vigorously. At the melting point, the particles have enough energy to overcome some of the forces holding them in fixed positions. They break free from their regular arrangement and begin to slide past each other, forming a liquid.
EXPLAINExplain the difference between a compound and a mixture.
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Model Answer
In a compound, two or more elements are chemically bonded together in fixed proportions, and the compound has different properties from its individual elements. In a mixture, two or more substances are combined but not chemically bonded, so they keep their own properties and can be separated by physical methods (like filtration or distillation).
IDENTIFYIdentify which separation technique you would use to separate salt from water. Explain why.
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Model Answer
You would use evaporation. Salt is dissolved in the water (it is a solution), so filtration would not work. By heating the solution, the water evaporates and the salt crystals are left behind. If you also want to collect the water, you would use distillation instead, which cools and collects the water vapour.
DESCRIBEDescribe how you would use distillation to obtain pure water from ink.
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Model Answer
Heat the ink in a flask until the water boils and turns to steam. The steam rises and passes through a condenser (a cold tube), where it cools down and condenses back into liquid water. The pure water is collected in a separate container. The ink dye remains behind in the original flask because it has a higher boiling point than water.
EXPLAINExplain why the temperature stays constant during melting, even though heat is still being added.
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Model Answer
During melting, the added heat energy is used to break the bonds (forces of attraction) between particles rather than increasing their speed. Since temperature measures the kinetic energy of particles, and the energy is going into breaking bonds instead, the temperature remains constant until all the bonds are broken and the substance has fully changed state.
IDENTIFYIdentify whether each change is physical or chemical: (a) rusting iron, (b) boiling water, (c) baking a cake.
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Model Answer
(a) Rusting iron — Chemical change. Iron reacts with oxygen and water to form iron oxide (rust), a new substance.
(b) Boiling water — Physical change. Water changes state from liquid to gas, but remains H₂O.
(c) Baking a cake — Chemical change. The ingredients react to form new substances; the cake cannot be turned back into flour, eggs, and sugar.
(b) Boiling water — Physical change. Water changes state from liquid to gas, but remains H₂O.
(c) Baking a cake — Chemical change. The ingredients react to form new substances; the cake cannot be turned back into flour, eggs, and sugar.
EXPLAINExplain why gases can be compressed but liquids cannot.
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Model Answer
Gas particles have large spaces between them because the forces of attraction are very weak. When pressure is applied, the particles can be pushed closer together into these spaces. Liquid particles are already close together with only small gaps between them, so there is very little space to compress into.
Chemistry · Review
Flashcard Review
Tap each card to reveal the answer. Try to answer from memory first.
What does the kinetic particle theory state?
All matter is made of tiny particles that are in constant motion. The amount of energy determines the state of matter.
Tap to reveal
How are particles arranged in a solid?
Close together in a regular, fixed pattern. They vibrate in place but do not move freely.
Tap to reveal
Why can gases be compressed but solids cannot?
Gas particles have large spaces between them that can be pushed closer together. Solid particles are already packed tightly with no room to compress.
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What is the melting point of water?
0 °C — the temperature at which ice changes to liquid water.
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What is the boiling point of water?
100 °C — the temperature at which liquid water changes to steam (water vapour).
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What is the difference between a physical change and a chemical change?
Physical change: no new substance is formed (reversible, e.g., melting). Chemical change: a new substance IS formed (usually irreversible, e.g., burning).
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What is an element?
A pure substance made of only one type of atom (e.g., oxygen, iron, gold). It cannot be broken down into simpler substances.
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What is a compound?
A substance made of two or more different elements chemically bonded together in fixed proportions (e.g., H₂O, CO₂).
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What is a mixture?
Two or more substances combined but NOT chemically bonded. They keep their own properties and can be separated by physical methods.
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When would you use filtration?
To separate an insoluble solid from a liquid (e.g., sand from water). The solid gets trapped in the filter paper.
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When would you use evaporation?
To get a dissolved solid back from a solution (e.g., salt from salt water). The liquid evaporates, leaving the solid behind.
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What is distillation?
A technique that separates liquids by boiling point. The liquid is boiled, and the vapour is cooled and collected separately.
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What is condensation?
The change of state from gas to liquid. It happens when gas particles lose energy and slow down enough to form a liquid.
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Is dissolving sugar in water a physical or chemical change?
Physical change — no new substance is formed. The sugar can be recovered by evaporating the water.
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What is sublimation?
The change from solid directly to gas, without becoming a liquid first (e.g., dry ice turning into carbon dioxide gas).
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Chemistry · Practice Test
Practice Test
20 questions covering the particle model, states of matter, changes of state, and separation techniques.
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