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Chemistry329Updated 18 Sept 202611 pages

Complete IGCSE EDEXCEL Chemistry Guide

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Ever wondered why ice melts or how smell travels across a room? States of matter explain how particles behave differently as solids, liquids, and gases - and understanding this will help you nail those chemistry exam questions whilst making sense of everyday phenomena around you.
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Solids: Particles That Stay Put

Think of a solid like students sitting in assigned seats during assembly - they're fixed in position and can only wiggle about a bit. The particles in solids have strong forces of attraction keeping them locked in a neat lattice arrangement, all close together.

These particles can only vibrate in their fixed positions. When you heat a solid, the particles vibrate more vigorously, which is why solids expand when warmed up. This is why railway tracks have gaps - the metal needs room to expand on hot days!

Solids have the lowest energy of all three states and maintain a fixed shape and volume. You can't pour a solid (try pouring a brick!) and they're not compressible - you can't squeeze them into a smaller space.

Quick Check: Remember that stronger forces = less movement. Solids have the strongest forces, so particles move the least!

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Liquids: The Middle Ground

Liquids are like students during break time - they stick together but can move about freely within the playground. Particles in liquids have weaker forces of attraction than solids and arrange themselves randomly whilst staying in contact.

The particles move in random motion and slide past each other easily. When heated, liquids expand and particles move faster, which explains why your drink bottle might overflow if left in a hot car.

Liquids have medium energy levels and take the shape of their container whilst keeping a fixed volume. Unlike solids, liquids do flow (you can pour water) but they're still not compressible - try squashing water in a syringe and you'll see what we mean!

Real-World Tip: Think of honey vs water - both are liquids but honey flows slower because its particles have slightly stronger attractions.

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Gases: Maximum Freedom

Gas particles are like students after school - they're far apart, not touching, and moving fast in random directions. They have the weakest forces of attraction and travel in straight lines until they bump into something.

When you heat a gas, particles move even faster, creating more pressure and causing expansion. This explains why car tyres can burst on extremely hot days - the air inside expands too much for the tyre to contain.

Gases have the highest energy of all three states and take both the shape and volume of their container. They flow easily and are the only state that's compressible - you can squeeze gas particles closer together because there's loads of empty space between them.

Memory Trick: Gas particles are like teenagers - they want maximum space, move fast, and are hard to control!

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Changing States: The Transformation Game

Matter is any physical substance with mass that takes up space, and it can transform between the three states by adding or removing heat energy. These changes are called interconversions and they're happening around you constantly.

The key interconversions include melting (solid to liquid), boiling (liquid to gas), freezing (liquid to solid), and condensation (gas to liquid). There's also sublimation where solids jump straight to gas (like dry ice) and deposition where gases become solids directly.

When you heat matter, you're giving particles more energy to move faster and overcome the forces holding them together. Cool it down, and you're taking energy away, making particles move slower and stick together more.

Exam Focus: Learn the names of all seven state changes - they're guaranteed to appear in your chemistry tests!

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The Complete Change Cycle

Melting happens when solids become liquids (ice to water), whilst boiling occurs when liquids become gases at a specific temperature (water boils at 100°C). Evaporation is different - it's when liquids become gases at any temperature below boiling point.

The reverse processes include freezing (liquid to solid) and condensation (gas to liquid - like water droplets on your bathroom mirror). The more unusual changes are sublimation (solid straight to gas) and deposition (gas straight to solid).

Understanding density helps explain these changes too. Remember the formula: Density = Mass ÷ Volume. As substances change state, their density changes because the particle arrangement changes, even though the mass stays the same.

Practical Tip: Water is unusual because ice is less dense than liquid water - that's why ice floats in your drink!

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Diffusion: Particles on the Move

Diffusion is the natural movement of particles from areas of high concentration to areas of low concentration - basically, particles love to spread out and mix up. It's like how the smell of fresh cookies spreads from the kitchen throughout your house.

Diffusion doesn't occur in solids because particles can't move from place to place - they can only vibrate. It happens faster in gases than liquids because gas particles have more energy and space to move around.

When you add water to coloured solutions, you're seeing diffusion in action. The colour particles spread out into areas with fewer colour particles (lower concentration), mixing with water molecules and causing dilution to occur.

Easy Example: Dropping food colouring into water shows diffusion perfectly - watch the colour spread without any stirring!

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Factors That Speed Up Diffusion

Temperature is the biggest game-changer for diffusion rates. Higher temperatures give particles more kinetic energy, making them move faster and mix more quickly. This is why hot tea bags colour the water much faster than cold ones.

The concentration gradient - the difference in concentration between two areas - also matters hugely. A steeper gradient (bigger difference) means faster diffusion, like how strong perfume spreads faster than weak perfume.

Particle mass plays a crucial role too. Lighter particles diffuse faster than heavier ones because they can zip around more easily. It's like comparing a feather to a bowling ball - the feather moves much more readily in a breeze.

Exam Strategy: Remember the three factors as TMC - Temperature, Mass, Concentration gradient. Higher temperature and concentration = faster diffusion. Lower mass = faster diffusion.

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Diffusion Experiments You Need to Know

The potassium manganate (VII) and water experiment is a classic that shows diffusion beautifully. When you drop this bright purple chemical into water, it slowly spreads throughout the beaker without any stirring - that's diffusion in action.

The purple particles randomly move among water particles until the colour spreads evenly. If you add more water to the final solution, the purple particles spread even further apart, making the solution less purple - this demonstrates dilution.

The ammonia and hydrogen chloride experiment creates a white ring when these gases meet in a glass tube. The ring forms closer to the hydrogen chloride end because ammonia particles are lighter and diffuse faster, travelling further in the same time.

Key Insight: The position of the white ring proves that particle mass affects diffusion speed - lighter particles always win the race!

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More Diffusion Demonstrations

The ammonia and hydrogen chloride experiment shows exactly how particle mass affects diffusion rates. Ammonia gas (NH₃) and hydrogen chloride gas (HCl) diffuse from opposite ends of a glass tube and react to form a white ring of ammonium chloride when they meet.

The ring doesn't form in the middle - it appears closer to the hydrochloric acid end because ammonia particles are smaller and lighter, so they diffuse through air more quickly and travel further.

The bromine gas and air experiment uses brown bromine gas to demonstrate diffusion visually. When you remove the glass plate separating bromine gas from air, the brown gas slowly spreads through the air due to random particle motion.

Visual Learning: These experiments are perfect for understanding diffusion because you can actually see the particles spreading out in real time!

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Solubility: How Much Will Dissolve?

Solubility measures how much of a substance (solute) will dissolve in 100g of water (solvent) - it's expressed as grams per 100g of solvent. Think of it as the maximum amount of sugar you can dissolve in your tea before it stops dissolving.

Solubility curves are graphs that show how solubility changes with temperature. For most solids, solubility increases when temperature increases - hot water dissolves more sugar than cold water. However, for gases, solubility increases when pressure increases.

Understanding these curves helps you predict what happens in solutions. Any mass below the line means the solution is unsaturated (you could dissolve more). Any mass above the line means the solution is supersaturated and unstable - excess solute will start to crystallise out.

Practical Application: This explains why hot tea can hold more sugar than cold tea, and why fizzy drinks go flat when opened (gas escapes due to pressure drop)!

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