Kinetic particle theory
The plain idea first: all matter is built from incredibly small particles that are always moving. The technical name is the kinetic particle theory (kinetic just means "to do with movement"). The theory rests on three claims:
- All matter is made of tiny particles.
- The particles are constantly moving, so they have energy.
- The hotter the substance, the more energy the particles have and the faster they move.
The amount of energy the particles carry, and how strongly they pull on each other, is what sets the state a substance is in.
- Kinetic particle theory
- The model that all matter is made of small particles in constant motion, with more heat giving the particles more energy.
Diffusion is the giveaway
When you smell perfume from across a room, that is particles moving and spreading out on their own. This spreading is called diffusion, and it is direct evidence that particles are moving.
The three states of matter
Solids, liquids and gases differ in how their particles are arranged, how close they sit, and how freely they move. This table is worth learning cold:
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement | Regular, packed tightly | Close but random | Far apart, random |
| Movement | Vibrate in fixed places | Slide past each other | Move quickly in all directions |
| Shape | Fixed | Takes the container's shape | Fills the container |
| Can it be compressed? | No | Almost none | Yes, easily |
The changes of state have names you should know: melting (solid to liquid), freezing (liquid to solid), evaporating or boiling (liquid to gas), condensing (gas to liquid), and subliming (solid straight to gas). Heating adds energy and pushes particles apart; cooling removes it and lets them settle closer.
Physical vs chemical change
A physical change alters the appearance or state of a substance but makes no new substance, and it can usually be reversed. Melting ice, dissolving sugar, and boiling water are all physical: the water molecules are still water molecules throughout.
A chemical change makes one or more new substances, and it is usually hard to reverse. Burning, rusting and cooking an egg are chemical. Clues that a chemical change has happened include a colour change, a gas being given off, a temperature change, or a precipitate forming.
- Chemical change
- A change in which new substances are made, normally difficult to reverse, often shown by a colour change, a gas, or a temperature change.
Common slip
Dissolving salt in water is a physical change, not a chemical one. No new substance forms, the salt is still there, and you can get it back by evaporating the water.
Separation techniques
A mixture contains substances that are not chemically joined, so we can separate them using differences in their physical properties. Pick the method that matches the difference:
- Filtration separates an insoluble solid from a liquid. The mixture passes through filter paper: the liquid (the filtrate) goes through and the solid (the residue) is trapped. Use it to get sand out of water.
- Evaporation gets a soluble solid back from its solution. The solution is heated so the water turns to vapour and leaves the solid crystals behind. Use it to get salt from salt water.
- Simple distillation collects the solvent itself from a solution. The solution is boiled, the vapour rises and passes through a cooled condenser where it turns back to liquid (the distillate). Use it to get pure water from salt water.
The trick with distillation is the condenser: cold water flows around the outside so the hot vapour loses energy, condenses, and drips out as a pure liquid.
Paper chromatography and Rf
Paper chromatography separates a mixture of soluble, coloured substances, such as the dyes in an ink. A spot of the mixture is placed near the bottom of the paper, which stands in a shallow layer of solvent. As the solvent soaks up the paper, each substance is carried a different distance depending on how soluble it is and how strongly it sticks to the paper.
To compare results fairly, we measure the Rf value (retardation factor). It has no units because it is a ratio of two distances:
On a chromatogram, the solvent front travels 8.0 cm from the start line. A blue dye spot moves 6.0 cm in the same time. Find its Rf value.
Where this is assessed
Choosing the right separation method and explaining why sits in Criterion A. Reading a chromatogram, calculating an Rf, and drawing a conclusion from it is classic Criterion C (processing and evaluating data). Always keep Rf between 0 and 1, a value above 1 means a measuring mistake.
Check yourself
1. Explain, using particles, why a gas can be compressed but a solid cannot. +
In a gas the particles are spread far apart with large gaps between them, so squeezing pushes them closer into those gaps. In a solid the particles are already packed tightly together with no room to move closer, so it cannot be compressed.
2. Which technique would you use to obtain pure water from sea water, and why? +
Use simple distillation. The salt water is boiled, the water evaporates and passes through a condenser where it cools back to liquid, leaving the dissolved salt behind in the flask. Evaporation alone would lose the water as vapour, so distillation is needed to collect it.
3. A red dye moves 4.5 cm while the solvent front moves 9.0 cm. Calculate its Rf value. +
\(R_f = \dfrac{\text{distance moved by substance}}{\text{distance moved by solvent}} = \dfrac{4.5}{9.0} = \mathbf{0.50}\).
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