TheIBAxolotl
Pure substances and mixtures
Chemists sort matter by what it is made of:
- Element: one type of atom (Fe, O\(_2\), S\(_8\)). It cannot be split into anything simpler by chemical means.
- Compound: two or more elements chemically bonded in a fixed ratio (H\(_2\)O, NaCl). Its properties are usually nothing like those of its elements: sodium is a reactive metal, chlorine is a toxic gas, but sodium chloride is table salt.
- Mixture: two or more substances physically combined in any ratio. Each part keeps its own properties.
- Homogeneous mixtures have a uniform composition throughout (salt water, air, brass).
- Heterogeneous mixtures have visibly different regions (sand in water, oil on water).
The key test: a compound needs a chemical reaction to separate it; a mixture can be separated by physical methods.
Separation techniques
Each method exploits one difference in physical property:
| Method | Property used | Typical use |
|---|---|---|
| Filtration | particle size / solubility | insoluble solid from liquid |
| Evaporation / crystallisation | volatility of solvent | dissolved solid from solution |
| Distillation | boiling point | solvent from solution |
| Fractional distillation | small differences in boiling point | miscible liquids (ethanol + water) |
| Solvent extraction / separating funnel | solubility in two immiscible solvents | organic product from water |
| Chromatography | relative attraction to stationary and mobile phases | dyes, amino acids |
| Magnetic separation | magnetism | iron filings from sulfur |
For a sand + salt + water mixture: filter out the sand, then evaporate the filtrate to recover the salt.
Kinetic molecular theory
The model treats every substance as tiny particles in constant random motion.
- Solid: particles closely packed in fixed positions, vibrating only. Fixed shape and volume.
- Liquid: particles close together but able to slide past each other. Fixed volume, shape of container.
- Gas: particles far apart, moving fast in straight lines between collisions. Fills any container; easily compressed.
Temperature and kinetic energy
Temperature is a measure of the average kinetic energy of the particles. At a given temperature, particles have a spread of speeds, but the average kinetic energy is directly proportional to the absolute temperature in kelvin. Absolute zero (0 K) is where particle motion would be at its minimum.
\[T/\text{K} = \theta/^\circ\text{C} + 273.15\]
(Using 273 is accepted in most exam answers.)
Changes of state
Melting, vaporisation (boiling or evaporation) and sublimation are endothermic: energy is absorbed to overcome attractions between particles. Freezing, condensation and deposition are exothermic.
On a heating curve, the temperature stays constant during a change of state even though heat is being supplied. The energy goes into weakening the forces between particles (increasing potential energy), not into making them move faster, so average kinetic energy does not change.
Evaporation happens at the surface at any temperature; boiling happens throughout the liquid at one temperature (when vapour pressure equals external pressure).
Worked example
A gas is warmed from 27 °C to 327 °C. By what factor does the average kinetic energy of its particles change?
- \(T_1 = 27 + 273 = 300\ \text{K}\), \(T_2 = 327 + 273 = 600\ \text{K}\)
- Ratio \(= 600/300 = 2.0\)
The average kinetic energy doubles. Using Celsius would wrongly give a factor of about 12.
Common mistakes
- Calling air or brass a compound: both are mixtures (variable composition).
- Saying temperature rises during melting: it stays constant for a pure substance.
- Saying particles in a solid do not move: they vibrate.
- Using °C in any ratio or proportionality calculation.
- Saying that heat "breaks bonds" when water boils: it overcomes intermolecular forces; covalent bonds stay intact.
Exam tips
- When asked to explain a flat section of a heating curve, mention both "energy used to overcome intermolecular forces" and "average kinetic energy constant".
- For separation questions, name the method and the property it relies on.
- A pure substance melts at a sharp, fixed temperature; an impure one melts lower and over a range.
- Convert to kelvin at the start of any gas or temperature calculation.