S1.1 Introduction to the particulate nature of matterIB Chemistry SL: Revision notes
Section 1
Elements, compounds and mixtures
An element is a substance that cannot be chemically broken down into simpler substances; elements are the primary constituents of matter. A compound contains atoms of different elements chemically bonded together in a fixed ratio, so water is always H₂O and always 11.2 % hydrogen by mass. A mixture contains two or more elements or compounds in no fixed ratio that are not chemically bonded, so it can be separated by physical methods such as filtration, evaporation, distillation, crystallisation or chromatography.
Section 2
Distinguishing their properties
- A compound usually has properties very different from its elements (sodium is a reactive metal, chlorine a toxic gas, sodium chloride a stable white solid).
- A mixture keeps the properties of its components, which is why they can be separated physically.
- Pure elements and compounds have sharp melting and boiling points; mixtures usually melt or boil over a range.
- Mixtures can be homogeneous (uniform composition throughout, e.g. air, salt solution, alloys) or heterogeneous (non-uniform, e.g. sand in water).
Containing more than one element does not make something a mixture - compounds do too. The test is whether the ratio is fixed and whether the components are chemically bonded.
Section 3
The kinetic molecular theory and states of matter
The kinetic molecular theory models matter as particles in constant motion, with attractive forces between them.
| State | Arrangement | Movement | Attractions |
|---|---|---|---|
| Solid | close, regular, fixed positions | vibrate about fixed points | strong |
| Liquid | close, irregular | slide past one another | weaker, constantly broken and reformed |
| Gas | far apart, random | fast, random, straight lines between collisions | negligible |
The model explains why solids have a fixed shape, liquids take the shape of their container but have a fixed volume, and gases are compressible and fill their container.
Section 4
Temperature and the Kelvin scale
Temperature in kelvin (K) is a measure of the average kinetic energy of the particles: average Ek is directly proportional to T in K. At absolute zero (0 K) particles have minimum kinetic energy.
T(K) = T(°C) + 273 (273.15 precisely). Kelvin values are never negative and are written without a degree sign.
Because Ek ∝ T(K), doubling the kelvin temperature doubles the average kinetic energy, but doubling the Celsius value does not: 20 °C → 40 °C is 293 K → 313 K, only a 7 % increase.
At the same temperature, all gases have the same average kinetic energy - heavier particles simply move more slowly.
Section 5
Changes of state and heating curves
Changes of state: melting (s → l), freezing (l → s), vaporisation/boiling (l → g), condensation (g → l), sublimation (s → g) and deposition (g → s). Melting, vaporisation and sublimation absorb energy; the reverse processes release it.
On a heating curve for a pure substance the temperature stays constant during a change of state: the energy supplied overcomes attractions between particles (increasing potential energy) instead of raising average kinetic energy. The boiling plateau is longer than the melting plateau because separating particles completely needs much more energy than loosening them. Evaporation, unlike boiling, happens only at the surface and at any temperature.
Section 6
State symbols
Equations should show the state of each species: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). For example: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g), and for melting ice: H₂O(s) → H₂O(l).
Water produced in a reaction is (l), not (aq) - it is the solvent itself, not something dissolved in water.
Must know
- Element: cannot be chemically broken down. Compound: fixed ratio, chemically bonded. Mixture: no fixed ratio, not bonded, separable physically.
- Pure substances have sharp melting/boiling points; mixtures usually melt over a range.
- Temperature (K) ∝ average kinetic energy; T(K) = T(°C) + 273.
- Temperature is constant during a change of state because energy goes into overcoming attractions.
- Use (s), (l), (g) and (aq) in every equation.
That's the notes covered.
Carry on to the next subtopic.