States of Matter & Solutions — Quick Notes

The Three States of Matter

Solid, liquid, gas: the three states of matter.
Particle arrangement: solid = regular, liquid = random, gas = random; movement: solid = vibrate about fixed position, liquid = move around each other, gas = move quickly in all directions.
Closeness of particles: solid very close, liquid close, gas far apart.
Substances can usually exist in all three states, depending on temperature (and pressure).
State changes occur at melting point (solid → liquid, and liquid → solid) and at boiling point (liquid → gas and gas → liquid).
Melting/freezing occur at the melting point; boiling/condensing at the boiling point.
Pure substances have the same melting and freezing point; and the same boiling and condensation point.
Changes of state are physical changes; the particles themselves do not change identity, only the forces between them.
A simple model represents particles as small spheres.
Energy to change state depends on interparticle forces: stronger forces → higher melting and boiling points.
Reversible arrows (⇌) in state-change diagrams indicate reversibility.

Melting: solid → liquid; energy absorbed as heat; particles gain kinetic energy; occurs at a specific temperature: the melting point (mp).
Boiling vs Evaporation:
- Boiling: liquid → gas; bubbles form inside the liquid; occurs at a specific temperature: the boiling point (bp).
- Evaporation: liquid → gas at the surface; can happen below the bp; occurs over a range of temperatures; surface area and temperature affect rate.
Freezing: liquid → solid; occurs at the mp (same as melting point for pure substances).
Condensation: gas → liquid; occurs on cooling over a range of temperatures.
Sublimation: solid → gas (and desublimation/deposition for the reverse); occurs for some solids (e.g., iodine, dry ice).

Diffusion: movement of particles from high to low concentration due to random motion; happens in gases and liquids.
Rate depends on temperature: higher temperature → faster diffusion.
Diffusion in gases is faster than in liquids because particles are more spread out and move freely.
Dilution: adding more solvent decreases concentration of solute; does not remove particles, just spreads them out.
Observations in liquids: diffusion of solutes (e.g., potassium manganate(VII)) is slower than in gases due to closer packing.

In water, purple potassium manganate(VII) diffuses, forming a gradient until equilibrium.
Explanation: particles move randomly and mix due to molecular motion; liquids allow sliding over each other, enabling mixing.

Key terms:
- Solvent: the liquid in which a solute dissolves (e.g., water in seawater).
- Solute: the substance dissolved (e.g., salt in seawater).
- Solution: the mixture formed when a solute is dissolved in a solvent.
- Saturated solution: maximum concentration of solute dissolved in solvent.
- Soluble: substance will dissolve in a solvent.
- Insoluble: substance will not dissolve.
Solubility: amount of solute that will dissolve in a given volume of solvent.
Solubility of solids generally increases with temperature; solubility of gases generally decreases with temperature and increases with pressure.
Solubility curves plot solubility (g per 100 g of water) against temperature.
Sodium chloride (table salt) shows little change in solubility with temperature compared to other solids.

KNO₃ solubility at 50 °C: 68\,\text{g per }100\,\text{g of water}
- Dissolve in 20 g water: 68 \times (20/100) = 13.6\,\text{g} dissolves.
Pb(NO₃)₂ solubility:
- At 90 °C: 118\,\text{g per }100\,\text{g water}
- At 40 °C: 64\,\text{g per }100\,\text{g water}
- Crystals formed per 100 cm³ of solution: 118 - 64 = 54\,\text{g}
- For 200 cm³ of solution: 2 \times 54 = 108\,\text{g}
Examiner tip: as temperature increases, solids become more soluble; gases become less soluble.

Aim: measure solubility of a salt at different temperatures.
Method (summary):
- Prepare hot and ice baths.
- Measure 4 cm³ distilled water in a boiling tube.
- Weigh 2.6 g ammonium chloride and add to water.
- Heat to dissolve, then cool in ice bath; note temperature when crystals first appear; record.
- Add 1 cm³ water, warm again to redissolve; repeat cooling and observation until a total of 10 cm³ water has been added.
Results are typically tabulated: Volume of water (cm³) vs Solubility (g per 100 g) vs Temperature at which crystals appear (°C).
Graph: plot solubility (g/100 g water) vs temperature to obtain a solubility curve.
Conclusion: shape of curve shows how solubility varies with temperature for the salt studied.

Solubility is often reported as grams of solute per 100 g of solvent.
For a solute S in solvent W:
- Solubility = \frac{m_{\text{solute}}}{100\,\text{g solvent}}
Solubility curves help predict how solubility changes with temperature.
Gas solubility: higher pressure increases solubility; higher temperature decreases solubility.

Solvent: liquid in which a solute dissolves.
Solute: substance dissolved.
Solution: homogeneous mixture of solvent and solute.
Saturated: no more solute can dissolve at given conditions.
Soluble: capable of dissolving.
Insoluble: not capable of dissolving.
Melting point: temperature at which a solid becomes a liquid.
Boiling point: temperature at which a liquid becomes a gas.
Evaporation: surface phenomenon where a liquid slowly loses particles to the gas phase.
Condensation: gas to liquid.
Diffusion: spreading of particles from high to low concentration due to random motion.
Dilution: added solvent reduces concentration of solute in solution.