(a) States of matter

Learning objectives

1.1States of matterUnderstand the three states of matter in terms of the arrangement, movement and energy of the particles 1.2State changesUnderstand the interconversions between the three states of matter in terms of:• the names of the interconversions• how they are achieved• the changes in arrangement, movement and energy of the particles.1.3Diffusion and dilutionUnderstand how the results of experiments involving the dilution of coloured solutions and diffusion of gases can be explained 1.4Solutions terminologyKnow what is meant by the terms:• solvent• solute• solution• saturated solution.15C SolubilityKnow what is meant by the term solubility in the units g per 100 g of solvent 16C Solubility curvesUnderstand how to plot and interpret solubility curves 17C Solubility practicalPractical: investigate the solubility of a solid in water at a specific temperature

The particle model explains a state of matter by three linked features: how the particles are arranged, how they move, and their relative energy. The particles themselves remain particles; it is their spacing, freedom of movement and energy that differ.

State	Arrangement	Movement	Relative particle energy
Solid	Very close together in a regular, fixed arrangement	Vibrate about fixed positions	Lowest
Liquid	Close together but irregularly arranged	Move randomly and slide past one another	Higher than in the solid
Gas	Far apart and irregularly arranged	Move rapidly and randomly in all directions	Highest

A solid keeps its shape because its particles cannot move from place to place. A liquid flows because close particles can move past one another. A gas fills its container because widely separated particles move freely in all directions.

Solid particles are not motionless: they vibrate. Do not describe liquid particles as widely separated, or gas particles as larger than particles of the same substance in another state. A particle diagram's empty space represents separation, not air between the particles.

A change of state is a physical change. Heating transfers energy to particles, so they move more and can overcome attractions. Cooling removes energy, so movement decreases and attractions hold particles closer or in fixed positions.

Interconversion	Name	How achieved	Particle change
Solid → liquid	Melting	Heating	Energy rises; vibrations increase until particles can move past one another and the regular arrangement breaks down.
Liquid → solid	Freezing	Cooling	Energy falls; movement slows and particles become fixed in a regular arrangement.
Liquid → gas	Boiling or evaporation	Heating	Energy rises; particles overcome attractions and become far apart, moving rapidly and randomly.
Gas → liquid	Condensation	Cooling	Energy falls; particles slow and come close enough for attractions to keep them together.
Solid ↔ gas directly	Sublimation	Heating for solid → gas; cooling for the reverse	Particles change directly between fixed, close positions and widely separated random motion.

State symbols record the before-and-after state: evaporation is (l) → (g), melting is (s) → (l), condensation is (g) → (l), and sublimation of iodine is (s) → (g).

The substance does not become a different chemical during a state change. Do not call dissolving a change of state, and do not explain faster evaporation only by saying 'it is hotter': link higher particle energy to particles escaping the liquid more readily.

Diffusion is the net spreading of particles from a region of higher concentration to a region of lower concentration, caused by their continuous random motion, until they are more evenly distributed.

Observation	Particle explanation
A coloured crystal forms a coloured solution	The solid first dissolves; its particles then diffuse through the water.
Adding water changes dark purple solution to pale purple	The same coloured particles diffuse through a larger volume, so there are fewer coloured particles per unit volume.
Ammonia and hydrogen chloride form a white ring	Both gases diffuse from opposite ends; ammonia travels farther in the same time, so the ring forms nearer the hydrogen chloride end.
A diffusion experiment is warmer	Particles have more kinetic energy and move faster, so diffusion and meeting occur sooner.

Gas particles move quickly but not straight from one end of a tube to the other. Their random directions and collisions with air particles and the tube walls make the visible result take time.

Dissolving releases solute particles into a solvent; diffusion spreads them. Dilution is adding solvent to reduce concentration. These ideas can occur in the same experiment, but they are not interchangeable names.

A solution forms when a solute dissolves in a solvent. The vocabulary identifies each part of that process and whether more solute can dissolve under the stated conditions.

Term	Precise meaning
Solute	The substance that dissolves.
Solvent	The liquid that dissolves the solute.
Solution	The homogeneous mixture formed when the solute dissolves in the solvent.
Saturated solution	A solution containing the maximum amount of dissolved solute at a particular temperature; no more solute will dissolve at that temperature.

In salt water, salt is the solute, water is the solvent and salt water is the solution. If added salt remains undissolved after thorough stirring at a fixed temperature, the solution above it is saturated.

A concentrated solution is not necessarily saturated. 'Concentrated' means much solute per volume; 'saturated' means the maximum has dissolved at that temperature. The liquid is the solvent, not the solution as a whole.

Solubility is the maximum mass of solute that dissolves in 100 g of solvent at a stated temperature. Its unit is g per 100 g of solvent because solubility usually changes with temperature.

$\text{solubility}=\frac{\text{mass of solute dissolved}}{\text{mass of solvent}}\times100\quad\text{g per 100 g solvent}$

Worked example: 17.6 g of dry salt was dissolved using 50.0 g of water. Solubility = (17.6 ÷ 50.0) × 100 = 35.2 g per 100 g of water. The numerical scaling factor is 2 because 100 g is twice 50.0 g.

Use the mass of solvent, not the mass of solution. State the temperature and unit with the result. Solubility describes the saturated maximum; a smaller mass that happens to dissolve does not by itself give the solubility.

A solubility curve shows how the maximum mass of solute that dissolves in 100 g of solvent changes with temperature. Temperature is the independent variable, so it goes on the horizontal axis.

Job	Method
Plot	Put temperature / °C on the x-axis and solubility / g per 100 g solvent on the y-axis; choose even scales, plot points accurately and draw a smooth best-fit curve.
Read	Move vertically from a temperature to the curve, then horizontally to the solubility axis; interpolate only within the measured range.
Compare	The higher curve at the same temperature represents greater solubility; an intersection means equal solubility at that temperature.
Cool	For the same mass of solvent, mass crystallised = solubility at the higher temperature − solubility at the lower temperature.

$\text{mass crystallised}=\frac{\Delta\text{solubility}}{100}\times\text{mass of solvent}$

If solubility falls from 90 to 30 g per 100 g water on cooling, 60 g crystallises from 100 g water. From 50 g water, the mass is (60 ÷ 100) × 50 = 30 g.

Do not join every point with straight zigzags when a smooth trend is expected; identify and do not force the curve through an anomalous point. Do not read beyond the plotted data as though an extrapolated value were measured.

To measure solubility, make a saturated solution at one specified temperature, find the mass of dissolved solid associated with a known mass of water, then scale the result to 100 g of water.

Order	Assessed method	Why it matters
1	Measure 50.0 cm³ of water into a beaker and keep it at the specified temperature.	Fixes the solvent amount and temperature. For water, 1.00 cm³ has a mass of 1.00 g.
2	Add the solid a little at a time, stirring after each addition until no more dissolves.	Reaches saturation without mistaking slow dissolving for the endpoint.
3	Filter to obtain the required volume of saturated solution.	Removes undissolved excess solid.
4	Weigh an empty, dry evaporating basin; add the saturated solution and weigh again.	Establishes the container and solution masses.
5	Heat gently to remove the water, cool the basin, then weigh the basin and dry salt.	The mass increase above the empty basin is the dissolved salt recovered.
6	Calculate the result in g per 100 g water.	Converts the measured ratio to the required solubility unit.

Control the temperature throughout saturation, measure the correct water volume, add enough solid, stir thoroughly, filter before sampling and evaporate gently. Reheat, cool and reweigh until the mass is constant if checking that all water has been removed.

Stirring changes how quickly the endpoint is reached, not the equilibrium solubility. A wrong water volume, wrong temperature, too little solid or inadequate stirring can produce an anomalous result. Do not include undissolved excess solid in the mass reported as dissolved solute.

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—(b) Elements, compounds and mixturesRead

(a) States of matter

Learning objectives

Compare particles in solids, liquids and gases

Explain every change of state with particles

Explain diffusion and dilution from particle motion

Use solution terms precisely

Calculate solubility in g per 100 g of solvent

Plot and interpret solubility curves

Measure solubility at a fixed temperature

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