SC1 · States of matter · Topic 2 — States of matter and mixtures | GCSE Edexcel Separate Sciences

Revise the key ideas

Matter can be modelled as particles: atoms, molecules or ions depending on the substance. Drawn circles are a model, not a literal picture of every particle.
In a solid, particles are close together and vibrate around fixed positions. In a crystalline solid they form a regular arrangement. A solid has a fixed shape and volume.
In a liquid, particles are close together but can move past one another. A liquid flows and takes the shape of its container, while keeping approximately the same volume.
In a gas, particles are much farther apart and move rapidly in random directions. A gas expands to fill its container and has no fixed shape or volume.
Same particles; different arrangement and spacing. A simplified model, not to scale.
Solids and liquids are difficult to compress because particles are already close together. Gases are readily compressed because there is much empty space between particles.
The particles themselves do not expand, melt or change identity during a physical change of state. Their arrangement, separation, movement and energy change.

Particles attract one another. The type and strength of attraction depend on the substance. For example, an ionic solid has strong attractions between ions, while a molecular solid has attractions between molecules (intermolecular forces).
Heating a substance within one state generally increases particle kinetic energy: particles move faster, or vibrate more strongly in a solid.
During melting or boiling, transferred energy helps overcome attractions and rearrange or separate particles. For a pure substance at constant pressure, temperature remains steady during the change.
Melting and boiling take in energy; freezing and condensation transfer energy to the surroundings. Cooling within one state reduces average kinetic energy.
Comparing states for one substance, gas particles usually have higher energy than liquid particles, which usually have higher energy than solid particles. Do not claim this comparison applies regardless of temperature or substance.
Gas pressure results from particles colliding with container walls. A hotter sealed gas can exert greater pressure when volume and amount of gas are unchanged.

Melting changes solid to liquid; freezing changes liquid to solid. These are reverse physical processes.
Boiling or evaporation changes liquid to gas; condensation changes gas to liquid.
Sublimation changes a solid directly into a gas, without becoming liquid first. The reverse change, from gas directly to solid, is deposition.
Each pair has a forward and reverse physical change.
Melting gives particles greater freedom to move while they remain close. Boiling separates particles much more widely.
Freezing restricts particles to fixed positions, while condensation brings gas particles close enough to form a liquid.
A physical change does not make a new substance. Water vapour, liquid water and ice all contain H₂O molecules; suitable cooling or heating can reverse the change.

Evaporation occurs at the surface of a liquid and can occur below its boiling point. Particles with sufficient energy escape into the gas phase.
Boiling happens throughout a liquid at its boiling point and forms bubbles of vapour. The boiling point depends on the pressure.
Both are physical changes from liquid to gas.
A puddle drying on a cool day illustrates evaporation, not necessarily boiling. Water on the outside of a cold glass usually comes from condensation of water vapour in surrounding air.
Dry ice is solid carbon dioxide and sublimes at normal atmospheric pressure. Frost can form by deposition of water vapour on a sufficiently cold surface.
The visible white cloud above hot water consists mainly of tiny liquid droplets formed by condensation. Water vapour itself is invisible.
Different substances have different melting and boiling points. Pressure also affects boiling point; always use the conditions supplied by the question.

At a temperature below its melting point, a substance is solid; between its melting and boiling points it is liquid; above its boiling point it is gas, assuming the stated pressure and no chemical decomposition.
For a substance melting at −10 °C and boiling at 80 °C, it is solid at −20 °C, liquid at 25 °C and gas at 100 °C.
At a transition temperature, two states can coexist during the change.
At exactly a melting or boiling point, two states can coexist while the change is taking place. The temperature alone does not tell you that the whole sample has completed the change.
Water at ordinary atmospheric pressure melts at about 0 °C and boils at about 100 °C. These familiar values are not the values for every substance.
On a heating curve, sloping sections show temperature rising within a state; flat sections show melting and boiling of a pure substance under suitable constant conditions.
Flat sections correspond to changes of state, rather than no energy input.
Take care with negative temperatures: −20 °C is colder than −10 °C because it is further below zero.

A useful state diagram should show spacing and arrangement as well as motion. Keep the same number and identity of particles when comparing one closed sample before and after a physical change.
A simple sphere model cannot show all internal structure, real particle sizes, the full range of forces or three-dimensional motion. Arrows and labels communicate information the circles alone cannot.
Gas particles move randomly between collisions; they do not all drift permanently upwards. Diffusion produces net movement from high to low concentration even though individual movements remain random.
When gases mix in a closed container, they spread through available space. Mixing does not by itself mean a chemical reaction has formed a new substance.
Use a substance’s properties to identify its state. A fixed shape suggests a solid; flowing while keeping a fixed volume suggests a liquid; easily compressing and expanding to fill a container suggests a gas.
Practical heating and cooling observations need suitable equipment, eye protection and care with hot surfaces; temperature measurements and controlled pressure support interpretation.