Unit 2: Properties of materials

GRADE 8 – Unit 2: Properties of materials — HIA International - Grade 8 Revision Notes & Topical Questions

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Lesson summary

GRADE 8 - Unit 2: Properties of materials — HIA INTERNATIONAL notes: Use particle theory to explain states of matter and changes of state, describe atoms and the Periodic Table, and explain bonding and how structure affects properties.PropertySolidLiquidGasShapeFixedTakes the sha…

Solid particles
Gas compression
Gas pressure
Evaporating or boiling
Condensing
Sublimation

Part 1 of 7States of matter

Property	Solid	Liquid	Gas
Shape	Fixed	Takes the shape of the container	Takes the shape of the container
Volume	Fixed	Fixed	Not fixed; fills the container
Arrangement	Regular pattern, touching	Close together, irregular	Far apart, random
Movement	Vibrate about fixed positions	Move past each other	Move quickly in all directions
Attractive forces	Strongest	Weaker	Weakest
Compressible	No	Almost none	Yes, easily
Flows	No	Yes	Yes

Part 2 of 7Particle ideas

Particle ideas

Solid particles

Solid particles they vibrate about fixed positions and do not stand still.

Gas compression

Gas compression a gas can be compressed because there are big gaps between its particles.

Gas pressure

Gas pressure it is caused by particles colliding with the walls of the container.

Heating

Heating heated particles move faster and further apart, so the substance expands.

Diffusion

Diffusion the net movement of particles from a higher to a lower concentration.

Part 3 of 7Changes of state

1Melting

Melting solid to liquid as the particles gain energy.

2Freezing

Freezing liquid to solid.

3Evaporating or boiling

Evaporating or boiling liquid to gas.

4Condensing

Condensing gas to liquid.

5Sublimation

Sublimation solid to gas directly, e.g. dry ice and iodine.

6Evaporation compared with boiling

Evaporation compared with boiling evaporation happens at the surface at any temperature,
- whereas boiling happens throughout the liquid at the boiling point.
Evaporation is faster with higher temperature, larger surface area, air movement and lower humidity.

7Clouds

Clouds warm moist air rises and cools, and water vapour changes back into tiny liquid droplets by condensation.

8Filtration

Filtration separates an insoluble solid from a liquid using filter paper.

Part 4 of 7Atomic structure

Atomic structure

Atomic number

Atomic number the number of protons, which identifies the element.

Mass number

Mass number protons plus neutrons, so neutrons = mass number − atomic number.

Charge

Charge protons are positive and electrons negative; an atom has equal numbers of each, so no overall charge.

Electron shells

Electron shells the first holds up to 2 electrons and the second and third up to 8 each, e.g. sodium is 2,8,1.

Density

Density the mass of a fixed volume of a substance, in g/cm³.

Part 5 of 7The Periodic Table and groups

The Periodic Table and groups

Position

Position metals are on the left and non-metals on the right of the table.

Group

Group elements in the same group have the same number of outer electrons.

Group 1 (alkali metals)

Group 1 (alkali metals) reactivity increases and melting point decreases going down the group.

Group 7 (halogens, Group 17)

Group 7 (halogens, Group 17) fluorine is the most reactive, and reactivity decreases down the group.

Group 8 (noble gases, Group 18)

Group 8 (noble gases, Group 18) the outer shell is full, so they are inert and hardly react.

Predictions

Predictions trends in a group let you predict properties of an element you have not met.

Naming compounds

Naming compounds names ending in -ide contain two elements, e.g. copper oxide;
- -ate also contains oxygen, e.g. copper sulfate has Cu, S and O.

Part 6 of 7Ions and covalent bonds

Stable atoms

Stable atoms atoms bond to obtain a full outer shell, which makes them more stable.

Ion

Ion a charged particle made when an atom gains or loses electrons.

Ionic bond

Ionic bond metals lose electrons to form positive ions and non-metals gain electrons to form negative ions,
- which attract each other, e.g. Na⁺ and Cl⁻ form NaCl.

Covalent bond

Covalent bond two non-metal atoms share a pair of electrons, as in H₂, Cl₂, H₂O and CH₄.

Double bond

Double bond oxygen, O₂, shares two pairs of electrons.

Molecule

Molecule a group of atoms joined by covalent bonds.

Part 7 of 7Structure and properties

Structure	Example	Properties
Simple molecular	CO₂	Weak forces between molecules, so low melting points; gases or low-melting solids; do not conduct
Giant ionic lattice	Sodium chloride	Strong forces between ions, so very high melting points; conducts when molten or dissolved
Giant covalent	SiO₂, diamond	Many strong covalent bonds, so very high melting points and hard
Graphite	Carbon	Layers slide over each other because bonds between layers are weak, so it is soft
Metal	Copper	Layers of atoms held by a sea of free electrons, so it conducts electricity and heat

Key terms

Term	Meaning
Sublimation	A change of state from solid directly to gas.
Diffusion	The net movement of particles from higher to lower concentration.
Atomic number	The number of protons in an atom.
Mass number	The number of protons plus neutrons.
Ion	A charged particle made when an atom gains or loses electrons.
Covalent bond	A bond formed when two non-metal atoms share a pair of electrons.
Filtration	Separating an insoluble solid from a liquid using filter paper.
Noble gas	A Group 8 element with a full outer shell that hardly reacts.

Avoid these mistakes

Particles in a solid do not move — they vibrate about fixed positions.

Particles expand when heated — they move faster and further apart, so the substance expands.

Evaporation and boiling are the same — evaporation is at the surface at any temperature; boiling is throughout the liquid at the boiling point.

Gas pressure comes from particles hitting each other — it comes from collisions with the container walls.

Compounds ending -ide and -ate are the same — -ate also contains oxygen.

Last updated: 12 October 2026