Matter and Materials – Grade 8 Summary

A comprehensive set of question-and-answer flashcards covering Grade 8 Matter and Materials topics: particle model, phase changes, density, pure vs impure substances, separation methods, atomic structure, periodic table, molecules, and physical vs chemical changes.

What are the three common phases of matter studied in Grade 8?

Solids, liquids and gases.

State the five main points of the Kinetic Theory of Matter.

1) All matter is made of tiny particles. 2) Particles are too small to see. 3) Empty spaces exist between particles. 4) Particles are in constant motion. 5) Forces of attraction/repulsion act between particles.

How are particles arranged in a solid?

In a regular, closely-packed pattern where particles only vibrate in fixed positions.

Why can’t solids be compressed easily?

Because their particles are densely packed with virtually no spaces between them.

Describe particle motion in liquids.

Particles slide past one another, move randomly and can change places, allowing liquids to flow and take the container’s shape.

Why are liquids almost incompressible?

Their particles are still fairly close together, leaving only small spaces between them.

Explain why gases fill any container completely.

Gas particles move very fast in all directions with large spaces between them and negligible attractive forces, so they spread out to fill the container.

Define melting point.

The temperature at which a substance changes from solid to liquid.

What is the reverse process of melting?

Freezing – change from liquid to solid.

Give the definition of boiling point.

The temperature at which a liquid changes into a gas.

What term describes gas changing to liquid, and at what point does it occur?

Condensation; it occurs at the condensation point (same temperature as boiling point for that substance).

What is sublimation?

Direct change from solid to gas without passing through the liquid phase.

Name the reverse process of sublimation.

Deposition (or desublimation) – gas to solid.

Which element has the lowest known melting point?

Helium.

Which element has the highest known melting point?

Carbon.

How does temperature affect particle energy according to the Kinetic Theory?

Increasing temperature increases particle energy and motion; decreasing temperature lowers energy and slows particles.

State the definition of diffusion.

The spontaneous movement of particles from an area of high concentration to an area of low concentration.

Why does diffusion occur faster in gases than in liquids?

Gas particles move faster and have larger spaces between them, reducing collisions and resistance.

Write the density formula.

Density (D) = mass (m) ÷ volume (V).

Give the SI unit commonly used for density in school science.

g/cm³ (or kg/m³ in SI).

How does temperature generally affect the density of a substance?

Rising temperature decreases density because volume expands while mass stays constant.

Define a pure substance.

Matter that contains only one type of particle (element or compound).

What is an element?

The simplest pure substance that cannot be chemically broken down into simpler substances; made of identical atoms.

Define a compound.

A pure substance formed when atoms of different elements chemically bond in a fixed ratio, with properties different from its components.

What is an impure substance?

Matter containing more than one kind of particle; a mixture.

State two key differences between mixtures and compounds.

Mixtures have no fixed ratio and can be separated physically; compounds have fixed ratios and can be separated only chemically.

Give three types of mixtures with examples.

Solution (salt water), suspension (sand in water), emulsion (oil in water).

Differentiate between homogeneous and heterogeneous mixtures.

Homogeneous mixtures have one visible phase; heterogeneous mixtures show more than one phase and components are visible.

Name four physical methods used to separate mixtures.

Filtration, distillation, evaporation, chromatography (also hand-sorting, magnetism, separating funnel).

What process purifies a solid from a solution by forming crystals?

Crystallization.

Explain the basic principle of distillation.

Separate liquids based on different boiling points by vaporizing and then condensing one component.

What laboratory device is commonly used to condense vapour in distillation?

A Liebig condenser.

Describe the simple steps of paper chromatography.

Place a dot of ink on chromatography paper, allow solvent to rise; different dyes travel different distances producing a chromatogram.

Define an atom.

The smallest particle of an element that can exist independently, consisting of nucleus and electron cloud.

Name the three sub-atomic particles with their charges.

Proton (+), neutron (0), electron (–).

Where are protons and neutrons located, and what do they contribute to?

Inside the nucleus; they account for almost all the atom’s mass.

What is meant by atomic number?

The number of protons in an atom’s nucleus.

Why is an atom electrically neutral?

Because it has an equal number of protons (+) and electrons (–).

State the maximum number of electrons that the first three Bohr orbitals can hold.

Orbital 1: 2, Orbital 2: 8, Orbital 3: 8.

Give the electron configuration of a neutral aluminium atom.

2 : 8 : 3.

Who is regarded as the father of the Periodic Table?

Dmitri Mendeleev.

How many groups and periods are on the modern Periodic Table?

18 groups (vertical) and 7 periods (horizontal).

List the names of Groups 1, 2, 17 and 18.

Group 1 – Alkali metals; Group 2 – Alkaline earth metals; Group 17 – Halogens; Group 18 – Noble gases.

Where are metals and non-metals located on the Periodic Table?

Metals on the left and centre; non-metals on the right (plus hydrogen).

Give three physical properties typical of metals.

Shiny (lustrous), good conductors of heat/electricity, malleable and ductile with high melting points.

State two common physical properties of non-metals.

Often gases or brittle solids with low density; generally poor conductors (insulators).

Why is graphite an exception among non-metals?

Graphite is a good conductor of electricity despite being a non-metal.

What information is found in each element block of the Periodic Table key?

Chemical symbol, atomic number (protons) and relative atomic mass (protons + neutrons).

Write the symbols for the first five elements.

H, He, Li, Be, B.

Why must chemical symbols use correct capitalisation (e.g., NaCl not NACL)?

Because the first letter is always capital and any second letter is lowercase; incorrect case represents different elements or nonsense.

Define a molecule.

Two or more atoms chemically bonded together; may consist of the same or different elements.

List the seven naturally occurring diatomic molecules.

H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.

What distinguishes a compound from an element at the molecular level?

Compounds contain atoms of different elements bonded together in fixed ratios; elements contain only one kind of atom.

Give the fixed atom ratio in water and in carbon dioxide.

Water H₂O – 2 H : 1 O; Carbon dioxide CO₂ – 1 C : 2 O.

Contrast physical and chemical changes regarding new substances.

Physical change forms no new substance; chemical change always forms one or more new substances.

Provide one example each of physical and chemical change.

Physical – Ice melting; Chemical – Vinegar reacting with baking soda producing CO₂.

What is electrolysis?

Chemical decomposition of a compound by passing an electric current through its liquid or solution (electrolyte).

During electrolysis of copper(II) chloride, where is copper deposited?

At the cathode (negative electrode).

Name the apparatus commonly used to electrolyse water and collect gases.

Hoffmann voltameter.

Which gases are produced at each electrode when water is electrolysed?

Oxygen at the anode (+) and hydrogen at the cathode (–).