Chemistry — Assumed Knowledge¶
Year 11 Chemistry assumes you remember a lot from Years 7–10 Science without teaching it again. This page is a quick refresher on those ideas, based on the chemistry in the Science 7–10 Syllabus (2023) — so if a Year 11 page mentions something that feels unfamiliar, it's probably here.
Each section says where the idea came from (Stage 4 = Years 7–8, Stage 5 = Years 9–10) and which Year 11 pages need it.
How to use this page
This is a refresher, not new content — no syllabus mapping and no Advanced/Extension boxes. Read the section a Year 11 page sent you to, try the quick question, then head back. If a whole section feels new rather than rusty, ask your teacher — it might not have been covered in your Years 7–10 course.
Jump to: Particle model · Mixtures · Solutions · Density · Atoms and the periodic table · Chemical change · Energy · Bonding · Equations · Reaction types · Acids and pH · Rates · Nuclear · Hydrocarbons · Maths: notation and sig figs · Maths: units · Maths: rearranging · Maths: graphs
States of matter and the particle model¶
From: Stage 4 — Solutions and mixtures. Needed for: Separation of Mixtures, Properties of Gases, Rates of Reactions.
All matter is made of tiny particles that are always moving. The three states differ in how the particles are arranged, how they move, and how strongly they attract each other:
| Solid | Liquid | Gas | |
|---|---|---|---|
| Arrangement | Packed closely in a regular pattern | Close together, but disordered | Far apart, random |
| Movement | Vibrate in fixed positions | Move past each other | Move fast in all directions |
| Attractive forces between particles | Strong | Medium | Very weak |
| Shape and volume | Fixed shape, fixed volume | Takes the container's shape, fixed volume | Fills the whole container |
Heating gives particles more kinetic energy, so they move faster. At the melting point a solid turns into a liquid; at the boiling point a liquid turns into a gas. While a substance is changing state, its temperature stays constant — the energy is going into overcoming the attractions between particles.
Try it yourself: Use the particle model to explain why a gas can be compressed but a liquid can't (easily).
Answer
Gas particles are far apart with lots of empty space between them, so pushing them closer together is easy. Liquid particles are already close together, so there's almost no space to squash them into.
Pure substances and mixtures¶
From: Stage 4 — Solutions and mixtures; Periodic table and atomic structure. Needed for: Separation of Mixtures.
- An element is made of only one kind of atom (e.g. iron, oxygen). It's listed on the periodic table.
- A compound is two or more elements chemically bonded in a fixed ratio (e.g. water, \(\ce{H2O}\)). It has completely different properties from the elements it's made from — sodium is a reactive metal and chlorine is a poisonous gas, but sodium chloride is table salt.
- A mixture is two or more substances mixed together but not chemically bonded. Each part keeps its own properties, which is why mixtures can be separated by physical methods (filtering, evaporating, distilling, chromatography, using a magnet).
Elements and compounds are pure substances; mixtures are impure.
Try it yourself: Classify each as an element, compound or mixture: (a) carbon dioxide, (b) copper, (c) seawater.
Answer
(a) Compound — carbon and oxygen chemically bonded. (b) Element. (c) Mixture — water with salts dissolved in it, not bonded together.
Solutions and solubility¶
From: Stage 4 — Solutions and mixtures. Needed for: Separation of Mixtures, Measures of Concentration, Classifying Chemical Reactions (precipitates).
| Word | Meaning |
|---|---|
| Solute | The substance that dissolves (e.g. salt) |
| Solvent | The substance it dissolves in (usually water) |
| Solution | The mixture of solute and solvent |
| Soluble / insoluble | Does / doesn't dissolve |
| Solubility | How much solute dissolves in a given amount of solvent at a given temperature |
| Dilute / concentrated | A little / a lot of solute in the solvent |
| Saturated | Holding as much solute as it can at that temperature — any more just sits at the bottom |
| Supersaturated | Temporarily holding more than its normal maximum (usually made by cooling a hot saturated solution carefully) — unstable, crystals form suddenly |
In particle terms, dissolving means solvent particles surround solute particles and spread them evenly through the solution. The solute hasn't disappeared — it's still there, just spread out as particles too small to see. For most solids, solubility increases with temperature (more sugar dissolves in hot tea than iced tea).
Try it yourself: What's the difference between "dissolving" and "melting"?
Answer
Dissolving is a solute spreading through a solvent to make a solution (sugar in water). Melting is a single substance changing from solid to liquid when heated (ice to water). No solvent is involved in melting.
Density¶
From: Stage 4 — Solutions and mixtures. Needed for: Separation of Mixtures (separating funnels, centrifuging), Atomic Structure (isotopes).
Density is how much mass is packed into a given volume:
In chemistry it's usually in g mL⁻¹ (the same as g cm⁻³). Water has a density of about \(1.00\ \text{g mL}^{-1}\) — so 1 mL of water has a mass of about 1 g, a shortcut you'll use often. Substances denser than water sink in it; less dense ones float (oil floats on water).
Try it yourself: A 10.0 mL sample of a liquid has a mass of 7.9 g. What is its density, and will it float on water?
Answer
\(\rho = \dfrac{7.9}{10.0} = 0.79\ \text{g mL}^{-1}\). It's less dense than water, so it floats. (It's ethanol.)
Atoms, elements and the periodic table¶
From: Stage 4 — Periodic table and atomic structure. Needed for: Atomic Structure, Periodic Trends.
An atom is the smallest unit of an element that still has that element's properties. Atoms are made of three subatomic particles:
| Particle | Location | Relative charge | Relative mass |
|---|---|---|---|
| Proton | Nucleus | +1 | 1 |
| Neutron | Nucleus | 0 | 1 |
| Electron | Shells around the nucleus | −1 | almost 0 |
The atomic number is the number of protons. In a neutral atom, the number of electrons equals the number of protons.
Electron shells (the planetary model): electrons fill shells from the inside out — up to 2 in the first shell, 8 in the second, and 8 in the third for the first 18 elements. So sodium (11 electrons) is 2, 8, 1 and chlorine (17 electrons) is 2, 8, 7. In Year 11 you'll extend this to the more detailed spdf notation.
The periodic table is arranged in order of atomic number:
- Rows are periods; columns are groups. Elements in the same group have the same number of outer electrons and react in similar ways.
- Metals are on the left and in the middle (shiny, conduct electricity and heat, malleable). Non-metals are on the right (usually dull, brittle, poor conductors). Metalloids sit along the staircase line between them (e.g. silicon) and have some properties of each.
- Group 1 (alkali metals) are very reactive and get more reactive down the group; group 18 (noble gases) are very unreactive.
Try it yourself: Write the electron arrangement for magnesium (atomic number 12) and say which group it's in.
Answer
2, 8, 2. It has 2 outer electrons, so it's in group 2.
Physical and chemical change¶
From: Stage 4 — Change. Needed for: Classifying Chemical Reactions, Conservation of Mass.
- A physical change doesn't make a new substance — e.g. melting, boiling, dissolving, cutting. It's often easy to reverse.
- A chemical change (a chemical reaction) makes new substances. It's usually hard to reverse.
Signs that a chemical change has probably happened:
- a colour change
- a gas given off (fizzing or bubbles)
- a precipitate (a solid) forming when two solutions are mixed
- a temperature change (getting hotter or colder) or light being given off
A word equation shows the reactants and products in words: magnesium + oxygen → magnesium oxide. Reactants go on the left, products on the right, and the arrow means "react to form".
Try it yourself: Is boiling water a physical or chemical change? What about burning paper?
Answer
Boiling water is physical — it's still water, just as a gas. Burning paper is chemical — new substances (carbon dioxide, water vapour, ash) are made, and you can't turn them back into paper.
Energy and conservation of energy¶
From: Stage 4 — Change; Stage 5 — Energy. Needed for: Energy Changes, Conservation of Mass (open and closed systems).
- Kinetic energy is energy of movement. Potential energy is stored energy — chemical energy stored in bonds is a type of potential energy.
- The law of conservation of energy: energy can't be created or destroyed, only transferred or transformed from one form to another. When a fuel burns, chemical energy is transformed into heat and light.
- Heat moves by conduction, convection and radiation.
- In an open system, energy and matter can get in and out; in a closed system, energy can get in and out but matter can't.
- Heat and temperature aren't the same thing: temperature is how hot something is; heat is energy being transferred because of a temperature difference. A bath of warm water holds more heat energy than a cup of boiling water, even though it's at a lower temperature.
Try it yourself: Describe the energy transformation in a hand warmer that heats up when it's activated.
Answer
Chemical potential energy stored in the substances is transformed into heat (thermal energy) as they react. The total amount of energy stays the same.
Ions, valency and chemical bonds¶
From: Stage 5 — Materials. Needed for: Periodic Trends, Chemical Bonding.
Atoms are most stable with a full outer shell — the same arrangement as a noble gas (a noble gas configuration). They get there by gaining, losing or sharing electrons.
- Atoms that lose electrons become positive ions (cations) — metals do this.
- Atoms that gain electrons become negative ions (anions) — non-metals do this.
Valency is the number of electrons an atom needs to gain, lose or share to get a full outer shell:
| Group | 1 | 2 | 13 | 14 | 15 | 16 | 17 | 18 |
|---|---|---|---|---|---|---|---|---|
| Outer electrons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 (full) |
| Valency | 1 | 2 | 3 | 4 | 3 | 2 | 1 | 0 |
| Usual ion | +1 | +2 | +3 | — | −3 | −2 | −1 | none |
The three types of bond:
- Ionic — between a metal and a non-metal; electrons are transferred, and the oppositely charged ions attract (e.g. sodium chloride).
- Covalent — between non-metals; electrons are shared (e.g. water, carbon dioxide).
- Metallic — between metal atoms; outer electrons are shared in a "sea" around positive metal ions.
Some elements exist as diatomic molecules (two atoms bonded together): \(\ce{H2}\), \(\ce{N2}\), \(\ce{O2}\), \(\ce{F2}\), \(\ce{Cl2}\), \(\ce{Br2}\), \(\ce{I2}\). Always write them this way in equations.
Try it yourself: What ion does oxygen form, and why?
Answer
Oxygen is in group 16, with 6 outer electrons. Gaining 2 electrons gives it a full outer shell (like neon), so it forms \(\ce{O^{2-}}\).
Formulas and balanced equations¶
From: Stage 5 — Materials; Reactions. Needed for: Chemical Bonding, Conservation of Mass, Classifying Chemical Reactions.
Formulas of ionic compounds: the charges must add up to zero. Magnesium (+2) and chlorine (−1) → you need two chlorides for each magnesium → \(\ce{MgCl2}\). In a name, the metal goes first and the non-metal ends in -ide (magnesium chloride).
Balanced equations: a chemical equation uses formulas instead of words. To balance it, put numbers in front of formulas (never change the small numbers inside a formula) until there are the same number of each type of atom on both sides — because atoms are only rearranged in a reaction, never created or destroyed (conservation of mass).
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous — dissolved in water.
Try it yourself: Balance \(\ce{Mg + O2 -> MgO}\).
Answer
\(\ce{2Mg + O2 -> 2MgO}\). Check: 2 Mg on each side, 2 O on each side ✓
Types of reaction¶
From: Stage 5 — Reactions. Needed for: Classifying Chemical Reactions.
| Type | What happens | Example (word equation) |
|---|---|---|
| Synthesis | Two or more substances join to make one | iron + sulfur → iron sulfide |
| Decomposition | One substance breaks down into two or more | copper carbonate → copper oxide + carbon dioxide |
| Displacement | A more reactive metal pushes a less reactive one out of a compound | zinc + copper sulfate → zinc sulfate + copper |
| Neutralisation | An acid reacts with a base to make a salt and water | hydrochloric acid + sodium hydroxide → sodium chloride + water |
| Combustion | A substance reacts with oxygen, releasing heat and light | methane + oxygen → carbon dioxide + water |
Year 11 adds precipitation reactions and explains displacement using electrons (redox).
Acids, bases and pH¶
From: Stage 5 — Reactions. Needed for: Classifying Chemical Reactions (neutralisation), and much of Year 12.
pH measures how acidic or basic a solution is, on a scale from about 0 to 14:
| pH | Meaning | Examples |
|---|---|---|
| Below 7 | Acidic (the lower, the more acidic) | Stomach acid (~1–2), lemon juice (~2), vinegar (~3) |
| 7 | Neutral | Pure water |
| Above 7 | Basic (alkaline) | Baking soda solution (~9), soapy water (~10), bleach (~12–13) |
Indicators change colour depending on pH. Litmus turns red in acid and blue in base. Universal indicator shows a range of colours — red for strong acid, green for neutral, purple for strong base. A pH meter gives a number directly.
In neutralisation, an acid and a base cancel each other out, so the pH moves towards 7.
Try it yourself: A solution turns universal indicator orange. Is it acidic, neutral or basic?
Answer
Acidic — orange is on the acid side of the universal indicator range (roughly pH 4–5).
Rates of reaction: the basics¶
From: Stage 5 — Reactions. Needed for: Rates of Reactions.
Four factors speed up a reaction:
- Higher concentration of a reactant in solution
- Larger surface area (smaller pieces, or a powder)
- Higher temperature
- Adding a catalyst — a substance that speeds up a reaction without being used up
In Year 10 you investigated these; in Year 11 you'll explain why each one works, using collision theory.
Nuclear reactions: the basics¶
From: Stage 5 — Reactions. Needed for: Atomic Structure.
- Some nuclei are unstable — they break down (decay) and give off radiation. Atoms with unstable nuclei are radioactive isotopes (radioisotopes).
- In alpha decay, the nucleus gives off an alpha particle (2 protons + 2 neutrons). In beta decay, a neutron turns into a proton and a fast electron (beta particle) is given off.
- Half-life is the time for half of the radioactive atoms in a sample to decay.
- Fission is a large nucleus splitting into smaller ones (nuclear power stations). Fusion is small nuclei joining to make a larger one (the Sun).
- Radioisotopes are used in medicine (imaging, cancer treatment), industry and environmental monitoring.
Try it yourself: A radioisotope has a half-life of 6 hours. What fraction of it is left after 18 hours?
Answer
18 hours is 3 half-lives: \(\tfrac{1}{2} \to \tfrac{1}{4} \to \tfrac{1}{8}\). One-eighth is left.
Hydrocarbons and combustion¶
From: Stage 5 — Materials. Needed for: Classifying Chemical Reactions (combustion), Energy Changes, and Year 12 Organic Chemistry.
Hydrocarbons are compounds of only hydrogen and carbon. The simplest family is the alkanes (formula \(\ce{C_nH_{2n+2}}\)), which come from crude oil:
| Carbons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Name | Methane | Ethane | Propane | Butane | Pentane | Hexane | Heptane | Octane |
| Formula | \(\ce{CH4}\) | \(\ce{C2H6}\) | \(\ce{C3H8}\) | \(\ce{C4H10}\) | \(\ce{C5H12}\) | \(\ce{C6H14}\) | \(\ce{C7H16}\) | \(\ce{C8H18}\) |
Combustion of hydrocarbons:
- Complete combustion (plenty of oxygen): makes carbon dioxide and water, releases the most energy, burns with a clean blue flame.
- Incomplete combustion (not enough oxygen): also makes carbon monoxide (toxic) and/or soot (carbon), releases less energy, burns with a smoky yellow flame.
Try it yourself: Write a word equation for the complete combustion of propane.
Answer
propane + oxygen → carbon dioxide + water
Scientific notation and significant figures¶
Maths skill. Needed for: Mole Concept (the Avogadro constant), every calculation, and Precision and Uncertainty.
Scientific notation writes a number as \(a \times 10^n\), where \(1 \le a < 10\). It's essential for the very big and very small numbers in chemistry.
- \(602\,200\,000\,000\,000\,000\,000\,000 = 6.022 \times 10^{23}\) (decimal point moves 23 places left → power \(+23\))
- \(0.00072 = 7.2 \times 10^{-4}\) (decimal point moves 4 places right → power \(-4\))
On a calculator, use the ×10ˣ (or EXP/EE) key — type 6.022 ×10ˣ 23, not 6.022 × 10 ^ 23, to avoid bracket mistakes.
Significant figures — the rules:
- All non-zero digits count.
- Zeros between non-zero digits count (\(505\) has 3 sig figs).
- Leading zeros never count (\(0.0032\) has 2 sig figs).
- Trailing zeros count only if there's a decimal point (\(3.20\) has 3 sig figs).
Give your answer to the same number of sig figs as the least precise measurement in the question.
Try it yourself: Write \(0.000\,045\,6\) in scientific notation. How many significant figures does it have?
Answer
\(4.56 \times 10^{-5}\). Three significant figures (4, 5 and 6 — the leading zeros don't count).
Units and conversions¶
Maths skill. Needed for: Measures of Concentration, Properties of Gases, Energy Changes.
The conversions you'll use most in Chemistry:
| Convert | How | Example |
|---|---|---|
| mL → L | ÷ 1000 | 250 mL = 0.250 L |
| L → mL | × 1000 | 0.050 L = 50 mL |
| g → kg | ÷ 1000 | 500 g = 0.500 kg |
| mg → g | ÷ 1000 | 25 mg = 0.025 g |
| °C → K | + 273.15 | 25 °C = 298.15 K |
| J → kJ | ÷ 1000 | 2840 J = 2.84 kJ |
| cm³ ↔ mL | Same thing | 1 cm³ = 1 mL |
The SI prefixes behind these:
| Prefix | Symbol | Factor |
|---|---|---|
| Mega | M | \(10^{6}\) |
| Kilo | k | \(10^{3}\) |
| Milli | m | \(10^{-3}\) |
| Micro | μ | \(10^{-6}\) |
| Nano | n | \(10^{-9}\) |
(The syllabus lists the full range from femto, \(10^{-15}\), to peta, \(10^{15}\) — the same table is on the Physics Assumed Knowledge page.)
The most common mistake in Year 11 Chemistry: forgetting to convert mL to L before using \(C = \dfrac{n}{V}\). Check every volume before you substitute.
Try it yourself: Convert (a) 75 mL to litres and (b) −10 °C to kelvin.
Answer
(a) \(75 \div 1000 = 0.075\ \text{L}\). (b) \(-10 + 273.15 = 263.15\ \text{K}\).
Rearranging formulas¶
Maths skill. Needed for: every calculation page — \(n = \dfrac{m}{MM}\), \(C = \dfrac{n}{V}\), \(PV = nRT\), \(q = mc\Delta T\).
To make a different quantity the subject, do the same thing to both sides until it's on its own.
Worked example
Rearrange \(n = \dfrac{m}{MM}\) to make \(m\) the subject.
Multiply both sides by \(MM\): \(n \times MM = m\), so \(m = n \times MM\).
Rearrange \(PV = nRT\) to make \(V\) the subject: divide both sides by \(P\), so \(V = \dfrac{nRT}{P}\).
Rearrange before you put the numbers in — it's much easier to spot a mistake.
Try it yourself: Rearrange \(C = \dfrac{n}{V}\) to make \(V\) the subject.
Answer
Multiply both sides by \(V\): \(CV = n\). Then divide both sides by \(C\): \(V = \dfrac{n}{C}\).
Graphs¶
Maths skill. Needed for: Periodic Trends, Properties of Gases, Rates of Reactions, and Representing Data.
- Put the independent variable (the one you change) on the x-axis and the dependent variable (the one you measure) on the y-axis.
- Label both axes with the quantity and unit, e.g. "Temperature (°C)".
- Draw a smooth line or curve of best fit — don't join the dots.
- The gradient (steepness) of a line is \(\dfrac{\text{rise}}{\text{run}} = \dfrac{\Delta y}{\Delta x}\). On a graph of product against time, a steeper line means a faster reaction.
Try it yourself: A line on a graph of volume of gas (mL) against time (s) goes from (0, 0) to (20, 50). What is its gradient, and what does it mean?
Answer
\(\dfrac{50 - 0}{20 - 0} = 2.5\ \text{mL s}^{-1}\). On average, 2.5 mL of gas was produced each second — the rate of the reaction.