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Every year, examiners report the same mistakes appearing in thousands of student scripts. These are not obscure errors — they are well-known misconceptions and sloppy habits that cost marks on the most common question types.
This lesson catalogues the most frequent Chemistry mistakes and teaches you how to avoid each one. Study this lesson carefully — correcting even a few of these could gain you several extra marks across both papers.
This is the most fundamental error in Chemistry, and it appears at every level.
| Term | Definition | Example |
|---|---|---|
| Atom | The smallest particle of an element that retains its chemical properties | A single carbon atom (C) |
| Molecule | Two or more atoms chemically bonded together | O₂ (two oxygen atoms bonded), H₂O (two hydrogen atoms bonded to one oxygen atom) |
| Element | A substance made of only one type of atom | Oxygen (O₂), iron (Fe), carbon (C) |
| Compound | A substance made of two or more different elements chemically bonded together | Water (H₂O), sodium chloride (NaCl) |
| Mixture | Two or more substances not chemically bonded — can be separated by physical methods | Air, salt water, crude oil |
Exam tip: "Oxygen" is an element, but oxygen gas exists as O₂ molecules. Be precise: an oxygen atom is not the same as an oxygen molecule.
| Wrong Statement | Correct Statement |
|---|---|
| "In ionic bonding, atoms share electrons" | In ionic bonding, electrons are transferred from the metal atom to the non-metal atom |
| "In covalent bonding, atoms transfer electrons" | In covalent bonding, atoms share pairs of electrons |
| "Ionic bonds are formed between two non-metals" | Ionic bonds are formed between a metal and a non-metal |
| "NaCl is a molecule" | NaCl is an ionic compound that forms a giant ionic lattice, not individual molecules |
Exam tip: The word "share" goes with covalent bonding. The word "transfer" goes with ionic bonding. If you mix these up in a 6-mark answer, you will drop to Level 1 immediately.
A catalyst speeds up a reaction without being used up or chemically changed. It is still present at the end of the reaction, unchanged.
| Wrong | Correct |
|---|---|
| "The catalyst is used up during the reaction" | "The catalyst increases the rate of reaction but is not used up — it can be reused" |
| "Adding more catalyst makes more product" | "Adding more catalyst makes the reaction faster, but it does not change the amount of product formed" |
The catalyst works by providing an alternative reaction pathway with a lower activation energy.
This is one of the most persistent misconceptions in Chemistry.
| Wrong | Correct |
|---|---|
| "When heated, the particles expand" | "When heated, the particles gain kinetic energy and move faster. The gaps between the particles increase, causing the substance to expand" |
| "Particles get bigger" | "Particles stay the same size — only the spaces between them change" |
Exam tip: If you write "particles expand" in an exam, you will lose the mark. The correct phrase is "particles move faster and the distance between them increases."
| Wrong | Correct |
|---|---|
| "Solid sodium chloride conducts electricity" | "Solid ionic compounds do not conduct electricity because the ions are held in a fixed lattice and cannot move" |
| "Electrolysis works on solid lead bromide" | "The ionic compound must be molten or dissolved in water so the ions are free to move and carry charge" |
When writing chemical equations, you may be asked to include state symbols. Omitting them costs marks.
| Symbol | Meaning |
|---|---|
| (s) | Solid |
| (l) | Liquid |
| (g) | Gas |
| (aq) | Aqueous (dissolved in water) |
Example with state symbols: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Exam tip: If the question says "write a balanced equation with state symbols," you will lose a mark for every missing or incorrect state symbol. If the question does not mention state symbols, you will not lose marks for omitting them — but including correct ones can earn credit.
An unbalanced equation will lose marks. The number of atoms of each element must be the same on both sides.
Example:
Exam tip: After balancing, always count every atom on both sides to verify. A common error is balancing one element while unbalancing another.
Although this is more commonly a Biology error, it appears in Chemistry when discussing catalysts and biological molecules.
| Wrong | Correct |
|---|---|
| "The enzyme is killed" | "The enzyme is denatured — the active site changes shape so the substrate can no longer fit" |
| "High temperatures destroy enzymes" | "High temperatures cause the enzyme to denature — the bonds holding the tertiary structure break, changing the shape of the active site" |
Enzymes are not alive, so they cannot be "killed."
These are two completely different concepts:
| Concept | What It Means | Example |
|---|---|---|
| Strong acid | Fully dissociates (ionises) in water — every acid molecule releases H⁺ ions | HCl, H₂SO₄, HNO₃ |
| Weak acid | Partially dissociates in water — only some molecules release H⁺ ions | CH₃COOH (ethanoic acid), citric acid |
| Concentrated | A large amount of acid dissolved in a small volume of water | 10 mol/dm³ HCl |
| Dilute | A small amount of acid dissolved in a large volume of water | 0.01 mol/dm³ HCl |
You can have a concentrated weak acid (lots of ethanoic acid in a small volume) or a dilute strong acid (a small amount of HCl in lots of water).
Exam tip: Strong/weak refers to dissociation. Concentrated/dilute refers to amount per volume. Confusing these is a Higher tier error that comes up every year.
Every numerical answer in Chemistry needs units. Writing "0.5" instead of "0.5 mol" will lose you the unit mark.
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