States of matter & properties of substances – GCSE Chemistry exam questions with mark scheme

1Grade 1–33 marks

The table gives the melting point and boiling point of three substances.

Substance	Melting point in °C	Boiling point in °C
A	−114	78
B	801	1413
C	−218	−183

Give the state of each substance at 25 °C. [3]

A: liquid(1)
B: solid(1)
C: gas(1)
Answer: A liquid, B solid, C gas
Examiner tip: Ask: is 25 °C below the melting point (solid), between mp and bp (liquid), or above the bp (gas)?

2Grade 4–54 marks

Sodium chloride has a high melting point. It conducts electricity when molten but not when solid.

(a) Explain why sodium chloride has a high melting point. [2]

(b) Explain why sodium chloride conducts electricity when molten but not when solid. [2]

(a) (giant ionic lattice with) strong electrostatic forces of attraction between oppositely charged ions allow many strong ionic bonds do not award if the answer refers to intermolecular forces, molecules or covalent bonds(1)
(a) a lot of energy is needed to overcome these forces / break these bonds(1)
(b) when molten the ions are free to move (and carry charge) allow ions can move(1)
(b) in the solid the ions are held in fixed positions / cannot move if the answer says electrons move or carry the charge, max 1 for (b)(1)
Answer: (a) strong attraction between oppositely charged ions needs lots of energy to overcome (b) ions free to move when molten; fixed in solid
Examiner tip: It is the ions that move in ionic compounds — mentioning electrons moving loses a mark.

3Grade 6–75 marks

The diagram shows the arrangement of atoms in a pure metal and in an alloy.

(a) Explain why the alloy is harder than the pure metal. Use the diagram. [3]

(b) Explain why metals are good conductors of electricity. [2]

(a) in the pure metal the atoms are (all the same size and) arranged in (regular) layers(1)
(a) the layers can slide over each other (so the pure metal is soft)(1)
(a) in the alloy the different-sized atom distorts the layers, so it is harder for them to slide allow disrupts the layers / the regular arrangement ignore 'stronger bonds'(1)
(b) metals have delocalised electrons allow sea of electrons(1)
(b) which can move through the (whole) structure and carry charge do not accept ions moving(1)
Answer: (a) different-sized atoms distort the layers so they cannot slide easily (b) delocalised electrons move through the structure
Examiner tip: The key word is “distort”: a different-sized atom stops the layers sliding — saying the alloy has “stronger bonds” earns nothing.

4Grade 6–73 marks

Methane, $\text{CH}_4$, and poly(ethene) are both made of molecules containing covalent bonds.

(a) Explain why methane has a low boiling point. [2]

(b) Poly(ethene) melts at a much higher temperature than methane. Suggest why. [1]

(a) weak intermolecular forces (between methane molecules) do not accept covalent bonds breaking / weak covalent bonds — 0 marks for (a)(1)
(a) which need little energy to overcome(1)
(b) the intermolecular forces (between poly(ethene) molecules) are stronger because the molecules are much larger/longer allow stronger intermolecular forces alone do not accept stronger covalent bonds(1)
Answer: (a) weak intermolecular forces need little energy to overcome (b) larger molecules → stronger intermolecular forces
Examiner tip: Boiling never breaks the covalent bonds inside a molecule — say “intermolecular forces”.

5Grade 1–33 marks

Sodium reacts with water to produce sodium hydroxide solution and hydrogen gas.

The equation for the reaction is:

$$2\text{Na}(\ldots) + 2\text{H}_2\text{O}(\ldots) \rightarrow 2\text{NaOH}(\ldots) + \text{H}_2(\ldots)$$

(a) Complete the equation by writing the correct state symbol in each bracket. [2]

(b) What does the state symbol (aq) mean? [1]

(a) two or three of the four state symbols correct: Na(s), H₂O(l), NaOH(aq), H₂(g)(1)
(a) all four state symbols correct (scores both marks)(1)
(b) dissolved in water / aqueous solution; allow 'in solution'; do not accept 'liquid' or 'wet'(1)
Answer: Na(s), H₂O(l), NaOH(aq), H₂(g); (aq) means dissolved in water
Examiner tip: Water itself is (l), not (aq) — (aq) only means something is dissolved in water.

6Grade 4–55 marks

A student heated a sample of solid substance X and measured its temperature every minute.

The graph shows the results.

(a) Use the graph to give the melting point of substance X. [1]

(b) What is the state of substance X at 12 minutes? [1]

Explain why. [2]

(d) Substance Y has stronger forces between its particles than substance X.

Suggest how the melting point of Y compares with the melting point of X. Give a reason for your answer. [1]

(a) 55 °C; allow 54–56 °C(1)
(b) liquid(1)
(c) (energy is being transferred to the substance but) the energy is used to overcome the forces (of attraction) between particles; allow to break bonds between particles; allow to change state / to melt the solid(1)
(c) so the energy is not used to increase the (kinetic energy of the particles / the) temperature; ignore 'the solid is melting' on its own for this mark(1)
(d) higher (melting point) and more energy needed to overcome the (stronger) forces between particles; both parts needed(1)
Answer: (a) 55 °C (b) liquid (c) energy overcomes forces between particles instead of raising the temperature (d) higher, more energy needed to overcome stronger forces
Examiner tip: Don't just say 'it is melting' — say where the energy goes: into overcoming forces between particles, not into raising the temperature.

7Grade 6–7Higher5 marks

Copper is used to make the bases of saucepans.

(a) Explain why copper is a good conductor of thermal energy. [2]

(b) A student says: “A single copper atom is a good conductor of thermal energy.”

Explain why the student is wrong. [1]

Give two limitations of the simple particle model. [2]

(a) (copper has) delocalised electrons; allow 'sea of free electrons'; do not accept 'free ions'(1)
(a) which (move through the structure and) transfer / carry thermal energy (through the metal)(1)
(b) individual atoms do not have the bulk properties of the material / conduction is a property of the whole structure (of many atoms with delocalised electrons); allow a single atom has no delocalised electrons that can move through a structure(1)
(c) any two from: there are no forces (of attraction) shown between the particles; particles are not (all) spherical; particles are not solid (atoms are mostly empty space); ignore 'does not show the particles moving'; ignore 'particles are too big' / 'not to scale'(1)
(c) second limitation from the list above(1)
Answer: (a) delocalised electrons carry thermal energy through the metal (b) one atom doesn't have bulk properties (c) e.g. no forces shown; particles not solid spheres
Examiner tip: For metals, always name delocalised electrons and say what they carry — 'electrons move' alone does not explain thermal conduction.

8Grade 4–54 marks

Fluorine, chlorine, bromine and iodine are made of small molecules, F₂, Cl₂, Br₂ and I₂.

The table shows information about these substances.

Substance	Relative formula mass $M_r$	Boiling point in °C
fluorine	38	−188
chlorine	71	−34
bromine	160	59
iodine	254	184

(a) Describe the pattern shown by the data. [1]

(b) Explain the pattern. [2]

(a) as the relative formula mass increases the boiling point increases; allow the bigger the molecule the higher the boiling point(1)
(b) (larger molecules have) stronger (weak) intermolecular forces; allow forces between molecules; do not accept stronger covalent bonds or 'bonds' unqualified(1)
(b) (so) more energy needed to overcome the intermolecular forces; do not accept 'to break covalent bonds' — max 1 mark for (b) if covalent bonds are said to break(1)
(c) the molecules have no overall (electric) charge; allow no charged particles (free to move) / no ions and no delocalised electrons; ignore 'no free electrons' or 'no delocalised electrons' on their own(1)
Answer: (a) higher M_r → higher boiling point (b) bigger molecules have stronger intermolecular forces, needing more energy to overcome (c) molecules have no overall charge
Examiner tip: When small molecules boil, intermolecular forces are overcome — saying covalent bonds break loses the mark.

9Grade 8–96 marks

A student tested four solid substances, W, X, Y and Z. The table shows the results.

Substance	Melting point in °C	Conducts electricity when solid?	Conducts electricity when molten?
W	1710	no	no
X	772	no	yes
Y	80	no	no
Z	1085	yes	yes

Each substance has a different type of structure: giant covalent, ionic, metallic or small molecules.

Identify the type of structure of each substance.

Use the data and your knowledge of bonding to explain your answers. [6]

W is giant covalent(1)
W: very high melting point because many strong covalent bonds must be broken(1)
W: no ions or delocalised electrons (free to move), so does not conduct solid or molten(1)
X is ionic(1)
X: high melting point because lots of energy needed to overcome strong electrostatic forces between oppositely charged ions (in all directions)(1)
X: ions fixed in solid so cannot conduct; ions free to move when molten so it conducts(1)
Y is (made of) small molecules / simple molecular(1)
Y: low melting point because only weak intermolecular forces are overcome (not covalent bonds)(1)
Y: molecules have no overall charge, so does not conduct(1)
Z is metallic(1)
Z: high melting point because of strong (metallic) bonds / strong attraction between positive metal ions and delocalised electrons(1)
Z: delocalised electrons free to move through the structure, so it conducts both as a solid and when molten(1)

Level	Marks	What it looks like
Level 3	5–6	All four structures correctly identified. Clear, linked explanations for each that connect the melting point to the strength of the bonds or forces overcome and the conductivity to whether charged particles are free to move. Correct terminology throughout (e.g. intermolecular forces, not covalent bonds, for Y).
Level 2	3–4	At least three structures correctly identified, with some correct explanation of melting point or conductivity for at least two of them. Explanations may be incomplete or not fully linked to the data.
Level 1	1–2	At least two structures correctly identified, or some relevant statements about bonding or conductivity that are not clearly linked to the data.
No relevant content	0	Nothing worthy of credit.

Answer: W giant covalent, X ionic, Y small molecules, Z metallic — each explained using bond/force strength (melting point) and mobile charged particles (conductivity).
Examiner tip: Conductivity is the key that splits the high-melting solids: conducts as solid = metal; only when molten = ionic; never = giant covalent.

10Grade 6–75 marks

The diagram shows the repeating unit of the polymer poly(chloroethene), PVC.

(a) One molecule of PVC has a value of $n = 1500$.

Calculate the relative formula mass ($M_r$) of this molecule.

Relative atomic masses ($A_r$): C = 12, H = 1, Cl = 35.5 [2]

(b) The monomer chloroethene is a gas at room temperature. PVC is a solid at room temperature.

Explain why. [2]

What is overcome when PVC melts? [1]

(a) $M_r$ of repeating unit = (2 × 12) + (3 × 1) + 35.5 = 62.5(1)
(a) 62.5 × 1500 = 93 750; allow 93 800 / 9.4 × 10⁴; correct answer with no working scores 2(1)
(b) PVC molecules are (very) large / long chains; allow polymer molecules are much bigger than monomer molecules(1)
(b) so the intermolecular forces (between polymer molecules) are (relatively) strong / stronger, needing more energy to overcome; do not accept covalent bonds break or 'strong covalent bonds' as the reason for being solid(1)
(c) the (relatively strong) intermolecular forces / forces between the polymer chains; do not accept covalent bonds(1)
Answer: (a) 93 750 (b) PVC molecules are very large so intermolecular forces are stronger and need more energy to overcome (c) intermolecular forces
Examiner tip: Count the atoms inside the brackets only — the bonds sticking out through the brackets link to the next repeating unit.

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