Pearson Edexcel GCSE Combined Science · C1

Key concepts in chemistry

50 revision questions with answers for C1 Key concepts in chemistry in Pearson Edexcel GCSE Combined Science (spec 1SC0). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below. Not official Pearson material.

All 50 questions and answers

Question 1 · Spec C1.1
Why did Dalton's atomic model change?
Answer:
Discovery of subatomic particles showed atoms were divisible and had an internal structure.
Question 2 · Spec C1.2
Describe an atom's basic structure.
Answer:
A nucleus containing protons and neutrons, surrounded by electrons in shells.
Question 3 · Spec C1.3
State the relative charges and masses of the three main subatomic particles.
Answer:
Proton: +1, mass 1. Neutron: 0, mass 1. Electron: -1, mass about 1/1836.
Question 4 · Spec C1.4
Why is a neutral atom uncharged?
Answer:
It has equal numbers of positive protons and negative electrons.
Question 5 · Spec C1.5
How does the nucleus compare with the whole atom?
Answer:
It is extremely small but contains almost all the atom's mass.
Question 6 · Spec C1.7
What does mass number count?
Answer:
Protons plus neutrons in the nucleus.
Question 7 · Spec C1.8
What determines an element's identity?
Answer:
Its proton number; every atom of the same element has the same number of protons.
Question 8 · Spec C1.10
A neutral atom has atomic number 12 and mass number 24. Count its particles.
Answer:
12 protons, 12 neutrons and 12 electrons.
Question 9 · Spec C1.9
What are isotopes?
Answer:
Atoms of the same element with equal proton numbers but different neutron numbers.
Question 10 · Spec C1.11
Why are some relative atomic masses not whole numbers?
Answer:
They are weighted means reflecting the masses and abundances of different isotopes.
Question 11 · Spec C1.12 · Higher only
An element is 75% mass-35 and 25% mass-37 isotopes. Calculate relative atomic mass.
Answer:
(75 × 35 + 25 × 37) / 100 = 35.5.
Question 12 · Spec C1.14
How did Mendeleev use gaps in his table?
Answer:
He left spaces for undiscovered elements and predicted their properties from surrounding patterns.
Question 13 · Spec C1.15
Why does increasing relative atomic mass not always match modern element order?
Answer:
Isotope abundances affect relative atomic masses; modern order follows proton number instead.
Question 14 · Spec C1.17
How are periods and groups arranged in the modern table?
Answer:
Periods are horizontal rows in increasing atomic number; groups are vertical columns of elements with similar properties.
Question 15 · Spec C1.18
Where are most metals and non-metals, and how do their atoms form ions?
Answer:
Metals lie mainly left and centre and lose electrons; non-metals lie mainly right and often gain electrons.
Question 16 · Spec C1.19
Give electron configurations for sodium and calcium.
Answer:
Sodium: 2.8.1. Calcium: 2.8.8.2.
Question 17 · Spec C1.20
What do period and main-group position reveal about electrons?
Answer:
Period gives occupied shell number; main-group position indicates outer-shell electron number.
Question 18 · Spec C1.22
What is an ion?
Answer:
An atom or group of atoms with an overall positive or negative charge.
Question 19 · Spec C1.23
How many electrons are in Mg²⁺ if magnesium has atomic number 12?
Answer:
10 electrons: the neutral atom has lost two.
Question 20 · Spec C1.21
Explain ionic bond formation between sodium and chlorine.
Answer:
Sodium transfers one electron to chlorine, forming Na⁺ and Cl⁻; electrostatic attraction holds the ions together.
Question 21 · Spec C1.26
Deduce magnesium chloride's formula from Mg²⁺ and Cl⁻.
Answer:
MgCl₂: two chloride ions balance one magnesium ion's charge.
Question 22 · Spec C1.25
What do the endings -ide and -ate commonly indicate?
Answer:
-ide usually indicates a compound of two elements; -ate commonly indicates an oxygen-containing compound with other elements.
Question 23 · Spec C1.27
Describe a giant ionic lattice.
Answer:
A regular three-dimensional arrangement of oppositely charged ions held by strong electrostatic attractions.
Question 24 · Spec C1.28
What is a covalent bond?
Answer:
A shared pair of electrons between two atoms.
Question 25 · Spec C1.30
What is the approximate scale of atoms and small molecules?
Answer:
Typically around 0.1 nm for atoms and roughly nanometre scale or smaller for small molecules.
Question 26 · Spec C1.31
How many shared electron pairs form each bond in H₂ and HCl?
Answer:
One shared pair forms a single bond in each molecule.
Question 27 · Spec C1.31
How many single covalent bonds are present in H₂O and CH₄?
Answer:
Water has two O-H bonds; methane has four C-H bonds.
Question 28 · Spec C1.31
Describe the bonds in O₂ and CO₂.
Answer:
O₂ has one O=O double bond; CO₂ has two C=O double bonds, with two shared pairs in each double bond.
Question 29 · Spec C1.32
Name the four main structure types used to classify substances.
Answer:
Ionic, simple molecular, giant covalent and metallic.
Question 30 · Spec C1.33
Why do ionic compounds melt at high temperatures and conduct only when molten or dissolved?
Answer:
Strong ion attractions need much energy to overcome; ions carry charge only when free to move.
Question 31 · Spec C1.34
Why do simple molecular substances often boil easily and conduct poorly?
Answer:
Weak intermolecular attractions are overcome during boiling; there are no mobile charged particles to conduct electricity.
Question 32 · Spec C1.36
Why is diamond suitable for cutting tools?
Answer:
Each carbon forms four strong covalent bonds in a rigid giant structure, making diamond very hard.
Question 33 · Spec C1.37
Why is graphite useful as both an electrode and lubricant?
Answer:
Delocalised electrons conduct electricity; weak forces between layers allow them to slide.
Question 34 · Spec C1.38
How do graphene and C₆₀ differ structurally?
Answer:
Graphene is one carbon-atom-thick sheet; C₆₀ is a hollow cage molecule containing 60 carbon atoms.
Question 35 · Spec C1.39
Describe the molecules in poly(ethene).
Answer:
Very large molecules containing long chains of covalently bonded carbon atoms.
Question 36 · Spec C1.40
Why are metals malleable and electrically conducting?
Answer:
Layers of metal particles can slide while bonding remains; delocalised electrons move and carry charge.
Question 37 · Spec C1.41
Give a limitation of a ball-and-stick model.
Answer:
The sticks are not real rods and the displayed gaps or relative particle sizes may be misleading.
Question 38 · Spec C1.43
Calculate Mr of CaCO₃. Ar: Ca = 40, C = 12, O = 16.
Answer:
Mr = 40 + 12 + 3 × 16 = 100.
Question 39 · Spec C1.43
Find oxygen's percentage by mass in H₂O. Ar: H = 1, O = 16.
Answer:
Mr = 18; oxygen percentage = 16 / 18 × 100 = 88.9%.
Question 40 · Spec C1.44
Find an empirical formula from 2.4 g Mg and 1.6 g O. Ar: Mg = 24, O = 16.
Answer:
Relative amounts: 2.4/24 = 0.10 and 1.6/16 = 0.10; ratio 1:1, so MgO.
Question 41 · Spec C1.45
A compound has empirical formula CH₂ and Mr 42. Find its molecular formula. Ar: C = 12, H = 1.
Answer:
Empirical formula mass = 14; multiplier = 42/14 = 3; molecular formula = C₃H₆.
Question 42 · Spec C1.46
How can magnesium oxide's empirical formula be found experimentally?
Answer:
Weigh magnesium, heat in a crucible with controlled air access to constant mass, find oxygen gain and calculate the simplest atom ratio.
Question 43 · Spec C1.47
Why does total mass stay constant in a sealed reaction?
Answer:
The same atoms remain in the system before and after the reaction.
Question 44 · Spec C1.47
Why can a burning metal gain measured mass in an open system?
Answer:
It combines with oxygen from the air; the additional oxygen becomes part of the solid product.
Question 45 · Spec C1.48
For 2Mg + O₂ → 2MgO, find product mass from 6 g Mg. Ar: Mg = 24, O = 16.
Answer:
48 g Mg forms 80 g MgO; 6 × 80/48 = 10 g MgO, with oxygen in excess.
Question 46 · Spec C1.49
Find concentration when 4.0 g solute makes 200 cm³ solution.
Answer:
200 cm³ = 0.200 dm³; concentration = 4.0/0.200 = 20 g/dm³.
Question 47 · Spec C1.50 · Higher only
What is one mole of a substance?
Answer:
An amount containing 6.02 × 10²³ stated particles; its mass in grams equals its relative particle mass.
Question 48 · Spec C1.51 · Higher only
Find moles and molecules in 9.0 g water. Mr = 18.
Answer:
Amount = 9.0/18 = 0.50 mol; molecules = 0.50 × 6.02 × 10²³ = 3.01 × 10²³.
Question 49 · Spec C1.52 · Higher only
What determines the maximum product when one reactant is in excess?
Answer:
The amount of the other, limiting reactant, which is completely used up.
Question 50 · Spec C1.53 · Higher only
4 g H₂ reacts with 32 g O₂ to form 36 g H₂O. Derive coefficients. Mr: 2, 32, 18.
Answer:
Amounts are 2, 1 and 2 mol, giving 2H₂ + O₂ → 2H₂O.

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Key concepts in chemistry

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