Original open focus revision; complete specification coverage remains unfinished.
1 · Particles, bonding and bulk properties
ionic bond · delocalized electron
edexcel_igcse_chemistry:bonding:open:1 · 2 marks
Why does molten sodium chloride conduct?
- Its ions can move
- Its atoms become electrons
- Its molecules contain no charge
Answer and reasoning · 答案与解析
Its ions can move
- To explain a bulk property, name the structure, particles, forces and mobile charge carriers. Simple molecular substances can have strong covalent bonds inside molecules but weak attractions between molecules.
edexcel_igcse_chemistry:bonding:open:2 · 4 marks
An atom has atomic number 17 and mass number 35. Find its number of neutrons.
Answer and reasoning · 答案与解析
18
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Melting a simple molecular substance usually overcomes intermolecular attractions; it does not require breaking all covalent bonds within each molecule.
2 · Amounts, equations and limiting reagents
mole · limiting reagent
edexcel_igcse_chemistry:moles:open:1 · 2 marks
Which step comes first in a reacting-mass calculation?
- Compare masses without an equation
- Use a balanced equation and convert mass to amount
- Assume every mole ratio is 1:1
Answer and reasoning · 答案与解析
Use a balanced equation and convert mass to amount
- Calculate the amount available for each reactant and divide by its coefficient. The smaller ratio limits the reaction. Use that reactant to calculate the maximum product before comparing actual yield.
edexcel_igcse_chemistry:moles:open:2 · 4 marks
Calculate amount in 5.0 g of a substance with molar mass 100 g per mole.
Answer and reasoning · 答案与解析
0.05 mol
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Excess acid means acid does not limit the stated calculation. A coefficient of 2 before HCl does not double the hydrogen amount.
3 · Redox and electrolysis
oxidation · reduction
edexcel_igcse_chemistry:electrolysis:open:1 · 2 marks
What happens to Cu²⁺ at the cathode?
- It gains electrons
- It loses two protons
- It gains neutrons
Answer and reasoning · 答案与解析
It gains electrons
- Predict products using the specified electrolyte and electrode material. In an aqueous solution, hydrogen or oxygen may form because water-related species compete. Molten salts contain only the ions of the salt.
edexcel_igcse_chemistry:electrolysis:open:2 · 4 marks
How many moles of electrons reduce 0.20 mol of Cu²⁺ to copper?
Answer and reasoning · 答案与解析
0.4 mol
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Electrode signs depend on the cell type. In an electrolytic cell the cathode is negative; reduction remains the defining process at a cathode in every cell.
4 · Chemical tests and analytical confidence
chromatography · Rf
edexcel_igcse_chemistry:analysis:open:1 · 2 marks
Why use pencil for a chromatography baseline?
- Graphite is less likely to dissolve and move with the solvent
- Pencil automatically identifies ions
- Ink is always invisible
Answer and reasoning · 答案与解析
Graphite is less likely to dissolve and move with the solvent
- Rf is distance travelled by a component divided by distance travelled by the solvent front, both measured from the baseline. Compare under the same conditions; an Rf value alone does not establish identity across different solvents.
edexcel_igcse_chemistry:analysis:open:2 · 4 marks
A spot travels 4.2 cm while the solvent front travels 7.0 cm. Find Rf.
Answer and reasoning · 答案与解析
0.6
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
A single spot can conceal substances that co-elute. Ink on the baseline can dissolve and create extra spots.
5 · Calorimetry and chemical energy
exothermic · enthalpy change
edexcel_igcse_chemistry:energy:open:1 · 2 marks
Which process requires energy?
- Breaking a covalent bond
- Forming every chemical bond
- Cooling a hot solution
Answer and reasoning · 答案与解析
Breaking a covalent bond
- Calculate energy transferred from the mass, specific heat capacity and temperature rise when these are given. At GCSE, use reaction profiles and bond-energy differences where your tier specifies them; this lesson does not introduce advanced molar enthalpy calculations.
edexcel_igcse_chemistry:energy:open:2 · 4 marks
50 g water rises 4 °C. Use c = 4.2 J per gram per degree to find q.
Answer and reasoning · 答案与解析
840 J
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Heat loss usually lowers the observed temperature rise. The solution gaining heat and the reaction losing heat have opposite signs.
6 · Rates, catalysts and reliable endpoints
activation energy · rate
edexcel_igcse_chemistry:rates:open:1 · 2 marks
Two reactions reach the same final gas volume. Which can still differ?
- Their rates
- The balanced equation must be different
- The product must have different molar mass
Answer and reasoning · 答案与解析
Their rates
- A product-time graph has a steeper gradient where rate is larger. A tangent estimates instantaneous rate; a secant gives average rate over an interval. The final plateau reflects the total collected product under the stated conditions.
edexcel_igcse_chemistry:rates:open:2 · 4 marks
A reaction makes 24 cubic centimetres of gas in 40 s. Find average rate.
Answer and reasoning · 答案与解析
0.6 cm³/s
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
A catalyst does not change the equilibrium constant at a fixed temperature. A mass-loss method cannot detect all reactions, and losing gas through a leak biases a collection experiment.
7 · Equilibrium and changing conditions
equilibrium · reversible reaction
edexcel_igcse_chemistry:equilibrium:open:1 · 2 marks
What is equal at dynamic equilibrium?
- Concentrations of all species
- Forward and reverse reaction rates
- Masses of every reactant and product
Answer and reasoning · 答案与解析
Forward and reverse reaction rates
- Changing conditions can change the equilibrium composition. A catalyst speeds both directions and shortens the time to equilibrium; it does not change the final equilibrium composition at a fixed temperature.
edexcel_igcse_chemistry:equilibrium:open:2 · 4 marks
In one second there are 12 forward events and 12 reverse events. Find the net product-count change.
Answer and reasoning · 答案与解析
0
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Do not use a catalyst to claim a larger equilibrium yield. For heterogeneous equilibria, pure solids are omitted from the usual equilibrium expression.
8 · Organic structures and reaction pathways
functional group · isomer
edexcel_igcse_chemistry:organic:open:1 · 2 marks
What happens to the carbon skeleton when bromine adds to ethene?
- It remains two carbons long
- It becomes one carbon long
- It becomes a six-carbon ring
Answer and reasoning · 答案与解析
It remains two carbons long
- Distinguish addition, substitution, oxidation and polymerization by tracing bonds before and after reaction. Conditions and reagents belong to the reaction arrow; they are not interchangeable labels.
edexcel_igcse_chemistry:organic:open:2 · 4 marks
How many moles of bromine react completely with 0.15 mol ethene?
Answer and reasoning · 答案与解析
0.15 mol
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Bromine decolourization provides evidence of unsaturation in an appropriate test. It is not proof that an unknown sample is specifically ethene.