Chemistry Made Easy

Section 1: Principles of Chemistry Exam Practice

Edexcel IGCSE Chemistry • Exam-style Practice Questions • Structured Questions

Exam Practice

Section 1: Principles of Chemistry Practice

Test your understanding of the Principles of Chemistry with exam-style questions. Attempt the multiple-choice section, then write your answers to the structured questions and compare them to the mark scheme.

Edexcel IGCSE Hub › Section 1 › Exam Practice

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📋 Structured Questions

These questions test key concepts from Section 1. Attempt each question on paper, then click "Show Mark Scheme" to check your answer.

Question 1: Gas Diffusion Experiment

4 marks

A cotton wool pad soaked in concentrated aqueous ammonia is placed at one end of a glass tube. Another pad soaked in concentrated hydrochloric acid is placed at the opposite end. After several minutes, a white solid forms inside the tube.

(a) State the chemical name and chemical formula of the white solid that forms. [2]

(b) Explain why the white solid forms closer to the end of the tube containing the hydrochloric acid. [2]

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(a)

  • Ammonium chloride [1]
  • NH4Cl [1]

(b)

  • Ammonia gas has a lower relative molecular mass (Mr = 17) than hydrogen chloride gas (Mr = 36.5), so ammonia diffuses faster [1]
  • Ammonia gas travels further along the tube than hydrogen chloride gas in the same amount of time [1]
Examiner tip: Ensure you write the name "ammonium chloride" and formula "NH4Cl" correctly. Do not confuse "ammonium" with "ammonia". For part (b), compare the relative molecular masses and diffuse speeds directly.

Question 2: Isotopes & Relative Atomic Mass

4 marks

(a) Explain what is meant by the term isotopes. Refer to subatomic particles in your answer. [2]

(b) A sample of chlorine is found to contain 75.0% of chlorine-35 and 25.0% of chlorine-37. Calculate the relative atomic mass (Ar) of this sample of chlorine. Show your working. [2]

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(a)

  • Atoms of the same element with the same number of protons / same atomic number [1]
  • But with a different number of neutrons / different mass number [1]

(b)

  • Working: [(35 x 75.0) + (37 x 25.0)] / 100 or 2625 + 925 / 100 [1]
  • Ar = 35.5 [1]
Examiner tip: In part (a), you must mention both protons and neutrons. In part (b), always show your full working to secure the calculation mark even if you make a simple calculator error.

Question 3: Bonding & Electrical Conductivity

4 marks

(a) Explain why sodium chloride conducts electricity when molten or dissolved in water, but ethene (C2H4) does not conduct electricity under any conditions. [4]

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  • Sodium chloride is an ionic compound with a giant ionic lattice, containing charged ions [1]
  • When molten or dissolved, these ions are free to move and carry electrical charge [1]
  • Ethene is a simple molecular (covalent) substance [1]
  • It does not contain any free-moving ions or delocalised electrons to carry charge [1]
Examiner tip: Clearly distinguish between ionic and covalent structures. Use the word "ions" for sodium chloride, and specify that they are "free to move". For covalent substances like ethene, state that there are "no mobile ions or free/delocalised electrons". Do not say that covalent bonds break when conducting electricity.

Question 4: Decomposing Calcium Carbonate

3 marks

When heated strongly, solid calcium carbonate decomposes into solid calcium oxide and carbon dioxide gas:
CaCO3(s) → CaO(s) + CO2(g)

(a) Calculate the volume of carbon dioxide gas, in dm3, produced at room temperature and pressure (RTP) when 5.0 g of calcium carbonate is completely decomposed.
(Relative atomic masses: C = 12, O = 16, Ca = 40. Molar volume of any gas at RTP = 24 dm3)

Show Mark Scheme
  • Calculate Mr of CaCO3: 40 + 12 + (3 x 16) = 100 [1]
  • Calculate moles of CaCO3: 5.0 g / 100 = 0.050 moles [1]
  • Calculate volume of CO2 (1:1 mole ratio): 0.050 moles x 24 dm3 = 1.2 dm3 (or 1200 cm3) [1]
Examiner tip: Ensure you determine the Mr of the reactant first. Since the equation shows a 1:1 ratio between CaCO3 and CO2, the number of moles of CO2 produced is equal to the moles of CaCO3. Finally, multiply the moles of gas by 24 to get the volume in dm3.

Question 5: Electrolysis of Copper(II) Chloride

4 marks

(a) Aqueous copper(II) chloride is electrolysed using inert graphite electrodes. Describe the observation and name the product formed at each electrode. [4]

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At the negative electrode (cathode):

  • Observation: Brown or pink solid deposit formed [1]
  • Product: Copper [1]

At the positive electrode (anode):

  • Observation: Effervescence / bubbles of green-yellow gas (which bleaches damp blue litmus paper) [1]
  • Product: Chlorine [1]
Examiner tip: An observation is what you actually see (e.g. "bubbles of gas" or "brown solid"), whereas the product is the chemical identity (e.g. "chlorine gas" or "copper"). Do not write "bubbles of chlorine" as the observation, write "bubbles of gas".

Question 6: Comparing Structures

6 marks

(a) Compare the bonding, structure, and physical properties of sodium chloride and diamond. In your answer, explain why they both have high melting points but differ in their electrical conductivity. [6]

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Indicative content:

  • Bonding & Structure: Sodium chloride has ionic bonding in a giant ionic lattice. Diamond has covalent bonding in a giant tetrahedral covalent macromolecular structure.
  • High Melting Points: Both have high melting points because they require a large amount of energy to break strong chemical bonds. In sodium chloride, strong electrostatic attractions between oppositely charged ions must be broken. In diamond, many strong covalent bonds between carbon atoms must be broken.
  • Electrical Conductivity: Sodium chloride does not conduct when solid because the ions are fixed in position, but it does conduct when molten or dissolved in water because the ions are free to move and carry charge. Diamond does not conduct electricity under any conditions because all outer-shell electrons are localized in strong covalent bonds, meaning there are no free-moving electrons or ions.

Marking guidance:

  • 5 to 6 marks: Detailed comparisons of bonding, structure, and both physical properties (melting point and conductivity) are explained clearly. Explanations of both high melting points and conductivity differences are correct.
  • 3 to 4 marks: Explains bonding/structure for both substances and attempts to explain either melting point or conductivity differences.
  • 1 to 2 marks: Simple statements identifying the bonding or structures, or identifying that both have high melting points.
Examiner tip: Ensure you explicitly structure your answer into sections covering bonding/structure, melting point, and electrical conductivity. This makes it easier for the examiner to award full marks. Always refer to "ions" for sodium chloride and "electrons" for diamond.