Chemistry Study Guide
Core chemistry concepts for GCSE, IGCSE, O Level, WAEC, JAMB, and A-Level students. Each topic includes clear explanations and a sample exam question with a model answer.
Atomic Structure
The atom is the basic unit of matter, made up of a nucleus containing protons and neutrons, surrounded by electrons arranged in shells.
Sample Exam Question: An atom of chlorine has atomic number 17 and mass number 35. How many protons, neutrons, and electrons does it have?
Answer: 17 protons, 18 neutrons (35 − 17), 17 electrons (equal to protons in a neutral atom).
Chemical Bonding
Atoms bond together to form compounds. The three main types of chemical bonding are ionic, covalent, and metallic bonding.
Sample Exam Question: Explain why sodium chloride has a high melting point but conducts electricity when dissolved in water.
Answer: NaCl has a giant ionic lattice with strong electrostatic forces requiring large energy to break (high melting point). When dissolved, the ions are free to move and carry charge (conducts electricity).
Chemical Reactions & Equations
Chemical reactions involve the rearrangement of atoms. Reactants are converted into products, and the total mass is conserved.
Sample Exam Question: Balance the equation: Mg + HCl → MgCl₂ + H₂
Answer: Mg + 2HCl → MgCl₂ + H₂ (need 2 HCl to provide 2 Cl for MgCl₂ and 2 H for H₂)
Acids, Bases & Salts
Acids produce H⁺ ions in solution and have pH below 7. Bases neutralise acids; alkalis are soluble bases with pH above 7.
Sample Exam Question: Write the word equation for sulfuric acid reacting with calcium hydroxide. Name the salt produced.
Answer: Sulfuric acid + calcium hydroxide → calcium sulfate + water. The salt is calcium sulfate.
Rates of Reaction
The rate of a chemical reaction can be increased by raising temperature, increasing concentration, using a catalyst, or increasing surface area.
Sample Exam Question: Describe two ways to increase the rate of marble chips reacting with hydrochloric acid, explaining each in terms of collision theory.
Answer: (1) Increase HCl concentration — more acid particles per unit volume, more frequent collisions. (2) Crush marble to powder — increases surface area, more particles exposed to the acid, increasing collision frequency.
The Periodic Table
Elements are arranged by increasing atomic number. Vertical columns are groups (similar properties); horizontal rows are periods.
Sample Exam Question: Potassium (K, Group 1, Period 4) is more reactive than sodium (Na, Group 1, Period 3). Explain why.
Answer: Potassium's outer electron is in a shell further from the nucleus with more electron shielding, so nuclear attraction on the outer electron is weaker. Potassium loses its outer electron more easily → greater reactivity.
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