WAEC Chemistry
Past Questions

AI Explanation

## The reasoning Think of an atom like a building: the **nucleus** is the central core (like the foundation), and electrons orbit around it (like people moving around outside). The nucleus contains two particles: - **Protons** — positively charged (+) - **Neutrons** — no charge (neutral) **Electrons** are negatively charged (−) and they orbit *outside* the nucleus in electron shells. They're never *in* the nucleus itself. This is the fundamental structure of every atom. The principle here is **atomic structure** — understanding where each subatomic particle lives. ## Why the wrong options tempt you **A) Electron only** — Tricks you if you forget electrons orbit *outside* the nucleus, not inside it. **B) Proton only** — Makes you overlook neutrons, which are also in the nucleus but often forgotten because they have no charge. **D) Electron and proton** — Sneaky! It correctly includes protons but wrongly puts electrons in the nucleus. ## Quick takeaway **"The nucleus = protons + neutrons; electrons circle outside."** Remember: the nucleus is the heavy center, electrons are the light orbiting particles.

AI Explanation

**The reasoning** When a gas transforms **directly** into a solid *without* passing through the liquid phase, we call this **deposition**. Think of it like this: the gas molecules slow down so much (usually due to cooling) that they arrange themselves into a solid structure immediately. A real-world example? Frost forming on your car windscreen on a cold harmattan morning! Water vapor in the air goes straight to ice crystals (solid) without becoming liquid water first. The key principle: **Phase transitions have specific names based on direction**: - Solid → Gas = Sublimation - **Gas → Solid = Deposition** ✓ **Why the wrong options tempt you** - **Sublimation (C)** is the reverse process (solid → gas), like camphor balls disappearing. Many students mix these two up because sublimation is more commonly discussed. - **Evaporation (A)** is liquid → gas (water drying from clothes). - **Condensation (B)** is gas → liquid (dew drops on grass in the morning). **Quick takeaway** Remember: "**Deposition deposits a solid from a gas**" — the word itself hints that something is being *deposited* or laid down in solid form.

AI Explanation

**The reasoning** When a substance changes **directly from gas to solid** without passing through the liquid state, this process is called **deposition**. Think of frost forming on a cold morning — water vapor in the air goes straight to ice crystals on surfaces, skipping the liquid water stage entirely. The principle here is understanding phase transitions: deposition is the reverse of sublimation (solid → gas). **Why the wrong options tempt you** - **Evaporation (A)** is liquid → gas, like water drying from clothes - **Condensation (B)** is gas → liquid, like water droplets forming on a cold bottle of Coke from the fridge - **Sublimation (C)** is the *opposite* direction: solid → gas directly (like camphor or dry ice disappearing without melting). This is the biggest trap because sublimation involves gas and solid, but in the wrong direction! **Quick takeaway** **Deposition = gas deposits as solid** (think frost on grass); sublimation goes the other way. Remember: "DEPosition = DEscending from gas DOWN to solid."

AI Explanation

**The reasoning** Common salt is the everyday substance we sprinkle on our jollof rice and use to cook soup. Its chemical name is **sodium chloride**, which tells us it contains sodium (Na) and chlorine (Cl). When these two elements combine, sodium donates one electron to chlorine, forming an **ionic bond**. Since sodium has a +1 charge and chlorine has a -1 charge, they combine in a 1:1 ratio, giving us **NaCl**. **Why the wrong options tempt you** - **KCl** (potassium chloride) looks similar because potassium is also in Group 1 like sodium, but it's used as a salt substitute, not our everyday table salt. - **CaCl₂** (calcium chloride) might confuse you because calcium is common (in bones, chalk), but notice the subscript 2 — calcium needs two chlorine atoms. - **MgCl₂** (magnesium chloride) has the same subscript issue; magnesium also needs two chlorines to balance its +2 charge. **Quick takeaway** Common salt = NaCl because **"Na" sounds like "nah" as in "nah, I need salt!"** — and it's the only option with a simple 1:1 ratio of metal to chlorine.

AI Explanation

**The reasoning** Let's break down common alloys by their base metals: **Bronze** = Copper + Tin. This is the classic combination used for thousands of years to make tools, weapons, and sculptures. The tin makes copper harder and more durable. **Brass** = Copper + Zinc (not tin!). It's yellowish and used in musical instruments and door fittings. **Steel** = Iron + Carbon. No copper at all. **Solder** = Typically Lead + Tin (older types) or Tin + other metals. Used for joining metals together. The question specifically asks for copper + tin, which is **Bronze**. **Why the wrong options tempt you** - **Brass** looks similar to bronze and also contains copper, but it's copper + *zinc*, not tin - **Steel** is super common, so students might guess it without thinking about the metals involved - **Solder** contains tin (half correct!), but it's usually with lead, not copper **Quick takeaway** Remember: **Bronze = Copper + Tin** (both end in 'n' if that helps!). Brass swaps the tin for zinc. This distinction appears in Chemistry, Physics, and even some Technical Drawing questions!

AI Explanation

**The reasoning** One mole of water (H₂O) contains Avogadro's number of *molecules* = 6.02 × 10²³ molecules. But the question asks for *atoms*, not molecules! Each water molecule has **3 atoms total**: 2 hydrogen atoms + 1 oxygen atom. So: Total atoms = 6.02 × 10²³ molecules × 3 atoms/molecule = **3 × 6.02 × 10²³ atoms** This is the principle of **molecular composition** meeting Avogadro's number. Always check whether they're asking for molecules or atoms! **Why the wrong options tempt you** **A** tricks you if you forget water is H₂O and just think "1 mole = 6.02 × 10²³" without counting atoms per molecule. **B** is what you'd get if you calculated it out (1.806 × 10²⁴), but they want the answer in factored form. **D** is completely wrong—probably confusing atomic mass concepts with atom counting. **Quick takeaway** One mole gives you 6.02 × 10²³ *molecules*; multiply by the number of atoms *per molecule* to get total atoms—for H₂O, that's always ×3!

AI Explanation

**The reasoning** When we collect hydrogen gas in the lab, we use the **water displacement method**. Here's how it works: You produce hydrogen (say, by reacting zinc with dilute acid), and the gas travels through a tube into an inverted container filled with water. As hydrogen bubbles in, it pushes the water down and out, taking its place at the top. This is called "displacement" because the gas literally *displaces* (pushes away) the water. We use this method because hydrogen is virtually insoluble in water and less dense than water, so it rises and collects at the top of the container without dissolving. **Why the wrong options tempt you** - **Upward/Downward** sound right because hydrogen does rise upward (it's lighter than air), but these describe the *direction* hydrogen moves, not the *collection method*. - **Diffusion** is how gases mix and spread out naturally — that's a process, not a collection technique. **Quick takeaway** "Displacement" means the gas pushes water out of the way to take its place — that's how we trap and collect gases like hydrogen over water.

AI Explanation

**The reasoning** A **mixture** contains two or more substances that are physically combined but not chemically bonded — think sand and salt, or beans and rice. Since no chemical bonds hold them together, you can separate them using **physical methods** like: - Filtration (solid from liquid) - Evaporation (salt from water) - Distillation (pure water from salty water) - Magnetic separation (iron filings from sulfur) - Handpicking (stones from rice) Notice: you're not breaking or forming chemical bonds. You're just using differences in physical properties (size, boiling point, magnetism, solubility) to pull components apart. **Why the wrong options tempt you** **A) Reaction** — This creates *new* substances through chemical change. That's not separation; that's transformation. **C) Heat alone** — Heat *can* help (like in evaporation), but it's not enough by itself for all mixtures. You need a complete physical method. **D) Catalysis** — Catalysts speed up *chemical reactions*, not separation processes. **Quick takeaway** Mixtures = physical combination → physical separation. No bonds broken, just properties exploited. Remember: garri and groundnut mixed? Just pick them apart! 🥜

AI Explanation

**The reasoning** When magnesium *burns*, it reacts with oxygen from the air. This is a **combustion reaction** (also called oxidation). The word "burning" is your key clue—it means reacting with O₂. The equation is simple: 2Mg + O₂ → 2MgO Magnesium metal combines directly with oxygen gas to produce magnesium oxide—a white powdery ash you'd see if you actually burned magnesium ribbon in the lab. That bright white light you see? That's MgO forming! **Why the wrong options tempt you** - **MgCl₂** forms when Mg reacts with *chlorine* or hydrochloric acid—not burning. - **Mg(OH)₂** forms when MgO reacts with *water*, or Mg reacts with water directly—not burning. - **MgSO₄** forms when Mg reacts with *sulfuric acid*—completely different reaction. All these are real magnesium compounds, but they require different reactants, not just burning in air. **Quick takeaway** **"Burning" always means reacting with oxygen from the air—so metals burning give you oxides (MgO, CaO, etc.), not chlorides, hydroxides, or sulfates.**

AI Explanation

**The reasoning** An **atom** is the smallest particle of an element that still retains the chemical identity of that element. Think of it like this: if you keep dividing a piece of iron into smaller and smaller bits, the tiniest piece you can get that's still iron is *one iron atom*. Go smaller (split the atom into protons, neutrons, electrons), and you no longer have iron — you have subatomic particles. This is the definition that sits at the foundation of chemistry: **Element → made of atoms**. **Why the wrong options tempt you** - **Molecule (A)**: This tricks you because molecules *are* small particles — but they're made of *two or more atoms* bonded together (like O₂ or H₂O). Not all elements exist as molecules naturally. - **Ion (C)**: An ion is just an atom that's gained or lost electrons. It's still an atom, just charged. Not smaller. - **Compound (D)**: A compound contains *different* elements combined (like NaCl). Way bigger than a single atom. **Quick takeaway** "Atom = smallest unit of an *element*; molecule = two or more atoms joined together."