JAMB UTME Chemistry
Past Questions

AI Explanation

**The reasoning** Let's decode the notation ⁴⁰₂₀Ca. The **top number (40)** is the **mass number (A)** — total protons + neutrons. The **bottom number (20)** is the **atomic number (Z)** — number of protons (which defines the element). The principle: **Number of neutrons = Mass number − Atomic number** So: Neutrons = 40 − 20 = **20 neutrons** Think of it like this: the atom's total "mass participants" (40) minus the protons (20) leaves you with neutrons (20). **Why the wrong options tempt you** - **B (40)**: You might pick the mass number directly, forgetting it's the *sum* of protons and neutrons, not neutrons alone. - **C (60)**: This trap catches students who *add* 40 + 20 instead of subtracting. - **D (10)**: Perhaps you divided 20 by 2 randomly, but there's no such rule here. **Quick takeaway** Neutrons are the "leftovers" after you remove protons from the mass number: **N = A − Z**. Always subtract the bottom from the top!

AI Explanation

## The reasoning pH measures how acidic or basic a solution is, and it's calculated using this formula: **pH = −log₁₀[H⁺]** Where [H⁺] is the hydrogen ion concentration. Given: [H⁺] = 1 × 10⁻⁴ mol/dm³ Substituting into the formula: pH = −log₁₀(1 × 10⁻⁴) pH = −(−4) *(because log₁₀ of 10⁻⁴ = −4)* **pH = 4** The negative sign in the formula flips that −4 to positive 4. ## Why the wrong options tempt you **Option A (2):** You might mistakenly square the exponent or confuse this with a different concentration like 10⁻². **Option C (10) & D (14):** These come from forgetting the negative sign or mixing up with pOH calculations. Remember, pH + pOH = 14, but that's a different calculation. Also, 10 and 14 would indicate basic solutions, but with lots of H⁺ ions, this solution is clearly acidic. ## Quick takeaway **Count the zeros after the decimal in standard form — that's your pH!** For 10⁻⁴, the exponent 4 *is* the pH. Simple!

AI Explanation

**The reasoning** An **alkene** is a hydrocarbon with at least one **carbon-carbon double bond (C=C)**. Alkenes follow the general formula **CₙH₂ₙ**. Let's check each option: - **A) CH₄** → C₁H₄ = C₁H₂(₁) ✓ But this is **methane**, an alkane (single bonds only) - **B) C₂H₆** → C₂H₂(₂)+₂ = C₂H₆ — This fits **alkanes (CₙH₂ₙ₊₂)**, so it's ethane - **C) C₃H₆** → C₃H₂(₃) = C₃H₆ ✓ This fits **CₙH₂ₙ** perfectly! This is **propene**, an alkene - **D) C₂H₂** → C₂H₂(₂)−₂ = C₂H₂ — This fits **alkynes (CₙH₂ₙ₋₂)**, so it's ethyne (acetylene) with a triple bond **Why the wrong options tempt you** Without knowing the formulas, they all look like random hydrocarbon combinations. **CH₄** and **C₂H₆** are common alkanes you've heard of. **C₂H₂** looks simple and might seem like it has fewer hydrogens because of unsaturation—but it's a *triple* bond (alkyne), not a double bond. **Quick takeaway** Alkenes = **CₙH₂ₙ** (equal number ratio: C₃H₆, C₄H₈). Just count: if hydrogen = 2 × carbon, you've found your alkene!

AI Explanation

## The reasoning The maximum number of electrons in any shell follows the formula **2n²**, where n is the shell number. For the second shell (L shell), n = 2: Maximum electrons = 2n² = 2(2)² = 2(4) = **8 electrons** Think of it this way: the L shell has two subshells (2s and 2p). The 2s holds 2 electrons max, and the 2p holds 6 electrons max. Together: 2 + 6 = **8 electrons**. This is why elements like neon (Ne) with 10 total electrons has a completely filled L shell with exactly 8 electrons (2 in the K shell, 8 in the L shell). ## Why the wrong options tempt you **A) 2** — That's the maximum for the *first* shell (K shell) only, where n = 1. **B) 6** — That's just the p-subshell capacity! You forgot to add the s-subshell's 2 electrons. **D) 18** — That's the *third* shell (M shell) where n = 3: 2(3)² = 18. You jumped ahead one shell. ## Quick takeaway **Remember "2n²" — for the second shell, that's 2(2²) = 8 electrons maximum, always.**

AI Explanation

**The reasoning** Isotopes are *versions* of the same element that differ only in their neutron count. What makes an element what it is? Its **atomic number** — the number of protons in the nucleus. Carbon is carbon because it has 6 protons. Whether it's Carbon-12, Carbon-13, or Carbon-14, they ALL have 6 protons. Change the proton number, and you've got a completely different element (like Nitrogen with 7 protons). - Same protons? Same element, just different isotopes. - Different neutrons? That's what creates the different isotopes. **Why the wrong options tempt you** **A & D)** These are literally what *change* between isotopes! Carbon-12 has 6 neutrons; Carbon-14 has 8 neutrons. Different neutron counts mean different atomic masses. **B)** Mass number = protons + neutrons. Since neutrons differ, mass numbers differ too (12 vs. 14). **Quick takeaway** Isotopes = same house (protons), different furniture (neutrons). The proton count defines the element; neutrons just add weight. **Protons = identity; neutrons = variety.**

AI Explanation

## The reasoning Sodium has 11 electrons (because Z = 11 means 11 protons, and atoms are neutral, so electrons = protons). We fill electron shells using the **2n² rule**: - **First shell** (n=1): Maximum 2 electrons → **2** - **Second shell** (n=2): Maximum 8 electrons → **8** - **Third shell** (n=3): Remaining electrons → **1** (since 2 + 8 + 1 = 11) So the configuration is **2,8,1**. This makes sodium highly reactive because that single electron in the outermost shell is easily lost to form Na⁺ ions (which is why sodium is in Group 1 of the periodic table!). ## Why the wrong options tempt you **B) 2,8,3** – You might confuse sodium with aluminum (Al, Z=13), which actually has 2,8,3. **C) 2,1,8** – This breaks the filling rule! You must completely fill inner shells before moving outward. The second shell can hold 8, so you can't leave it with just 1. **D) 1,8,2** – Same mistake—you can't put 8 electrons in the second shell before filling the first shell's 2 spots. ## Quick takeaway **Fill shells in order: 2 → 8 → 8 → ... and never skip ahead!** Count electrons = atomic number, then distribute properly.

AI Explanation

**The reasoning** A **strong acid** is one that **completely ionizes** (breaks apart 100%) in water, releasing all its H⁺ ions. Hydrochloric acid (HCl) does exactly this: HCl → H⁺ + Cl⁻ (complete dissociation) When HCl dissolves, virtually every molecule splits, making it extremely acidic. This is why it's used in labs and even in your stomach for digestion! The others (acetic, carbonic, citric) are **weak acids** — they only partially ionize in water, so most molecules stay intact. For example, acetic acid (vinegar) releases maybe 1-5% of its H⁺ ions. **Why the wrong options tempt you** - **Acetic acid** (vinegar) tastes sour and seems strong, but it's actually weak - **Carbonic acid** (in soft drinks) fizzes aggressively, which feels "strong," but it barely ionizes - **Citric acid** (in oranges) has a sharp taste, fooling you into thinking it's strong The confusion: we mix up *corrosiveness* or *sourness* with *chemical strength*. They're different! **Quick takeaway** The common strong acids you MUST memorize: **HCl, H₂SO₄, HNO₃** — they ionize completely; everything else you meet in secondary school is likely weak.

AI Explanation

## The reasoning Think of litmus paper as a **color-coded messenger** that tells you whether a solution is acidic or basic. This is based on a simple chemical principle: **acid-base indicators change color depending on pH**. Litmus is a natural dye extracted from lichen. In an **acidic solution** (like lemon juice, vinegar, or dilute HCl), litmus paper turns **red**. In a **basic solution** (like soap water or NaOH), it turns **blue**. The memory trick: **"Acids Are Red"** — both start with 'A'. Simple! ## Why the wrong options tempt you **Blue (A)** — This is what happens in a *base*, not an acid. Students often confuse the two directions. **Green (C) & Black (D)** — These aren't litmus colors at all. They're just distractors playing on uncertainty. Litmus only goes between red and blue. ## Quick takeaway **Acids turn litmus RED, bases turn it BLUE** — remember "Acids Are Red" and you'll never mix them up again!

AI Explanation

## The reasoning The pH scale measures how acidic or basic (alkaline) a solution is, ranging from 0 to 14. It's based on the concentration of hydrogen ions (H⁺) in the solution. **A neutral solution means neither acidic nor basic** — it has equal amounts of H⁺ ions and OH⁻ ions. Pure water at 25°C is the perfect example. At this balance point, the pH is exactly **7**. Think of pH like a number line: - pH < 7 = **acidic** (more H⁺ ions) — like lemon juice, vinegar - pH = 7 = **neutral** (balanced) — like pure water - pH > 7 = **basic/alkaline** (more OH⁻ ions) — like soap, bleach ## Why the wrong options tempt you **A (pH 1)** — This is *strongly acidic* (like battery acid). Students might confuse "starting point" with "neutral point." **B (pH 5)** — This is *weakly acidic* (like rain water). It's close to 7, so it can trick you if you're unsure. **D (pH 14)** — This is *strongly basic* (like drain cleaner). Some think extremes = neutral, but that's wrong. ## Quick takeaway **Seven is heaven — perfectly neutral, perfectly balanced, like pure water.**

AI Explanation

**The reasoning** Methane (CH₄) is an **alkane** because it follows the general formula CₙH₂ₙ₊₂ for alkanes. Let's check: if n = 1 (one carbon), then H = 2(1) + 2 = 4 hydrogens. Perfect match! Alkanes are hydrocarbons with **only single bonds** between carbon atoms. Look at methane's structure: one carbon connected to four hydrogens by single bonds (C–H). No double bonds, no triple bonds, no other functional groups. That's the signature of the alkane family (also called **saturated hydrocarbons** or **paraffins**). **Why the wrong options tempt you** - **Alkenes (B)** contain C=C double bonds and follow CₙH₂ₙ (example: ethene, C₂H₄). Methane has no double bond. - **Alkynes (C)** have C≡C triple bonds and follow CₙH₂ₙ₋₂ (example: ethyne, C₂H₂). Methane has no triple bond. - **Alcohols (D)** contain an –OH group (example: methanol, CH₃OH). Methane has no oxygen at all. **Quick takeaway** If every bond is single (C–C or C–H) and it fits CₙH₂ₙ₊₂, it's an alkane — remember: **All Single = Alkane**.

AI Explanation

**The reasoning** Ethanol's formula is C₂H₅OH. Breaking it down: C₂H₅ is the ethyl group (the carbon chain), and —OH is the functional group attached to it. The —OH group is called a **hydroxyl group**, and it defines **alcohols**. Any organic compound with —OH attached to a carbon chain is an alcohol. So ethanol = ethyl + alcohol = C₂H₅OH. Think of functional groups as the "identity cards" of organic compounds — they determine the compound's name and properties. **Why the wrong options tempt you** - **A) —COOH** (carboxyl group) defines carboxylic acids like ethanoic acid (CH₃COOH). Students mix this up because both have "OH" in them, but —COOH has a C=O attached. - **C) —CHO** (aldehyde group) is found in compounds like ethanal (CH₃CHO). The confusion comes from similar names (ethanol vs ethanal). - **D) —NH₂** (amino group) defines amines. Some guess this randomly without analyzing the formula. **Quick takeaway** Always look for the functional group *directly in the formula* — if you see —OH alone, it's an alcohol; the compound name will end in "-ol."

AI Explanation

## The reasoning Alkenes are hydrocarbons with **one carbon-carbon double bond** (C=C). This double bond is the key to their formula. Start with alkanes (single bonds only): their formula is CₙH₂ₙ₊₂. For example, ethane is C₂H₆ (which is C₂H₂₍₂₎₊₂). Now, when you introduce a double bond, you **remove two hydrogen atoms** because two carbons now share four electrons instead of two. So from CₙH₂ₙ₊₂, subtract 2H → **CₙH₂ₙ**. Check it: Ethene (simplest alkene) = C₂H₄ (fits C₂H₂₍₂₎ ✓). Propene = C₃H₆ (fits C₃H₂₍₃₎ ✓). ## Why the wrong options tempt you **A) CₙH₂ₙ₊₂** — This is alkanes (no double bonds), like petrol or cooking gas. **C) CₙH₂ₙ₋₂** — This is alkynes (triple bond C≡C), like the gas in welding torches. One more degree of unsaturation means two more H atoms lost. **D) CₙH₂ₙ₋₁** — This doesn't match any major hydrocarbon family. It's a distractor. ## Quick takeaway **Each double bond costs you 2 hydrogens**: alkanes are CₙH₂ₙ₊₂, alkenes lose 2H → CₙH₂ₙ, alkynes lose another 2H → CₙH₂ₙ₋₂.

AI Explanation

**The reasoning** In electrolysis of water, we're using electricity to split H₂O molecules. Here's the key principle: **cathodes attract cations (positive ions)**. The reaction: 2H₂O → 2H₂ + O₂ Water contains H⁺ ions (from H₂O dissociation). At the **cathode (negative electrode)**, these H⁺ ions are attracted and gain electrons: 2H⁺ + 2e⁻ → H₂ ↑ At the **anode (positive electrode)**, OH⁻ ions lose electrons: 4OH⁻ → O₂ + 2H₂O + 4e⁻ So **hydrogen gas bubbles at the cathode**, oxygen at the anode. **Why the wrong options tempt you** **A) Oxygen** — This is the biggest trap! Oxygen *is* produced, but at the anode (positive electrode), not the cathode. Students mix up which gas goes where. **C) Chlorine & D) Nitrogen** — These aren't even in water's formula (H₂O), so they can't form. **Quick takeaway** Remember: **"Cats are positive"** — **Cat**hodes attract **positive** H⁺ ions, producing hydrogen gas. The anode gets oxygen because it attracts negative OH⁻ ions.

AI Explanation

**The reasoning** Think of "exo-" like "exit" — heat **exits** the system. In an exothermic reaction, chemical bonds break and form, and when the new bonds release more energy than was needed to break the old ones, that extra energy escapes as heat into the surroundings. Classic examples: burning wood, rusting iron, or mixing acid and water (the container gets hot, right?). The reaction vessel feels warm because heat is being **released** to your hand. **Why the wrong options tempt you** **A (Absorbs heat)** — This describes *endothermic* reactions (think "endo-" = "into"). Easy to mix up if you don't anchor the prefixes. **C (No temperature change)** — Sounds scientific, but reactions that don't release or absorb noticeable heat are rare and wouldn't be called "exothermic." **D (Requires light)** — This confuses reaction conditions with energy flow. Some reactions need light to *start* (like photosynthesis), but that doesn't define whether they release or absorb heat. **Quick takeaway** **Exo = Exit:** exothermic reactions release heat and warm their surroundings, just like how exhaling pushes air *out* of your body.

AI Explanation

**The reasoning** Carbon-12 is the **standard reference** for atomic mass. Scientists chose it as the benchmark against which all other atomic masses are measured. By international agreement, one atom of Carbon-12 is assigned a mass of **exactly 12 atomic mass units (amu)**. This is a definition, not a calculation. Think of it like this: just as we defined 1 meter or 1 kilogram as our measurement standards, we defined Carbon-12 = 12 amu as our atomic mass standard. Everything else (hydrogen ≈ 1, oxygen ≈ 16, etc.) is measured *relative to* this Carbon-12 standard. **Why the wrong options tempt you** - **Option A (1)**: You might confuse this with hydrogen, which has atomic mass ≈ 1, or think "relative" means starting from 1. - **Option B (6)**: This is carbon's *atomic number* (number of protons), not its atomic mass. Classic mix-up! - **Option D (14)**: This is Carbon-14, a different isotope used in radiocarbon dating. Wrong carbon! **Quick takeaway** Carbon-12 is the *definition* of atomic mass — it equals exactly 12 amu by international agreement, making it the ruler by which all other atoms are measured.

AI Explanation

**The reasoning** When carbon dioxide (CO₂) gas is bubbled through lime water (a clear solution of calcium hydroxide, Ca(OH)₂), a chemical reaction occurs: **CO₂ + Ca(OH)₂ → CaCO₃ + H₂O** The product, calcium carbonate (CaCO₃), is **insoluble** — it forms tiny white particles suspended in the water, making it appear milky or cloudy. This is a classic test for CO₂ in the lab. Think of it like chalk dust in water! **Why the wrong options tempt you** - **Oxygen** is what we breathe and supports burning, but it's chemically unreactive with lime water. - **Nitrogen** makes up 78% of air but is extremely inert — it won't react with lime water at all. - **Hydrogen** burns with a "pop" sound (that's its test!), but doesn't interact with lime water either. These gases might sound scientific, but only CO₂ produces that telltale milky precipitate. **Quick takeaway** Carbon dioxide turns lime water milky because it forms insoluble calcium carbonate — this is THE definitive test for CO₂ you'll see in WAEC/JAMB practicals!

AI Explanation

**The reasoning** The formula connecting mass, moles, and molar mass is: **Number of moles (n) = Mass (m) / Molar mass (M)** You're given: - Mass of water = 36 g - Molar mass of H₂O = 18 g/mol Substituting into the formula: n = 36 g ÷ 18 g/mol = **2 moles** This principle is called the **mole concept** — it's the bridge between the mass you can weigh and the number of particles (atoms/molecules) you're working with. **Why the wrong options tempt you** - **Option A (0.5)**: You might flip the formula and do 18÷36 instead of 36÷18. Always keep "what you have ÷ what one mole weighs." - **Option B (1)**: Possibly confusing the molar mass (18) with the answer, or miscalculating. - **Option D (4)**: Maybe doubling 2 accidentally, or multiplying instead of dividing. **Quick takeaway** "To find moles, always divide the mass you have by the molar mass — think of it as 'how many 18 g portions fit into 36 g?'"

AI Explanation

## The reasoning The atmosphere is like a mixture of gases wrapped around Earth. By volume, **nitrogen makes up about 78%** of the air we breathe, while oxygen is only about 21%. The remaining 1% includes argon, carbon dioxide, and trace gases. Think of it this way: if you had 100 bottles of air, 78 would be pure nitrogen, 21 would be oxygen, and just 1 would contain everything else combined. Nitrogen is chemically stable and doesn't react easily, so it dominates our atmosphere. ## Why the wrong options tempt you **Oxygen (A)** — We breathe oxygen to survive, so our brains trick us into thinking it must be the most common. But being essential ≠ being most abundant. **Carbon dioxide (C)** — Always in the news about climate change, making it *feel* important and widespread. Actually, it's only about 0.04% of the atmosphere. **Argon (D)** — A noble gas that makes up roughly 0.93% — significant compared to CO₂, but nowhere near nitrogen's 78%. ## Quick takeaway **Remember "78-21-1": Nitrogen dominates at 78%, oxygen supports life at 21%, and everything else shares the last 1%.**

AI Explanation

**The reasoning** The **atomic number** is simply the number of protons in an atom's nucleus. It's what defines the element itself — it's like the element's ID number on the periodic table. Carbon always has **6 protons** in its nucleus. This never changes. If an atom has 6 protons, it's carbon. If it has a different number, it's a different element entirely. So carbon's atomic number = **6**. This is a fundamental definition you need to memorize: Carbon = 6 protons = atomic number 6. **Why the wrong options tempt you** - **Option A (4):** This might confuse you if you're thinking of carbon's valency (4 bonds it can form), but valency ≠ atomic number. - **Option C (8):** You might mix this up with oxygen's atomic number (8). - **Option D (12):** This is carbon-12's **mass number** (protons + neutrons = 6 + 6 = 12). Mass number and atomic number are different! The atomic number is always the smaller number and appears at the bottom left of the element symbol. **Quick takeaway** Atomic number = number of protons = the element's permanent identity; for carbon, it's always 6.

AI Explanation

**The reasoning** The maximum number of electrons in any shell follows the formula **2n²**, where n is the shell number. For the second shell (L-shell), n = 2: - Maximum electrons = 2n² = 2(2)² = 2 × 4 = **8 electrons** Think of it this way: the L-shell has two subshells — 2s (holds 2 electrons) and 2p (holds 6 electrons). Total = 2 + 6 = 8. **Why the wrong options tempt you** - **A) 2** — This is the K-shell (first shell) capacity, or just the 2s subshell alone. Easy mix-up! - **B) 6** — This is only the 2p subshell. You forgot to add the 2s electrons. - **D) 18** — This is the third shell (M-shell) capacity using 2(3)² = 18. You jumped ahead one shell. **Quick takeaway** Remember: **2n² is your best friend** — K-shell (n=1) holds 2, L-shell (n=2) holds 8, M-shell (n=3) holds 18. Master this formula and you'll never miss electron configuration questions again!

AI Explanation

**The reasoning** Isotopes are atoms of the **same element** but with different numbers of neutrons. The key word here is "same element." What defines an element? Its **atomic number** — which is the number of protons. For example, Carbon-12 and Carbon-14 are both carbon because they both have **6 protons**. But Carbon-12 has 6 neutrons while Carbon-14 has 8 neutrons. Same element, different masses. The principle: **The number of protons determines the element's identity.** Change the protons, you get a different element entirely! **Why the wrong options tempt you** - **A) Neutrons** — This is the opposite! Isotopes differ specifically because they have different numbers of neutrons. - **C) Mass number** — Mass number = protons + neutrons. Since neutrons differ, mass numbers must differ too (12 vs 14 in our carbon example). - **D) Atomic mass** — Same trap as C. Different neutrons = different total mass. **Quick takeaway** Isotopes are like siblings from the same family (same protons/element) but with different weights (different neutrons) — the family name (element) never changes!

AI Explanation

**The reasoning** Sodium (Na) has 11 electrons that need to be arranged in shells around the nucleus. Remember the **2-8-8 rule** (also called the octet rule): - First shell (K) holds maximum **2 electrons** - Second shell (L) holds maximum **8 electrons** - Third shell (M) holds maximum **8 electrons** So we fill them in order: - K shell: 2 electrons (9 remaining) - L shell: 8 electrons (1 remaining) - M shell: 1 electron (0 remaining) **Electronic configuration = 2,8,1** **Why the wrong options tempt you** **B) 2,8,3** — You might miscount or confuse sodium with aluminum (Al, Z=13), which is actually 2,8,3. **C) 2,1,8** — This breaks the filling rule! You can't leave the second shell half-empty and jump to fill the third shell. Shells fill sequentially from innermost to outermost. **D) 1,8,2** — This violates the first shell rule. The K shell MUST be filled first with its maximum of 2 electrons before moving outward. **Quick takeaway** Fill electron shells like filling seats on a bus: **complete the front rows (inner shells) before moving to back rows (outer shells)** — always 2, then 8, then 8.

AI Explanation

**The reasoning** A **strong acid** is one that completely dissociates (breaks apart 100%) into ions when dissolved in water. Hydrochloric acid (HCl) does exactly this: HCl → H⁺ + Cl⁻ (complete dissociation) When HCl hits water, virtually every single molecule splits into hydrogen ions and chloride ions. This complete ionization is what makes it "strong." Other common strong acids you should know: H₂SO₄ (sulfuric), HNO₃ (nitric), HBr, HI. **Why the wrong options tempt you** All the other options are **weak acids** — they only partially dissociate in water: - **Acetic acid** (CH₃COOH, found in vinegar) — only about 1% breaks apart - **Carbonic acid** (H₂CO₃, in soft drinks) — very weak, mostly stays intact - **Citric acid** (in oranges/lemons) — weak despite tasting sour The trap: students confuse "sour taste" or "corrosive" with "strong acid." Lemon juice tastes very sour but contains weak citric acid! **Quick takeaway** Strong ≠ concentrated or sour; strong means **complete ionization** — and you only need to memorize about 6 strong acids (HCl, H₂SO₄, HNO₃, HBr, HI, HClO₄).

AI Explanation

**The reasoning** The pH scale measures how acidic or basic (alkaline) a solution is, ranging from 0 to 14. A **neutral solution** is neither acidic nor basic — it's perfectly balanced. Pure water at 25°C is the classic example. In neutral water, the concentration of H⁺ ions equals the concentration of OH⁻ ions. Mathematically: [H⁺] = [OH⁻] = 1 × 10⁻⁷ M Since pH = −log[H⁺], we get: pH = −log(1 × 10⁻⁷) = 7 So pH 7 is neutral. Anything below 7 is acidic (more H⁺ ions), anything above 7 is basic (more OH⁻ ions). **Why the wrong options tempt you** - **pH 1** is *extremely acidic* — like battery acid. Students sometimes confuse "starting point" with "neutral point." - **pH 5** is *weakly acidic* (like black coffee or rain water). It's closer to neutral but not quite there. - **pH 14** is *extremely basic* — like drain cleaner. The high number might seem "balanced" but it's the opposite extreme from pH 1. **Quick takeaway** Remember: **7 is heaven** — perfectly neutral, like pure water at room temperature.

AI Explanation

**The reasoning** Litmus is a natural indicator extracted from lichens. It's designed to show us whether a solution is acidic or basic by changing color. The rule is simple and must be memorized: - **Blue litmus → turns RED in acid** - **Red litmus → turns BLUE in base (alkali)** So when you dip blue litmus paper into an acidic solution (like lemon juice, vinegar, or dilute HCl), it turns **red**. This is your test for acidity. **Why the wrong options tempt you** **A) Blue** — This is what litmus turns in a *base*, not an acid. Students often mix up the two because both involve litmus, but the colors are opposite. **C) Green & D) Black** — These aren't litmus colors at all. You might be thinking of other indicators like universal indicator (which shows green at neutral pH), but litmus only switches between red and blue. **Quick takeaway** **"Acid = Red litmus"** — think of the 'A' in **A**cid and the 'R' in **R**ed. Lock this into your memory and you'll never confuse it again!