Acids, Bases & Salts — Grade 8/9 Science Notes

📋 Table of Contents

Acids — Definitions & Examples
Bases — Definitions & Examples
Salts — Definitions & Examples
Types: Organic & Inorganic
The pH Scale
Indicators
Properties of Acids, Bases & Salts
Key Chemical Reactions
Uses in Daily Life
30 Objective Questions
20 Structured Questions

Acids

What is an Acid?

An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water. There are different ways scientists define acids:

Arrhenius Definition

An acid is a substance that produces H⁺ ions (protons) when dissolved in water.

Example: HCl → H⁺ + Cl⁻

Brønsted-Lowry Definition

An acid is a proton (H⁺) donor — it gives protons to another substance.

Simple Definition

Acids are sour-tasting, corrosive substances with a pH below 7.

Key Definition An acid is a substance that donates hydrogen ions (H⁺) to a solution, resulting in a pH less than 7. Acids have a sour taste, turn blue litmus red, and react with metals and bases.

Common Examples of Acids

Acid Name	Chemical Formula	Where Found	Strength
Hydrochloric acid	HCl	Stomach (gastric acid)	Strong
Sulfuric acid	H₂SO₄	Car batteries, fertilisers	Strong
Nitric acid	HNO₃	Explosives, fertilisers	Strong
Citric acid	C₆H₈O₇	Lemons, oranges	Weak
Ethanoic acid	CH₃COOH	Vinegar	Weak
Carbonic acid	H₂CO₃	Fizzy drinks, rainwater	Weak
Lactic acid	C₃H₆O₃	Sour milk, muscles during exercise	Weak
Tartaric acid	C₄H₆O₆	Grapes, baking powder	Weak

Remember! Strong acids completely dissociate (break apart) in water, releasing all their H⁺ ions. Weak acids only partially dissociate.

Bases

What is a Base?

A base is the chemical opposite of an acid. Bases accept hydrogen ions (H⁺) or produce hydroxide ions (OH⁻) in solution.

Key Definition A base is a substance that accepts hydrogen ions (H⁺) or donates hydroxide ions (OH⁻). Bases have a pH greater than 7, feel soapy or slippery to touch, and turn red litmus blue.

Alkalis vs. Bases: All alkalis are bases, but not all bases are alkalis. An alkali is specifically a base that dissolves in water to form a solution with pH > 7.

Common Examples of Bases

Base Name	Chemical Formula	Where Found	Strength
Sodium hydroxide	NaOH	Soap making, drain cleaners	Strong
Potassium hydroxide	KOH	Soap, batteries	Strong
Calcium hydroxide	Ca(OH)₂	Lime water, cement, whitewash	Moderate
Ammonia solution	NH₃(aq)	Cleaning products, fertilisers	Weak
Magnesium hydroxide	Mg(OH)₂	Milk of magnesia (antacid)	Weak
Sodium bicarbonate	NaHCO₃	Baking soda, antacids	Weak
Copper(II) oxide	CuO	Laboratory reagent	Insoluble

Salts

What is a Salt?

Key Definition A salt is an ionic compound formed when an acid reacts with a base (or alkali). The H⁺ from the acid is replaced by a metal ion or ammonium ion (NH₄⁺). Table salt (NaCl) is the most common example.

How Salts Are Named

A salt takes its name from the metal of the base and the acid used:

Hydrochloric acid → chlorides (e.g. sodium chloride, NaCl)
Sulfuric acid → sulfates (e.g. copper sulfate, CuSO₄)
Nitric acid → nitrates (e.g. potassium nitrate, KNO₃)
Carbonic acid → carbonates (e.g. calcium carbonate, CaCO₃)

Common Salts and Their Uses

Salt Name	Formula	Formed From	Use
Sodium chloride	NaCl	NaOH + HCl	Table salt, food preservation
Calcium carbonate	CaCO₃	Ca(OH)₂ + CO₂	Chalk, limestone, cement
Copper sulfate	CuSO₄	CuO + H₂SO₄	Fungicide, electroplating
Potassium nitrate	KNO₃	KOH + HNO₃	Fertiliser, gunpowder
Ammonium sulfate	(NH₄)₂SO₄	NH₃ + H₂SO₄	Fertiliser
Sodium nitrate	NaNO₃	NaOH + HNO₃	Fertiliser, food preservative
Calcium sulfate	CaSO₄	Ca(OH)₂ + H₂SO₄	Plaster of Paris
Zinc sulfate	ZnSO₄	Zn + H₂SO₄	Galvanising, medicine

Types: Organic & Inorganic

Organic Acids

Definition Organic acids contain carbon atoms and are produced naturally by living organisms. Most are weak acids. They typically contain the carboxyl group (–COOH).

Organic Acid	Formula	Natural Source	Use
Citric acid	C₆H₈O₇	Citrus fruits (lemons, oranges)	Food flavouring, preservative
Ethanoic acid (Acetic acid)	CH₃COOH	Vinegar (fermentation)	Food preservation, cleaning
Lactic acid	C₃H₆O₃	Sour milk, yoghurt, muscles	Food production, cosmetics
Tartaric acid	C₄H₆O₆	Grapes, tamarind	Baking powder, wine making
Malic acid	C₄H₆O₅	Apples, tomatoes	Food flavouring
Formic acid	HCOOH	Ant stings, nettles	Textile processing
Oxalic acid	C₂H₂O₄	Spinach, rhubarb	Cleaning agent, bleaching
Ascorbic acid	C₆H₈O₆	Vitamin C — fruits, vegetables	Nutrition, antioxidant

Inorganic (Mineral) Acids

Definition Inorganic acids do NOT contain carbon (except carbonic acid, H₂CO₃). They are usually strong acids derived from non-living sources (minerals). Also called mineral acids.

Inorganic Acid	Formula	Properties	Main Use
Hydrochloric acid	HCl	Strong, colourless, pungent gas/solution	Stomach acid, PVC production
Sulfuric acid	H₂SO₄	Strong, dense oily liquid, highly corrosive	Batteries, fertilisers, explosives
Nitric acid	HNO₃	Strong, colourless/yellow, oxidising	Fertilisers, explosives, dyes
Phosphoric acid	H₃PO₄	Moderate, used in food	Fertilisers, cola drinks, dental
Carbonic acid	H₂CO₃	Weak, unstable, formed in water + CO₂	Fizzy drinks, carbonation

Organic Bases

Definition Organic bases contain carbon and are found in living organisms. Many contain nitrogen atoms. They are generally weak bases.

Organic Base	Formula	Source	Use
Ammonia (organic form)	NH₃	Protein breakdown, fertilisers	Cleaning products
Aniline	C₆H₅NH₂	Synthetic	Dyes, polymers
Adenine / Guanine	–	DNA and RNA	Genetic code components
Caffeine	C₈H₁₀N₄O₂	Coffee, tea, cocoa	Stimulant
Nicotine	C₁₀H₁₄N₂	Tobacco leaves	Insecticide (historically)

Inorganic Bases

Definition Inorganic bases do not contain carbon. They are typically metal hydroxides or metal oxides. Many are strong bases.

Inorganic Base	Formula	Type	Use
Sodium hydroxide	NaOH	Metal hydroxide (strong)	Soap, paper, drain cleaners
Potassium hydroxide	KOH	Metal hydroxide (strong)	Soap, alkaline batteries
Calcium hydroxide	Ca(OH)₂	Metal hydroxide	Cement, soil treatment
Magnesium oxide	MgO	Metal oxide	Refractory material, antacid
Ammonia solution	NH₃(aq)	Alkali	Fertilisers, cleaning agents

The pH Scale

The pH scale measures how acidic or alkaline a solution is. It ranges from 0 to 14.

Figure 1: The pH Scale (0–14). Values below 7 are acidic, 7 is neutral, above 7 is alkaline.

pH Range	Classification	Ion Dominant	Example
0 – 2	Strongly Acidic	H⁺ ions	Stomach acid (pH 1–2), Battery acid
3 – 6	Weakly Acidic	H⁺ ions	Vinegar (pH 3), Lemon (pH 2–3)
7	Neutral	Equal H⁺ & OH⁻	Pure water, blood (≈7.4)
8 – 10	Weakly Alkaline	OH⁻ ions	Baking soda, sea water
11 – 14	Strongly Alkaline	OH⁻ ions	Bleach (pH 12), NaOH (pH 14)

Indicators

An indicator is a substance that changes colour depending on whether a solution is acidic or alkaline.

Figure 2: Colour changes of common indicators in acids and bases.

Natural indicators include red cabbage juice (turns red in acid, green in alkali), turmeric (stays yellow in acid, turns red in alkali), and hibiscus flower extract.

Properties of Acids, Bases & Salts

Properties of Acids

Physical Properties

Sour taste (e.g. lemon, vinegar)
pH less than 7
Turn blue litmus red
Conduct electricity (ionic)
Corrosive to skin and metals

Chemical Properties

React with metals → salt + hydrogen gas
React with bases → salt + water (neutralisation)
React with carbonates → salt + water + CO₂
React with metal oxides → salt + water
Ionise in water to release H⁺

Properties of Bases

Physical Properties

Bitter taste
Soapy / slippery feel
pH greater than 7
Turn red litmus blue
Conduct electricity (if soluble)

Chemical Properties

React with acids → salt + water
React with ammonium salts → salt + water + NH₃
Strong bases attack fats (saponification → soap)
Some react with metals (amphoteric metals)
Ionise in water to release OH⁻

Properties of Salts

Physical Properties

Generally crystalline solids
High melting/boiling points
Most are soluble in water
Many are coloured (e.g. CuSO₄ is blue)
Conduct electricity when dissolved or molten

Chemical Properties

Form from acid + base reactions
Can be acidic, alkaline, or neutral in solution
Some decompose on heating
React with other salts (double displacement)
Can form insoluble precipitates

Key Chemical Reactions

1. Neutralisation Reaction

When an acid and a base react together, they neutralise each other to form a salt and water.

Acid + Base → Salt + Water
HCl + NaOH → NaCl + H₂O
// Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water

2. Acid + Metal → Salt + Hydrogen Gas

Acid + Metal → Salt + H₂(g)
H₂SO₄ + Zn → ZnSO₄ + H₂↑
// Sulfuric acid + Zinc → Zinc sulfate + Hydrogen gas (↑ = gas evolved)

3. Acid + Carbonate → Salt + Water + Carbon Dioxide

Acid + Carbonate → Salt + H₂O + CO₂(g)
2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑

4. Acid + Metal Oxide → Salt + Water

Acid + Metal Oxide → Salt + Water
H₂SO₄ + CuO → CuSO₄ + H₂O

5. Preparation of Salts

Figure 3: Neutralisation — acid and base react to form salt and water.

Solubility Rules for Salts (Simple Guide)

Salt Group	Solubility	Exceptions
All nitrates (NO₃⁻)	Soluble	None
All chlorides (Cl⁻)	Soluble	AgCl, PbCl₂ (insoluble)
All sulfates (SO₄²⁻)	Soluble	BaSO₄, PbSO₄, CaSO₄ (insoluble)
Carbonates (CO₃²⁻)	Insoluble	Na₂CO₃, K₂CO₃, (NH₄)₂CO₃ (soluble)
Hydroxides (OH⁻)	Insoluble	NaOH, KOH, NH₄OH (soluble)

Uses in Daily Life

Uses of Acids

Acid	Industry / Area	Use
H₂SO₄ (Sulfuric acid)	Industry	Manufacturing fertilisers (ammonium sulfate), car batteries, detergents
HCl (Hydrochloric acid)	Food & Industry	Gastric digestion, cleaning metal surfaces (pickling), PVC production
HNO₃ (Nitric acid)	Agriculture	Making fertilisers (ammonium nitrate), explosives (TNT), dyes
CH₃COOH (Ethanoic acid)	Food	Vinegar — food preservation, condiment, cleaning
Citric acid	Food & Pharma	Flavouring, preservative, effervescent tablets
H₃PO₄ (Phosphoric acid)	Food & Agric	Cola beverages, fertiliser production
Carbonic acid	Beverages	Carbonation of fizzy drinks
Ascorbic acid (Vit C)	Health	Dietary supplement, antioxidant, preservative

Uses of Bases

Base	Industry / Area	Use
NaOH (Sodium hydroxide)	Industry	Making soap, paper, textiles, drain cleaners
Ca(OH)₂ (Calcium hydroxide)	Agriculture	Treating acidic soil (liming), making cement, whitewash
Mg(OH)₂ (Magnesium hydroxide)	Health	Antacid (milk of magnesia) to treat indigestion
NaHCO₃ (Sodium bicarbonate)	Cooking & Health	Baking soda (makes cakes rise), antacid, fire extinguisher
NH₃ (Ammonia)	Industry	Fertilisers, cleaning products, refrigerant
Al(OH)₃ (Aluminium hydroxide)	Health	Antacid tablets, water purification

Uses of Salts

Salt	Industry / Area	Use
NaCl (Sodium chloride)	Food & Industry	Table salt, food preservation, making chlorine & NaOH
CaCO₃ (Calcium carbonate)	Construction	Cement, glass, limestone, chalk, toothpaste
CuSO₄ (Copper sulfate)	Agriculture	Fungicide (Bordeaux mixture), electroplating
KNO₃ (Potassium nitrate)	Agriculture & Food	Fertiliser, food preservative, gunpowder
Na₂CO₃ (Sodium carbonate)	Industry	Glass making, water softening, detergents
CaSO₄ (Calcium sulfate)	Construction & Health	Plaster of Paris, setting bones, chalk
AgNO₃ (Silver nitrate)	Medicine & Photography	Antiseptic, photography film, staining
ZnSO₄ (Zinc sulfate)	Industry	Galvanising steel, wood preservative

30 Objective Test Questions

Multiple Choice Quiz Section A — 30 Questions

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Section A — Multiple Choice

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Q 01

Which of the following correctly defines an acid according to Arrhenius?

AA substance that accepts protons
BA substance that donates OH⁻ ions
CA substance that produces H⁺ ions in water
DA substance with a pH greater than 7

Q 02

What is the pH of a neutral solution?

Q 03

Which indicator turns red in acid and blue in alkali?

APhenolphthalein
BMethyl orange
CLitmus
DUniversal indicator

Q 04

Which of the following is a STRONG acid?

ACitric acid
BEthanoic acid
CCarbonic acid
DHydrochloric acid

Q 05

The reaction between an acid and a base produces:

AAcid + water
BSalt + water
CSalt + hydrogen gas
DBase + carbon dioxide

Q 06

Which salt is formed when hydrochloric acid reacts with sodium hydroxide?

ASodium sulfate
BSodium carbonate
CSodium chloride
DSodium nitrate

Q 07

Which of the following is an organic acid?

AHydrochloric acid
BSulfuric acid
CNitric acid
DCitric acid

Q 08

What gas is produced when an acid reacts with a metal?

AOxygen
BCarbon dioxide
CNitrogen
DHydrogen

Q 09

Vinegar contains which type of acid?

ACitric acid
BLactic acid
CEthanoic acid
DTartaric acid

Q 10

A solution has a pH of 2. This solution is:

AStrongly alkaline
BWeakly acidic
CStrongly acidic
DNeutral

Q 11

Which of the following bases is used as an antacid?

ANaOH
BMg(OH)₂
CKOH
DCa(OH)₂

Q 12

What does the term "alkali" specifically mean?

AAny acid with pH < 7
BA base that dissolves in water
CA neutral salt
DA metal oxide only

Q 13

Which gas is produced when an acid reacts with a carbonate?

AHydrogen
BOxygen
CCarbon dioxide
DSulfur dioxide

Q 14

Sodium chloride (NaCl) is a salt formed from:

AHCl + KOH
BH₂SO₄ + NaOH
CHCl + NaOH
DHNO₃ + NaCl

Q 15

The colour of phenolphthalein indicator in an alkaline solution is:

AYellow
BRed
CBlue
DPink / Magenta

Q 16

Which of the following is an inorganic (mineral) acid?

ALactic acid
BMalic acid
CSulfuric acid
DAscorbic acid

Q 17

What is the chemical formula of sulfuric acid?

AHCl
BHNO₃
CH₂SO₄
DH₃PO₄

Q 18

Calcium hydroxide Ca(OH)₂ is commonly known as:

ABaking soda
BSlaked lime
CCaustic soda
DMilk of magnesia

Q 19

Which correctly describes how a salt is formed?

AWhen acid reacts with another acid
BWhen H⁺ of an acid is replaced by a metal ion
CWhen any compound dissolves in water
DWhen a base decomposes alone

Q 20

Acid rain is mainly formed when which gases dissolve in rainwater?

ACO₂ and O₂
BSO₂ and NOₓ from burning fossil fuels
CH₂ and N₂
DCl₂ and HF

Q 21

Which of the following salts is used in agriculture as a fertiliser?

ANaCl
BCaCO₃
CNH₄NO₃
DAgNO₃

Q 22

Adding calcium hydroxide to acidic farmland soil is called:

ALeaching
BLiming
CSaponification
DElectrolysis

Q 23

The acid found in the human stomach is:

ASulfuric acid
BCitric acid
CHydrochloric acid
DLactic acid

Q 24

Which of the following is NOT a property of acids?

ASour taste
BTurn blue litmus red
CSlippery feel
DReact with metals to release H₂

Q 25

CuSO₄ is the formula for:

ACalcium sulfate
BCopper sulfate
CCobalt sulfate
DCarbon sulfate

Q 26

Which of the following substances has the LOWEST pH?

AMilk (pH 6.5)
BStomach acid (pH 1.5)
CLemon juice (pH 2.5)
DVinegar (pH 3.0)

Q 27

What does the limewater test detect?

AHydrogen gas
BOxygen gas
CCarbon dioxide
DWater vapour

Q 28

Sodium hydroxide (NaOH) is commonly known as:

ASlaked lime
BWashing soda
CCaustic soda
DBaking soda

Q 29

Organic acids characteristically contain the functional group:

A–OH (hydroxyl)
B–COOH (carboxyl)
C–NH₂ (amino)
D–SO₄ (sulfate)

Q 30

Which reaction produces a salt, water, AND carbon dioxide?

AAcid + metal oxide
BAcid + metal
CAcid + carbonate
DAcid + alkali

20 Structured Questions

Structured Response Quiz Section B — 20 Questions

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Section B — Structured Questions

Question 1 of 20

Define the term "acid" and give THREE examples of acids you would find in everyday life. State whether each is organic or inorganic.

6 marks

✅ Model Answer An acid is a substance that produces hydrogen ions (H⁺) when dissolved in water; it has a pH below 7 and tastes sour. [2 marks]

Examples: (i) Citric acid — found in lemons and oranges — organic [1]; (ii) Ethanoic acid — found in vinegar — organic [1]; (iii) Hydrochloric acid — gastric acid / car batteries — inorganic [1]. [Total: 6]

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Question 2 of 20

Explain the difference between a BASE and an ALKALI, using one example of each.

4 marks

✅ Model Answer A base is any substance that neutralises an acid — it accepts H⁺ or donates OH⁻ [1]. An alkali is a base that dissolves in water to form a solution with pH > 7 [1].

Example base (not alkali): Copper(II) oxide CuO — neutralises acids but doesn't dissolve in water [1]. Example alkali: Sodium hydroxide NaOH — dissolves in water, pH > 7 [1]. All alkalis are bases, but not all bases are alkalis. [Total: 4]

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Question 3 of 20

The pH scale ranges from 0 to 14. (a) What pH value indicates a neutral solution? (b) Classify these pH values: 2, 7, 9, and 13.

5 marks

✅ Model Answer (a) pH 7 is neutral [1].

(b) pH 2 → Strongly acidic [1] | pH 7 → Neutral [1] | pH 9 → Weakly alkaline [1] | pH 13 → Strongly alkaline [1]. [Total: 5]

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Question 4 of 20

Describe what happens when litmus, methyl orange, and phenolphthalein are each placed in (i) an acidic solution and (ii) an alkaline solution. Present as a table.

6 marks

✅ Model Answer

Indicator	In Acid	In Alkali
Litmus	Red	Blue
Methyl orange	Red	Yellow
Phenolphthalein	Colourless	Pink / Magenta

Award 1 mark per correct colour per condition (2 conditions × 3 indicators = 6). [Total: 6]

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Question 5 of 20

Write balanced equations for (a) sulfuric acid + zinc metal, and (b) hydrochloric acid + calcium carbonate. State the reaction type for each.

6 marks

✅ Model Answer (a) H₂SO₄ + Zn → ZnSO₄ + H₂↑ [2] — Type: Acid + Metal reaction [1].

(b) 2HCl + CaCO₃ → CaCl₂ + H₂O + CO₂↑ [2] — Type: Acid + Carbonate reaction [1]. [Total: 6]

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Question 6 of 20

Distinguish between organic acids and inorganic acids, giving THREE examples of each with their sources or uses.

8 marks

✅ Model Answer Organic acids contain carbon, are produced by living organisms, generally weak, and contain the –COOH (carboxyl) group [2]. Examples: Citric acid (lemons) [1], Lactic acid (sour milk/muscles) [1], Ethanoic acid (vinegar) [1].

Inorganic acids do not contain carbon, derived from minerals, usually strong [2]. Examples: Hydrochloric acid HCl (gastric juice) [1], Sulfuric acid H₂SO₄ (car batteries) [1], Nitric acid HNO₃ (fertilisers) [1]. [Total: 8]

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Question 7 of 20

A student pours dilute HCl into a beaker of NaOH. (a) Write the chemical equation. (b) What type of reaction? (c) How to find the exact neutralisation point?

5 marks

✅ Model Answer (a) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) [2]
(b) Neutralisation reaction [1]
(c) Add a few drops of indicator (e.g. phenolphthalein) to the alkali first; add acid drop-by-drop until indicator changes from pink → colourless (pH = 7) [2]. [Total: 5]

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Question 8 of 20

Give FOUR physical properties and THREE chemical properties of acids.

7 marks

✅ Model Answer Physical (4 marks): (i) Sour taste [1]; (ii) pH below 7 [1]; (iii) Turn blue litmus RED [1]; (iv) Corrosive — damage skin and metals [1].

Chemical (3 marks): (i) React with metals → salt + hydrogen gas [1]; (ii) React with bases → salt + water (neutralisation) [1]; (iii) React with carbonates → salt + water + CO₂ [1]. [Total: 7]

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Question 9 of 20

Explain why farmers add calcium hydroxide to their fields. Include a word equation.

4 marks

✅ Model Answer Farmers add slaked lime because the soil is too acidic [1], which prevents crops from growing well. Calcium hydroxide (a base) neutralises the excess acid [1], raising the pH to around 6–7 [1].

Word equation: Calcium hydroxide + Acid (in soil) → Calcium salt + Water [1]. [Total: 4]

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Question 10 of 20

Name the salt formed and write the equation for each: (a) HNO₃ + KOH; (b) H₂SO₄ + MgO; (c) HCl + ZnCO₃.

9 marks

✅ Model Answer (a) Salt: Potassium nitrate (KNO₃) [1] — HNO₃ + KOH → KNO₃ + H₂O [2]
(b) Salt: Magnesium sulfate (MgSO₄) [1] — H₂SO₄ + MgO → MgSO₄ + H₂O [2]
(c) Salt: Zinc chloride (ZnCl₂) [1] — 2HCl + ZnCO₃ → ZnCl₂ + H₂O + CO₂↑ [2] [Total: 9]

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Question 11 of 20

List FIVE uses of salts in everyday life, with one specific example for each.

5 marks

✅ Model Answer (i) Food seasoning/preservation — NaCl (table salt) [1]
(ii) Fertiliser — KNO₃ or NH₄NO₃ [1]
(iii) Medicine — Mg(OH)₂ (antacid) or MgSO₄ (Epsom salt) [1]
(iv) Construction — CaCO₃ in cement / CaSO₄ in plaster of Paris [1]
(v) Agriculture fungicide — CuSO₄ in Bordeaux mixture [1] [Total: 5]

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Question 12 of 20

Explain why drinking fizzy drinks can cause tooth decay. Reference the chemistry of acids.

4 marks

✅ Model Answer Fizzy drinks contain carbonic acid (H₂CO₃) and citric/phosphoric acids [1]. These give a low pH of about 2.5–3.5 [1]. The acid reacts with calcium phosphate in tooth enamel, dissolving it [1]. This weakens and erodes the enamel, causing cavities and tooth decay [1]. [Total: 4]

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Question 13 of 20

What is acid rain? Describe how it is formed and give TWO harmful effects.

5 marks

✅ Model Answer Acid rain is precipitation with a pH below 5.6 due to dissolved acidic gases [1]. Burning fossil fuels releases SO₂ and NOₓ [1]; these dissolve in water vapour to form H₂SO₄ and HNO₃ which fall as acid rain [1]. Harmful effects: kills trees and plants [1]; acidifies lakes/rivers, killing aquatic organisms [1]. [Total: 5]

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Question 14 of 20

A solution has pH 11. (a) Is it acidic, neutral, or alkaline? (b) Litmus colour change? (c) Suggest a chemical with this pH. (d) What would phenolphthalein show?

4 marks

✅ Model Answer (a) Alkaline — pH 11 > 7 [1]
(b) Red litmus turns blue [1]
(c) NaOH or ammonia solution [1]
(d) Phenolphthalein turns pink / magenta [1] [Total: 4]

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Question 15 of 20

Explain neutralisation and how it is used to treat indigestion. Include a word equation.

5 marks

✅ Model Answer Neutralisation is when an acid and a base react to form a salt and water, bringing pH towards 7 [2]. Excess HCl causes indigestion; antacids like Mg(OH)₂ neutralise it [2].

Word equation: Magnesium hydroxide + Hydrochloric acid → Magnesium chloride + Water [1]. [Total: 5]

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Question 16 of 20

Compare strong acids and weak acids. Explain what "dissociation" means and give one example of each.

6 marks

✅ Model Answer Dissociation = breaking of acid molecules into H⁺ and anion when dissolved in water [1].

Strong acids fully dissociate — all molecules ionise [1]; very low pH, very corrosive. Example: HCl → H⁺ + Cl⁻ (completely) [1].
Weak acids partially dissociate — only some molecules ionise [1]; higher pH, less corrosive. Example: CH₃COOH ⇌ CH₃COO⁻ + H⁺ (partially) [1].
Similarity: both have pH < 7 and release H⁺ [1]. [Total: 6]

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Question 17 of 20

Identify the salt, acid, and base in: Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂. Name the reaction type.

4 marks

✅ Model Answer Acid: H₂SO₄ (sulfuric acid) [1]
Carbonate/Base: Na₂CO₃ (sodium carbonate) [1]
Salt formed: Na₂SO₄ (sodium sulfate) [1]
Reaction type: Acid + Carbonate reaction (produces CO₂ gas) [1]. [Total: 4]

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Question 18 of 20

State THREE uses each of (a) sulfuric acid and (b) sodium hydroxide in industry.

6 marks

✅ Model Answer (a) Sulfuric acid — any 3:
(i) Car/lead-acid battery electrolyte [1]; (ii) Making fertilisers (ammonium sulfate) [1]; (iii) Manufacturing detergents and dyes [1]

(b) Sodium hydroxide — any 3:
(i) Making soap (saponification) [1]; (ii) Paper and pulp manufacturing [1]; (iii) Drain/oven cleaning [1] [Total: 6]

How did you do?

Question 19 of 20

A piece of zinc is placed in dilute HCl. (a) Describe your observations. (b) Write the chemical equation. (c) How would you test the gas produced?

5 marks

✅ Model Answer (a) Rapid bubbling at zinc surface; zinc gradually dissolves; solution stays colourless; heat produced [2 — any 2].

(b) Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) [2]

(c) Hold a burning splint near the tube opening — hydrogen makes a squeaky pop [1]. [Total: 5]

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Question 20 of 20

Design a simple experiment using natural indicators to test five household substances (lemon juice, milk, baking soda solution, bleach, water) for acidity or alkalinity. Describe the procedure, expected results, and recording method.

8 marks

✅ Model Answer Materials: Red cabbage juice, 5 test tubes, dropper, the 5 substances [1].
Procedure: Boil red cabbage and collect purple juice [1]. Place 5 mL of each substance in separate labelled test tubes [1]. Add 3–5 drops of indicator to each and observe colour changes [1].

Expected results:
Lemon juice → Red/pink → Acidic [0.5] | Milk → Pink → Weakly acidic [0.5]
Baking soda → Green → Alkaline [0.5] | Bleach → Yellow-green → Strongly alkaline [0.5]
Water → Purple (unchanged) → Neutral [0.5]

Recording: Draw a results table: Substance | Colour | Classification [1]. Compare to reference colour chart for conclusions [0.5]. [Total: 8]

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