Acids, Bases & Salts
The complete chapter explained like a friend sitting next to you — every theory point, every in-text question, all 15 exercise solutions and hands-on virtual labs you can actually play with.
Curated with love by Brain Booster Academy — boost your brain, one chapter at a time!
What’s inside these notes?
Exactly follows your NCERT book — same order, same activities, everything solved.
Indicators
Litmus, turmeric, onion smell tests + the virtual indicator lab.
Dive in →
Reactions of Acids & Bases
Metals, carbonates, neutralisation, oxides — with animations.
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Acids & Bases in Water
H⁺, OH⁻, hydronium, dilution rules & the danger of water→acid.
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The pH Scale
0–14 scale explorer, pH in stomach, teeth, soil & stings.
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More About Salts
Salt families, chlor-alkali, baking soda, washing soda, POP.
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NCERT Solutions
Every in-text question + all 15 exercise questions, step by step.
Jump to answers →
Understanding Acids & Bases — Indicators
How do we even KNOW if something is an acid or a base?
First, a quick taste test story…
You already know acids and bases from daily life:
- • Sour things → curd, lemon juice, orange juice, vinegar → they contain acids. (Fun fact: the word “acid” comes from the Latin acidus/acere = sour!)
- • Bitter + soapy to touch → baking soda solution → these are bases.
What is an indicator?
Acid–base indicators are special dyes that change their colour (or smell!) in acidic and basic media. They “indicate” — they point out — what the substance is.
Table 2.1 — The Magic Colour Chart
| Indicator | In Acid | In Base |
|---|---|---|
| Blue litmus | turns RED | stays blue |
| Red litmus | stays red | turns BLUE |
| Turmeric (haldi) | stays yellow | turns RED |
| Phenolphthalein | colourless | turns PINK |
| Methyl orange | turns RED | turns YELLOW |
Virtual Lab 1 — The Indicator Mixer
play with me!Pick any substance + any indicator. Watch the test tube change colour — then check WHY.
Observation:
Blue litmus turns red — the substance is an ACID!
indicator (before)
before
+ substance (after)
after
Olfactory indicators (smell-test!)
Some substances lose or change their smell in a base — these are called olfactory indicators (“olfactory” = related to smell).
- • Onion — pungent smell in acid, smell disappears in base
- • Vanilla essence — pleasant smell in acid, gone in base
- • Clove oil — smell in acid, gone in base
Question idea: Can vanilla be used as an olfactory indicator? → Try mentally with acid & base!
Olfactory = Odour!
Onion & Vanilla lose their perfume only in Bases.
Think: “Base se badboo band” — base switches the smell OFF.
In-Text Questions — Set 1 (Indications)
You have three test tubes. One contains distilled water and the other two contain an acidic and a basic solution. If you are given only red litmus paper, how will you identify the contents of each test tube?
Dip the red litmus paper in all three solutions, one by one:
- 1 The solution that turns red litmus BLUE → it is the basic solution.
- 2 Now you have blue-dipped paper from that base! Dip it in the remaining two: the one turning it back to RED → acidic solution.
- 3 The last one causes no change → it is the distilled water (neutral).
How do Acids & Bases React?
The 5 golden reactions — boards love all of them.
2.1.2 • Acid + Metal → Salt + Hydrogen gas
Activity 2.3 (NCERT): Take 5 mL dilute sulphuric acid in a test tube. Add a few zinc granules. You see bubbles of gas rising!
Pass the gas into soap solution → soap bubbles fill with gas. Bring a burning candle near → the gas burns with a POP sound = hydrogen (H₂)!
Generally: Acid + Metal → Salt + Hydrogen. Metal displaces hydrogen from the acid — it’s a displacement reaction. Mg strips (ribbon) react fastest here; Au & Cu stay cool and don’t react.
Do bases react with metals? Yes, some! e.g.
Virtual Lab 2 — The POP Test
H₂SO₄ + Zn
burning splinter
POP!
Step 1: drop zinc into the acid…
2.1.3 • Acid + Metal Carbonate/Hydrogencarbonate → Salt + Water + CO₂
Activity 2.4: Take sodium carbonate (Na₂CO₃) in a test tube, add dilute HCl — instant effervescence (fizzing). Pass the gas through lime water → lime water turns milky. That gas is CO₂!
The milky test: CO₂ reacts with lime water to form a white precipitate of calcium carbonate:
But wait — pass excess CO₂ and the milkiness disappears (white ppt dissolves into soluble calcium hydrogencarbonate):
Same logic = why antacids & soda-acid fire extinguishers work, and why chalk/eggshell fizz with acids.
Virtual Lab 3 — Lime Water Test
Na₂CO₃ + HCl
(CO₂ made here)
lime water
Ca(OH)₂
Click “Pass CO₂” and watch the lime water…
2.1.4 • Acid + Base → Salt + Water (Neutralisation)
Activity 2.5 (the reverse magic!): Take 2 mL NaOH in a test tube, add 2 drops of phenolphthalein → solution becomes pink. Now add dilute HCl drop by drop → pink slowly fades until it becomes colourless!
Why? The acid and base cancel (“neutralise”) each other, forming a salt and water:
Reverse it: add NaOH again to the colourless solution → pink colour comes back. Flashback: this is the most asked reaction of the chapter!
Virtual Lab 4 — Neutralisation Beaker
Beaker: NaOH + phenolphthalein (starts pink). Slide to add HCl ↓
HCl added: 0 mL
pH ≈ 13.0
Basic — pink shows!
2.1.5 • Metallic Oxide + Acid
Activity 2.6: black copper(II) oxide (CuO) + dilute HCl → mixture is warmed → black powder dissolves, solution turns blue-green (copper(II) chloride!):
See the pattern? Same as acid + base → salt + water. So… metallic oxides (like CuO) are BASIC in nature.
2.1.6 • Non-metallic Oxide + Base
Activity 2.7: You already saw it — CO₂ through lime water:
It behaves just like base + acid. So non-metallic oxides (like CO₂, SO₂) are ACIDIC in nature. Similarly: 2NaOH + CO₂ → Na₂CO₃ + H₂O.
In-Text Questions — Set 2 (Reactions)
Why should curd and sour substances not be kept in brass and copper vessels?
Curd & sour items contain acids (lactic acid etc.). These acids react with the metal of brass/copper vessels, forming harmful metal salts. That makes the food poisonous/unfit to eat — and also damages (corrodes) the vessel. That’s why we use steel or glass containers!
Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Hydrogen gas (H₂) is liberated. Example:
Test: Bring a burning splinter/candle near the mouth of the test tube. The gas burns with a POP sound → confirms hydrogen. (Go try the POP lab above!)
Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation if one of the compounds formed is calcium chloride.
Gas that puts out a candle + effervescence = CO₂. Since CaCl₂ is a product, compound A must be calcium carbonate (CaCO₃):
What do ALL acids & bases have in common?
The secret lives in water — ions! (Section 2.2)
Acids → give H⁺ ions in water
An acid’s whole personality comes from one tiny thing: the hydrogen ion H⁺. But H⁺ never swims alone — it hugs a water molecule and becomes the hydronium ion H₃O⁺:
Thinks of it as: H⁺ + H₂O → H₃O⁺. From now, reading “H⁺(aq)” = “H₃O⁺” is fine.
Activities 2.8 says: solutions of HCl, H₂SO₄, HNO₃ conduct electricity (the bulb glows!) because they produce ions in water. But solutions of glucose and alcohol do NOT — no ions, no glow, not acids even though they contain H!
Bases → give OH⁻ ions in water
Bases generate hydroxide ions OH⁻ in water:
Bases vs Alkalis — not all bases dissolve in water! Bases that DO dissolve in water are called alkalis: NaOH, KOH, Ca(OH)₂, Mg(OH)₂, NH₄OH.
What happens to an acid or base in water? → DILUTION
Mixing an acid/base with water decreases the concentration of ions (H₃O⁺/OH⁻) per unit volume — this process is called dilution. The acid gets weaker but the total number of ions stays the same — they just get more room!
No ions disappear. Only per mL, there are fewer H₃O⁺ soldiers.
Golden Safety Rule: dissolving an acid in water is HIGHLY exothermic (releases heat).
✅ Always add ACID → WATER, slowly, with constant stirring.
❌ Never add water → acid: the sudden heat can make the water boil & splash acid on your face. Right way = safe way!
Memory: “Acid into Water — the Awesome Way.
Virtual Lab 5 — Dilute the Acid
10 mL acid is fixed. Slide to pour water (safely, acid → water!)
beaker
Acid: 10 mL
Water: 0 mL
Total: 10 mL
H₃O⁺ concentration = 100% ↓
dots = same 12 ions… just spread out. Concentration per mL keeps falling!
In-Text Questions — Set 3 (Water & Ions)
Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not?
HCl, HNO₃ etc. ionise/dissociate in water to produce H⁺(aq)/H₃O⁺ ions — those ions are why they behave acidic. Alcohol & glucose contain hydrogen but do NOT ionise in water, so no H⁺ ions → no acidic character.
Why does an aqueous solution of an acid conduct electricity?
Electric current in liquids is carried by moving charged particles. An aqueous acid solution is full of free ions (H⁺/H₃O⁺ and negative ions), so it conducts electricity. Dry/molecular acid has no free ions → no conduction.
Why does dry HCl gas not change the colour of the dry litmus paper?
Dry HCl gas is just neutral molecules — no ions at all. Acids show what they are only in the presence of water (where they form H₃O⁺). No water → no H⁺ ions → no colour change on dry litmus. Wet the same litmus and watch it go red!
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Mixing acid + water is highly exothermic. When acid → water (slowly, stirring): the large amount of water absorbs the heat gradually — safe! When water → acid: all that heat hits a tiny amount of water at once, making it boil suddenly and splash concentrated acid out — dangerous burns!
How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
On dilution, the same number of H₃O⁺ ions spreads into a larger volume. So the concentration (H₃O⁺ per unit volume) DECREASES and the acid becomes weaker. (You literally just saw this in Lab 5 above.)
How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
More base → more dissociation → the OH⁻ concentration INCREASES per unit volume. The solution becomes a stronger base.
How STRONG is an acid or base? — the pH Scale
One rainbow ruler from 0 to 14 measures everything. (Section 2.3)
The pH Ruler — Super Simple Idea
A universal indicator is a mixture of several indicators that shows different colours at different H⁺ concentrations. That gives us a scale from 0 → 14, called the pH scale (p = potenz, German for “power” — the power of hydrogen!).
pH < 7
ACIDIC
(more H⁺ than OH⁻)
pH = 7
NEUTRAL
(H⁺ = OH⁻)
pH > 7
BASIC
(more OH⁻ than H⁺)
Closer to 0 → stronger acid. Closer to 14 → stronger base. Each step down = 10× more H⁺! (pH 2 is 10× more acidic than pH 3, 100× more than pH 4.)
| Strength words | pH range |
|---|---|
| Strong acid | 0 – 2 |
| Weak acid | 3 – 6 |
| Neutral | 7 |
| Weak base | 8 – 10 |
| Strong base | 11 – 14 |
pH Scale Explorer
drag the knob!7.0
Neutral
H⁺ concentration ≈ 1×10⁻⁷ mol/L
universal indicator colour:
Things around this pH:
Pure water · Human blood (7.35–7.45)
Why does pH matter in real life? (Exam favourite!)
1. Your stomach (antacids!)
Your stomach makes HCl to digest food. During indigestion it makes too much acid → pain, burning (“acidity”). The fix? An antacid — a mild base like milk of magnesia Mg(OH)₂ — which neutralises the excess acid.
2. Tooth decay — the 5.5 danger line
Tooth enamel = calcium hydroxyapatite, the hardest substance in your body. But it corrodes when mouth pH falls below 5.5 (bacteria in leftover sugar make acid). Cure: rinse after meals + brush with toothpaste — which is basic — to neutralise the acid.
3. Soil pH in farming
Plants grow best only in a certain pH window. If the soil is too acidic, the farmer treats it with bases like quick lime (CaO), slaked lime Ca(OH)₂ or chalk (CaCO₃). Too basic? Add organic manure that releases acids on decaying.
4. Nature’s own defence — stings!
Honey-bee sting injects an acidic liquid (burning pain) → rub baking soda (mild base) on it for relief. Nettle leaves sting using methanoic acid → the traditional cure is rubbing a dock leaf growing nearby. Nature always has the antidote next door!
• Our body works between pH 7.0 – 7.8.
• Rain with pH below 5.6 = acid rain — it lowers river pH and makes survival of aquatic life difficult.
• Even the Taj Mahal’s marble (CaCO₃) gets eaten slowly by acid rain — engineers call it “marble cancer”.
In-Text Questions — Set 4 (pH Power)
You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which is acidic and which is basic?
Solution A (pH 6) — the lower the pH, the higher the H⁺ concentration — so A has more hydrogen ions (actually 10× more than B… and 100× more per-log trick you just learned!). A is acidic (pH < 7) and B is basic (pH > 7).
What effect does the concentration of H⁺(aq) ions have on the nature of the solution?
It decides how acidic the solution is: higher H⁺ concentration → lower pH → more acidic. Lower H⁺ concentration → higher pH → moves towards neutral, then basic.
Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?
Yes! Every aqueous solution has both H⁺ and OH⁻ (even pure water). A solution is basic because it has MORE OH⁻ than H⁺ — the extra hydroxide ions win the tug-of-war, so the overall character is basic.
Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (CaO) or slaked lime Ca(OH)₂ or chalk (CaCO₃)?
When the soil becomes too acidic for healthy crop growth. All three are bases, and adding a base to acidic soil neutralises it, bringing pH back to the friendly range plants love.
More About SALTS
Not just namak! Salt = the product of acid + base. (Section 2.4)
2.4.1 • Family of Salts
Salts sharing the same positive or same negative ion belong to one “family”:
| Family name | Members | Common ion |
|---|---|---|
| Sodium salts | NaCl, Na₂SO₄ | Na⁺ |
| Chloride salts | NaCl, KCl | Cl⁻ |
| Sulphate salts | Na₂SO₄, CuSO₄, ZnSO₄ | SO₄²⁻ |
2.4.2 • pH of Salts — the parent rule
A salt inherits the strength of its parents:
| Parent acid | Parent base | Salt is… | Example |
|---|---|---|---|
| Strong (HCl) | Strong (NaOH) | Neutral, pH 7 | NaCl |
| Strong (HCl) | Weak (NH₄OH) | Acidic, pH < 7 | NH₄Cl |
| Weak (CH₃COOH) | Strong (NaOH) | Basic, pH > 7 | CH₃COONa |
Chant it: “Strong + Strong = chill. Strong acid’s kid = sour. Strong base’s kid = soapy.”
Bonus Lab — The Salt Fortune-Teller
pick the parents!+
Salt formed:
NaCl
Neutral salt • pH = 7
Both parents are strong → their child is perfectly balanced. This is your everyday table-salt family!
2.4.3 • Chemicals from Common Salt — the VIP section
Learn these 4 by heart — at least one WILL appear on your board paper.
1. Sodium Hydroxide — Chlor-alkali Process
Pass electricity through brine (concentrated NaCl solution):
- • Named chlor-alkali: chlorine + alkali (NaOH)
- • NaOH forms near the cathode
- • Uses of NaOH: de-greasing metals, making soaps & detergents, paper, artificial fibres
2. Bleaching Powder — CaOCl₂
Chlorine (from chlor-alkali!) is passed over dry slaked lime:
Formula: CaOCl₂ — calcium oxychloride.
Uses: bleaching cotton/linen in textiles, wood pulp in paper factories, washed clothes in laundry • as an oxidising agent • disinfecting drinking water (kills germs).
3. Baking Soda — NaHCO₃
Made from brine (with ammonia + CO₂):
Full name: sodium hydrogencarbonate. On mild heating it puffs things up:
Baking powder = NaHCO₃ + a mild edible acid (like tartaric acid). When heated/mixed with water, CO₂ is released → makes bread & cake soft and spongy. Also an antacid and used in soda-acid fire extinguishers.
4. Washing Soda — Na₂CO₃·10H₂O
Recrystallisation of sodium carbonate gives sodium carbonate decahydrate:
It’s a basic salt (from weak acid H₂CO₃ + strong base NaOH → pH>7!).
Uses: glass, soap & paper industries • making borax • domestic cleaning agent • removing permanent hardness of water.
2.4.4 • Are salt crystals really dry?
Nope! Many hide water of crystallisation — a fixed number of water molecules locked inside each formula unit:
- • Blue vitriol CuSO₄·5H₂O — gorgeous blue crystals. Heat → water leaves → white powder. Add water → blue again!
- • Washing soda Na₂CO₃·10H₂O
- • Gypsum CaSO₄·2H₂O — the parent of Plaster of Paris.
Plaster of Paris — CaSO₄·½H₂O
Heat gypsum to 373 K:
POP + water → sets as hard gypsum again:
Uses: supporting fractured bones in the right position, moulds for toys/statues, smoothing surfaces. Store it moisture-proof or it quietly sets!
The “number” exam trick: bleaching powder = CaOCl₂ (weird but easy), baking soda = NaHCO₃, washing soda = Na₂CO₃·10H₂O, gypsum = CaSO₄·2H₂O, POP = CaSO₄·½H₂O, blue vitriol = CuSO₄·5H₂O. Make a 10-2-½-5 chant: “Ten-two-half-five, salts stay alive!”
In-Text Questions — Set 5 (Salt Squad)
What is the common name of the compound CaOCl₂?
Bleaching powder (chemical name: calcium oxychloride). Formula: CaOCl₂.
Name the substance which on treatment with chlorine yields bleaching powder.
Slaked lime — Ca(OH)₂ (calcium hydroxide). Check the reaction: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O.
Name the sodium compound which is used for softening hard water.
Washing soda — Na₂CO₃·10H₂O (sodium carbonate decahydrate). It removes the permanent hardness of water.
What will happen if a solution of sodium hydrogencarbonate is heated? Give the equation of the reaction involved.
It decomposes into sodium carbonate + water + carbon dioxide:
Write an equation to show the reaction between Plaster of Paris and water.
POP grab the water and sets back to hard gypsum:
NCERT Exercise — all 15 solved
Read the question → think for 30 seconds → tap to reveal. Best revision method ever.
A solution turns red litmus blue, its pH is likely to be:
Red litmus turns blue only in a base, and bases have pH > 7. Among the options, only 10 is greater than 7. The others (1, 4, 5) are all acids → they’d turn blue litmus red.
A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains:
Egg shells = CaCO₃ (a carbonate). Gas turning lime-water milky = CO₂. Only an acid reacts with a carbonate to release CO₂, and the only acid among the options is HCl: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂↑.
10 mL of a solution of NaOH is completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same NaOH solution, the amount of HCl solution required will be:
It’s simple ratio in the same concentration: double the NaOH (10 → 20 mL) needs double the HCl (8 × 2 = 16 mL) to reach the neutralisation point.
Which one of the following types of medicines is used for treating indigestion?
Indigestion = excess HCl in the stomach. An antacid (anti-acid!) is a mild base like Mg(OH)₂ (milk of magnesia) that neutralises the extra acid and removes the burning pain.
Write word equations and then balanced equations for the reaction taking place when — (a) dilute sulphuric acid reacts with zinc granules (b) dilute hydrochloric acid reacts with magnesium ribbon (c) dilute sulphuric acid reacts with aluminium powder (d) dilute hydrochloric acid reacts with iron filings.
(a) zinc + sulphuric acid → zinc sulphate + hydrogen
Zn + H₂SO₄ → ZnSO₄ + H₂↑
(b) magnesium + hydrochloric acid → magnesium chloride + hydrogen
Mg + 2HCl → MgCl₂ + H₂↑
(c) aluminium + sulphuric acid → aluminium sulphate + hydrogen
2Al + 3H₂SO₄ → Al₂(SO₄)₃ + 3H₂↑
(d) iron + hydrochloric acid → iron(II) chloride + hydrogen
Fe + 2HCl → FeCl₂ + H₂↑
Pattern for all: Metal + Acid → Salt + H₂ (it’s a displacement reaction).
Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
- 1. Take a beaker and fix two carbon electrodes on a cork; connect them in series with a battery, a switch and a bulb.
- 2. Fill the beaker with glucose solution and switch on → the bulb does not glow.
- 3. Repeat with alcohol solution → again, no glow.
- 4. But with HCl or H₂SO₄ → the bulb glows brightly!
Conclusion: alcohol & glucose do not ionise in water, so no current flows and no H⁺ ions exist — officially not acids, even though they contain hydrogen.
Why does distilled water not conduct electricity, whereas rain water does?
Distilled water is almost pure H₂O — nearly no ions → no charge carriers → no conduction.
Rain water dissolves gases & dust as it falls (CO₂ → carbonic acid, plus dissolved salts/minerals), so it is full of ions → conducts electricity.
Why do acids not show acidic behaviour in the absence of water?
Acidic behaviour = giving H⁺(aq)/H₃O⁺ ions. Acids ionise only in water. Without water there are only molecules — no free H⁺ ions — so an acid shows no acidic properties at all when dry. (Exactly why dry HCl gas fails on dry litmus!)
Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration.
| Soln | pH | Nature |
|---|---|---|
| A | 4 | weakly acidic |
| B | 1 | strongly acidic |
| C | 11 | strongly alkaline |
| D | 7 | neutral |
| E | 9 | weakly alkaline |
(a) neutral → D (b) strongly alkaline → C (c) strongly acidic → B (d) weakly acidic → A (e) weakly alkaline → E
Increasing H⁺ concentration = decreasing pH: 11 < 9 < 7 < 4 < 1 → i.e. C < E < D < A < B
Equal lengths of magnesium ribbons are taken in test tubes A and B. HCl is added to A, while acetic acid (CH₃COOH) is added to B. Amount and concentration of both acids are the same. In which test tube will the fizzing occur more vigorously and why?
Fizzing = H₂ gas, and its speed depends on H⁺ concentration. HCl is a strong acid — it ionises completely, flooding the tube with H⁺ ions. CH₃COOH is weak — only partly ionised, few H⁺ ions. Same volume, same “label” concentration… but very different H⁺ armies → HCl fizzes much more vigorously.
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
pH will decrease below 6. While curdling, bacteria change milk sugar (lactose) into lactic acid. More acid means more H⁺ → the solution becomes more acidic → pH drops. That’s also why old curd tastes extra sour!
A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd?
(a) Baking soda (NaHCO₃) is basic. Making the milk slightly alkaline delays it turning sour — the milk stays “fresh” for more hours before curdling (and the curd comes out sweeter/whiter!).
(b) Curd forms only when lactic acid makes the milk acidic. Now the bacteria first have to neutralise the extra base before the milk can even begin to become acidic — so setting takes much longer.
Plaster of Paris should be stored in a moisture-proof container. Explain why?
POP (CaSO₄·½H₂O) eagerly reacts with any moisture — even humid air — and turns into hard, solid gypsum (CaSO₄·2H₂O), losing its power to set when you actually need it:
CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O
So air-tight, moisture-proof boxes it is!
What is a neutralisation reaction? Give two examples.
A reaction in which an acid and a base react with each other to form a salt and water, cancelling each other’s effect, is called neutralisation. Acid’s H⁺ and base’s OH⁻ unite to form H₂O.
Examples:
NaOH + HCl → NaCl + H₂O
KOH + HNO₃ → KNO₃ + H₂O
(Also counts: acid + metal oxide, e.g. CuO + 2HCl → CuCl₂ + H₂O, since metal oxides are basic.)
Give two important uses of washing soda and baking soda.
Washing soda (Na₂CO₃·10H₂O):
(i) Used in the manufacture of glass, soap and paper. (ii) Used for removing permanent hardness of water (also a household cleaning agent / making borax).
Baking soda (NaHCO₃):
(i) Main ingredient of baking powder — makes bread/cakes soft and spongy. (ii) Used in soda-acid fire extinguishers (also as an antacid to reduce stomach acidity).
5-Minute Recap Sheet
Read this on exam morning. That’s it. That’s the cheat code (a legal one).
The Big Ideas
- • Indicators = dyes that change colour in acids/bases (litmus, turmeric; onion/vanilla = olfactory)
- • Acids taste sour, bases are bitter & soapy
- • Acid + metal → salt + H₂ (POP test!)
- • Acid + carbonate → salt + H₂O + CO₂ (lime water → milky)
- • Acid + base → salt + water (neutralisation)
- • Metal oxides = basic • non-metal oxides = acidic
- • Acids give H⁺/H₃O⁺ in water • bases give OH⁻ • alkalis = water-soluble bases
- • Dilution: ions per mL ↓ • ACID → WATER, never the reverse
pH & Salts quick-map
- • pH < 7 acid • = 7 neutral • > 7 base; each unit = 10×
- • Antacid = Mg(OH)₂ • enamel corrodes below pH 5.5 • bee sting acid → rub baking soda
- • Acidic soil → add CaO / Ca(OH)₂ / CaCO₃
- • Strong+strong salt = neutral (NaCl); strong acid+weak base = acidic (NH₄Cl); weak acid+strong base = basic (CH₃COONa)
- • Chlor-alkali: brine + electricity → NaOH + Cl₂ + H₂
- • Ca(OH)₂ + Cl₂ → CaOCl₂ (bleaching powder)
- • NaHCO₃ = baking soda • Na₂CO₃·10H₂O = washing soda
- • Gypsum CaSO₄·2H₂O ⇋ POP CaSO₄·½H₂O (373 K)
- • Water of crystallisation: 5 (blue vitriol) · 10 (washing soda) · 2 (gypsum) · ½ (POP)
Rapid-Fire Quiz
6 questions. No pressure. Beat your own score!
Your score: 0 / 6
Tap an option on each card to answer.