Acids, Bases and Salts Notes Class 10 Science
Acids, Bases and Salts
Why this chapter matters
One lemon tastes sour. One spoon of soap feels slippery.
Both are everyday acids and bases. This chapter explains what makes them different, how we test them, and why the pH of your mouth, soil and stomach matters so much.
These Class 10 Science Chapter 2 notes cover the full syllabus prescribed by CBSE for the 2026–27 session.
Chapter Learning Map
Follow the spine top to bottom. Four groups, all equally important.
1 · Acids and bases — what they really are
An acid gives hydrogen ions, H+, when dissolved in water. A base gives hydroxide ions, OH−, when dissolved in water.
HCl in water → H+ + Cl− NaOH in water → Na+ + OH−
This one idea runs through the whole chapter. Acids supply H+ ions. Bases supply OH− ions. The strength of an acid or a base depends on how many ions it gives in water.
| Common acid | Found in / used for | Common base | Found in / used for |
|---|---|---|---|
| Hydrochloric acid (HCl) | Gastric juice in the stomach | Sodium hydroxide (NaOH) | Soap making; drain cleaner |
| Sulphuric acid (H2SO4) | Car batteries; industry | Calcium hydroxide, slaked lime | Whitewashing; neutralising acidic soil |
| Nitric acid (HNO3) | Fertilisers; laboratory work | Magnesium hydroxide | Milk of magnesia, an antacid |
| Citric acid | Lemon, orange, amla | Ammonium hydroxide | Cleaning agents |
| Lactic acid | Curd and sour milk | Calcium carbonate | Chalk, marble, limestone |
| Acetic acid | Vinegar | Sodium hydrogencarbonate | Baking soda, antacid |
Hydrogen chloride gas is not acidic by itself. It becomes acidic only after it dissolves in water and gives H+ ions.
Copper sulphate solution is a salt. A base such as sodium hydroxide reacts with it to give a blue precipitate.
Acid gives H+. Base gives OH−. Water is the stage where the acting happens.
A base that dissolves in water is called an alkali. All alkalis are bases, but not all bases are alkalis. Copper oxide is a base but not an alkali.
Acids taste sour and bases taste bitter. Never test a laboratory chemical by tasting it. Always use an indicator.
2 · Indicators — how we test a substance
An indicator is a substance that changes colour or smell in an acidic or a basic solution.
| Indicator | Type | In an acid | In a base |
|---|---|---|---|
| Litmus solution | Natural (from lichen) | Turns red | Turns blue |
| Turmeric | Natural | Stays yellow | Turns red-brown |
| China rose (gudhal) petal extract | Natural | Turns dark pink or magenta | Turns green |
| Methyl orange | Synthetic | Turns red | Turns yellow |
| Phenolphthalein | Synthetic | Remains colourless | Turns pink |
| Vanilla essence, onion | Olfactory | Smell stays | Smell is destroyed |
| Universal indicator | Mixture of indicators | Red to orange to yellow | Blue to violet |
Dip a strip of blue litmus paper into lemon juice. It turns red, so lemon juice is acidic.
Add a drop of phenolphthalein to soap solution. It turns pink, so soap solution is basic.
Blue litmus to red means acid. Red litmus to blue means base.
Learn methyl orange and phenolphthalein carefully. Together with litmus, they cover almost every indicator question.
Phenolphthalein stays colourless in an acid. Students often write “turns pink in acid”. Pink appears only in a base.
3 · Reactions of acids — metals, carbonates and bases
Dilute acids react with metals, metal carbonates and bases. Each reaction gives a different product, and each has a test.
| Reaction | Products | How to test the gas |
|---|---|---|
| Dilute acid + metal | Salt + hydrogen gas | Bring a burning candle near the gas — it burns with a pop sound |
| Dilute acid + metal carbonate or bicarbonate | Salt + water + carbon dioxide | Pass the gas through lime water — it turns milky |
| Dilute acid + metal oxide (a base) | Salt + water | No gas is formed; the solution warms up |
| Dilute acid + base | Salt + water (neutralisation) | Both solutions lose their nature; heat is released |
| Base + metal (like zinc) | Sodium zincate + hydrogen gas | Pop sound with a burning candle |
| Base + non-metal oxide (like CO2) | Salt + water | Example: 2NaOH + CO2 → Na2CO3 + H2O |
Zn + 2HCl → ZnCl2 + H2
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
Zinc granules are dropped into dilute hydrochloric acid. Bubbles of hydrogen rise up. A burning candle at the mouth gives a pop sound.
Baking soda is added to vinegar. Carbon dioxide bubbles out and the gas turns lime water milky. This is the soda-acid reaction.
Metal gives hydrogen — pop sound. Carbonate gives carbon dioxide — milky lime water.
The word “dilute” matters. Metals do not give hydrogen gas with concentrated nitric acid.
Hydrogen burns with a pop sound. Carbon dioxide turns lime water milky. Never swap the two tests in your answer.
4 · Neutralisation and salt formation
Neutralisation is the reaction of an acid with a base to give a salt and water. Heat is released, so it is an exothermic reaction.
Acid + Base → Salt + Water
NaOH + HCl → NaCl + H2O
The salt produced depends on the acid used. Hydrochloric acid gives chlorides. Sulphuric acid gives sulphates. Nitric acid gives nitrates.
| Acid used | Base used | Salt formed | Name of the salt |
|---|---|---|---|
| HCl | NaOH | NaCl | Sodium chloride |
| H2SO4 | NaOH | Na2SO4 | Sodium sulphate |
| HNO3 | KOH | KNO3 | Potassium nitrate |
| HCl | Ca(OH)2 | CaCl2 | Calcium chloride |
An antacid tablet contains a mild base. It neutralises the extra acid in the stomach and gives relief from indigestion.
A farmer adds slaked lime, which is a base, to acidic soil. The acid and base neutralise each other and the soil becomes suitable for crops.
Acid plus base gives salt plus water. Say it every time you write a neutralisation equation.
A neutralisation reaction does not always make the solution exactly neutral. It becomes neutral only when the acid and the base are taken in the right amounts.
5 · The pH scale
pH is a number from 0 to 14 that tells us how acidic or how basic a solution is. A lower pH means more H+ ions.
| pH value | Nature of the solution | What it means |
|---|---|---|
| Less than 7 | Acidic | More H+ ions than OH− ions |
| Exactly 7 | Neutral | H+ and OH− ions are balanced; pure water is neutral |
| More than 7 | Basic | More OH− ions than H+ ions |
The scale is balanced on both sides. There is only one neutral point, at pH 7, but there are several values on each side of it.
How many pH values fall in each zone of the 0–14 scale. The neutral point is a single value.
A solution with pH 3 has more H+ ions than a solution with pH 5. So pH 3 is more acidic, even though both are acids.
Pure water has pH 7. It is neutral because the H+ and OH− ions are present in equal amounts.
Below seven is acid. Seven is neutral. Above seven is base. Lower the number, stronger the acid.
The mathematical definition of pH uses logarithms. The syllabus clearly says that this is not required for Class 10.
Strength and concentration are different. A strong acid gives away all its H+ ions. A concentrated acid simply has less water. Do not mix the two words.
6 · pH in everyday life
Every living process works best in a particular pH range. When the pH goes outside that range, problems begin.
| Situation | What happens | What we do about it |
|---|---|---|
| Tooth decay | Bacteria make acid. When mouth pH falls below about 5.5, the enamel starts to corrode. | Brush with a basic toothpaste, which neutralises the extra acid |
| Stomach | The stomach makes hydrochloric acid to digest food. Too much acid causes indigestion. | Take an antacid — a mild base such as milk of magnesia |
| Soil for plants | Plants grow well only in a particular pH range of soil. | Add lime or organic matter to acidic soil; add organic matter to basic soil |
| Aquatic life | Acid rain lowers the pH of lakes and rivers. Fish and other animals cannot survive in very low pH. | Reduce the gases that cause acid rain |
| Bee sting | A bee injects an acid into the skin. | Rub a mild base such as baking soda on the sting |
| Nettle sting | The nettle injects methanoic acid. | Rub the leaf of the dock plant, which is basic |
Rain becomes acidic when gases such as carbon dioxide, sulphur dioxide and nitrogen oxides dissolve in it. This is called acid rain.
A marble statue is made of calcium carbonate. Acids in rain react with it slowly, and the surface of the statue gets damaged.
Acid in the mouth — brush with a base. Acid in the stomach — antacid. Acid in the soil — add lime.
Write the reason before the remedy. “Toothpaste is basic, so it neutralises the acid in the mouth” scores better than only naming toothpaste.
Never say that a base “kills” the acid. The correct word is neutralise. The acid and the base cancel each other’s nature.
7 · The five salts you must know
A salt is formed when the hydrogen of an acid is replaced by a metal. These five salts are named in the syllabus, so learn their preparation and uses.
| Common name | Chemical name and formula | How it is made | Main uses |
|---|---|---|---|
| Caustic soda | Sodium hydroxide, NaOH | Chlor-alkali process — electricity passed through brine | Soaps and detergents, paper, artificial fibres, degreasing metals |
| Bleaching powder | Calcium oxychloride, CaOCl2 | Chlorine gas passed over dry slaked lime | Bleaching cotton and linen, disinfecting drinking water, oxidising agent |
| Baking soda | Sodium hydrogencarbonate, NaHCO3 | From brine: sodium chloride, water, carbon dioxide and ammonia | Cooking, antacid, soda-acid fire extinguisher |
| Washing soda | Sodium carbonate decahydrate, Na2CO3·10H2O | Recrystallisation of sodium carbonate | Glass, soap and paper industry; laundry; removing permanent hardness of water |
| Plaster of Paris | Calcium sulphate hemihydrate, CaSO4·½H2O | Gypsum heated to 373 K | Plaster for fractured bones, toys, decorative moulds, smooth surfaces |
2NaCl + 2H2O → 2NaOH + Cl2 + H2 (chlor-alkali process)
Ca(OH)2 + Cl2 → CaOCl2 + H2O
NaCl + H2O + CO2 + NH3 → NH4Cl + NaHCO3
2NaHCO3 → Na2CO3 + H2O + CO2 (on heating)
CaSO4·2H2O → CaSO4·½H2O + 1½H2O (at 373 K)
Baking soda is added to cake batter. On heating it gives off carbon dioxide, and the bubbles make the cake soft and fluffy.
Plaster of Paris is mixed with water and poured around a fractured arm. It sets into a hard solid and supports the bone.
Water of crystallisation is the fixed number of water molecules attached to one formula unit of a salt. Washing soda has 10 such water molecules.
Bleaching powder is oxychloride. Washing soda is decahydrate. Plaster of Paris is hemihydrate. The name tells you the formula.
Baking soda is NaHCO3 and washing soda is Na2CO3·10H2O. They are two different salts. Do not use one formula for the other.
Plaster of Paris must be stored in a moisture-proof container. If it absorbs water, it sets and becomes useless.
Diagram Board — the four tests that carry marks
Most laboratory questions ask you to name the gas or the colour change. These four panels cover them.
Draw the test tube, not an artistic glass. Label the gas and the observation. Two labels and one observation fetch full marks.
⚡ Formula Vault
Revise these before every test. Each carries marks.
Important Question Bank
Questions in sequence. Tap "Show answer".
A · Multiple Choice (1 mark each)
(a) Oxygen (b) Hydrogen (c) Carbon dioxide (d) Nitrogen
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Answer: (b). Metals above hydrogen in the reactivity series displace hydrogen from dilute acids. The gas burns with a pop sound.
(a) 0 (b) 5 (c) 7 (d) 14
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Answer: (c). pH 7 is the single neutral point. Pure water is neutral, and the H+ and OH− ions are balanced.
(a) Litmus (b) Methyl orange (c) Phenolphthalein (d) Turmeric
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Answer: (c). Phenolphthalein is colourless in an acid and turns pink in a base.
(a) Sodium carbonate (b) Sodium hydrogencarbonate (c) Calcium sulphate (d) Calcium oxychloride
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Answer: (b). Baking soda is NaHCO3, sodium hydrogencarbonate. Sodium carbonate is washing soda.
(a) 273 K (b) 373 K (c) 573 K (d) 100 K
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Answer: (b). Gypsum, CaSO4·2H2O, is heated to 373 K to give calcium sulphate hemihydrate.
(a) Hydrogen (b) Oxygen (c) Carbon dioxide (d) Chlorine
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Answer: (c). Carbon dioxide reacts with lime water, calcium hydroxide, to form a white precipitate of calcium carbonate.
B · Assertion–Reason (1 mark each)
(a) Both true, R explains A. (b) Both true, R doesn’t explain A. (c) A true, R false. (d) A false, R true.
Assertion: Dry hydrogen chloride gas does not turn blue litmus red.
Reason: Hydrogen chloride becomes acidic only after it dissolves in water and gives H+ ions.
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Answer: (a). Both statements are true. Acidic behaviour needs water, because H+ ions are produced only in the aqueous solution.
Assertion: A solution of pH 3 is more acidic than a solution of pH 5.
Reason: A lower pH means a higher concentration of H+ ions.
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Answer: (a). Both are true, and the reason explains why a lower pH value means a stronger acidic nature.
Assertion: Washing soda is chemically the same as baking soda.
Reason: Washing soda is used in the glass and soap industry.
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Answer: (d). The assertion is false — washing soda is Na2CO3·10H2O and baking soda is NaHCO3. The reason is true.
Assertion: A concentrated acid is always a strong acid.
Reason: Concentration depends on the amount of water present in the solution.
Show answer
Answer: (d). The assertion is false. Strength depends on how completely the acid gives H+ ions; concentration depends on the amount of water. The reason is true.
C · Case-Based (4 marks)
A student tests a colourless liquid with a universal indicator. The colour matches a pH value of 3. He then adds sodium hydroxide solution to it slowly, drop by drop.
(i) Is the liquid acidic, basic or neutral?
(ii) Name the ion responsible for its nature.
(iii) What change will the student observe in the pH value as he keeps adding the base?
(iv) Name the reaction that takes place and state the products formed.
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(i) The liquid is acidic, because its pH is less than 7.
(ii) Hydrogen ions, H+, are responsible for the acidic nature.
(iii) The pH value will increase and move towards 7. The base neutralises the acid.
(iv) Neutralisation takes place. A salt and water are formed.
D · Short & Long Answer
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(i) Tooth decay: Bacteria present in the mouth produce acids. When the pH of the mouth falls below about 5.5, the enamel starts to corrode. Toothpaste is basic and neutralises this acid.
(ii) Stomach: The stomach produces hydrochloric acid for digestion. Excess acid causes indigestion and pain. An antacid, which is a mild base, neutralises the extra acid.
(iii) Plants: Plants grow well only in a particular pH range of soil. If the soil is too acidic, lime or organic matter is added. If it is too basic, organic matter is added.
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1. Caustic soda: Sodium hydroxide, NaOH. Used in the manufacture of soaps and detergents.
2. Bleaching powder: Calcium oxychloride, CaOCl2. Used for disinfecting drinking water.
3. Baking soda: Sodium hydrogencarbonate, NaHCO3. Used in cooking to make cakes soft and fluffy.
4. Washing soda: Sodium carbonate decahydrate, Na2CO3·10H2O. Used in the glass and soap industry.
5. Plaster of Paris: Calcium sulphate hemihydrate, CaSO4·½H2O. Used by doctors to support fractured bones.
E · Board-Pattern Practice
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(i) Zn + 2HCl → ZnCl2 + H2. The gas produced is hydrogen.
(ii) NaOH + HCl → NaCl + H2O. This is a neutralisation reaction.
Show answer
Name: Plaster of Paris, chemically calcium sulphate hemihydrate.
Formula: CaSO4·½H2O. It is prepared by heating gypsum, CaSO4·2H2O, to 373 K.
Common Mistakes Table
| Mistake students make | Correct idea |
|---|---|
| Writing “phenolphthalein turns pink in acid” | It is colourless in acid and turns pink in a base. |
| Swapping the gas tests | Hydrogen gives a pop sound. Carbon dioxide turns lime water milky. |
| Writing the formula of baking soda as Na2CO3 | Baking soda is NaHCO3. Na2CO3·10H2O is washing soda. |
| Using “strong” and “concentrated” as the same word | Strength is about giving H+ ions. Concentration is about the amount of water. |
| Writing “pH 0 means neutral” | Neutral is pH 7. pH 0 is strongly acidic. |
| Calling litmus a synthetic indicator | Litmus is a natural indicator obtained from lichen. |
| Saying neutralisation always gives a neutral solution | It gives a salt and water. The solution is neutral only if the acid and base are in the right amounts. |
| Writing “base kills the acid” | The correct term is neutralise. Both lose their nature. |
Active Recall & Revision
Cover the answers. Say them aloud. Then check.
Q: What ions do an acid and a base give in water?
An acid gives H+ ions. A base gives OH− ions.
Q: What is the colour of methyl orange in an acid and in a base?
Red in an acid and yellow in a base.
Q: Name the gas and the test for acid plus metal carbonate.
Carbon dioxide. It turns lime water milky.
Q: Write the general equation for neutralisation.
Acid + Base → Salt + Water. Heat is released, so it is exothermic.
Q: A solution has pH 9. Is it acidic or basic?
Basic, because the pH is above 7. It has more OH− ions than H+ ions.
Q: Why is toothpaste used to clean teeth?
Toothpaste is basic, so it neutralises the acid formed in the mouth by bacteria.
Q: Write the formula of bleaching powder and washing soda.
Bleaching powder is CaOCl2. Washing soda is Na2CO3·10H2O.
Q: Why must Plaster of Paris be stored in a moisture-proof container?
It absorbs water and sets into a hard mass. After setting, it cannot be used.
Revise after 2 days, then 3 days, then 5 days, then 7 days. This locks the memory.
Continue the Class 10 Science Chemistry journey:
• Metals and Non-metals — complete notes
• Carbon and its Compounds — complete notes
• Complete Chemistry — 130 questions with answers
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