Acids, Bases, Salts and pH – NDA Chemistry Notes

Every day you use things that are acids or bases. Lemon juice is sour because it has acid in it. Soap feels slippery because it is a base. The medicine you take for stomach pain works because it neutralises acid. Even the tea you drink is slightly acidic.

This chapter is the single most important chapter in Chemistry for NDA. NDA asks more questions from this chapter than from any other Chemistry topic. Questions come every year, sometimes three or four questions in a single paper.

NDA tests this chapter through direct questions, match-list questions, assertion-reason questions, and NOT-correct statement questions. The most important facts to know are: the formula of common salts, the water of crystallisation counts, the colour of turmeric with soap, the pH of Milk of Magnesia, and why NaOH cannot be used as an antacid.

1. What is an Acid?

An acid is a substance that tastes sour. It turns blue litmus paper red. When dissolved in water, it releases H⁺ ions. Its pH is less than 7.

The simple test for an acid: put blue litmus paper in the solution. If it turns red, the substance is an acid.

Acids react with metals and release hydrogen gas. They react with carbonates and release carbon dioxide gas. They react with bases to form salt and water.

Acid	Formula	Source or Use
Hydrochloric acid	HCl	Stomach (gastric juice)
Sulphuric acid	H₂SO₄	Car batteries, industry
Nitric acid	HNO₃	Fertilisers, explosives
Acetic acid	CH₃COOH	Vinegar
Citric acid	C₆H₈O₇	Lemon, orange
Lactic acid	C₃H₆O₃	Curd, sour milk
Tartaric acid	C₄H₆O₆	Tamarind
Formic acid (Methanoic acid)	HCOOH	Ant stings, nettle leaf hairs
Oxalic acid	C₂H₂O₄	Spinach, tomato

The acid in our stomach is hydrochloric acid (HCl). It helps digest food and kills harmful bacteria. [NDA 2013-I]

Nettle leaf hairs contain methanoic acid (formic acid). When you touch a nettle plant, this acid enters your skin and causes a burning pain. The same acid is found in ant stings. [NDA 2024-I]

When zinc metal is placed in dilute sulphuric acid, hydrogen gas is produced. The reaction is: Zn + H₂SO₄ → ZnSO₄ + H₂↑ [NDA 2013-II]

2. What is a Base?

A base is a substance that tastes bitter. It feels soapy or slippery when touched. It turns red litmus paper blue. When dissolved in water, it releases OH⁻ ions. Its pH is more than 7.

A base that dissolves in water is called an alkali. All alkalis are bases. But not all bases are alkalis. Some bases do not dissolve in water, for example, Cu(OH)₂ and Fe(OH)₃ are bases but they do not dissolve in water, so they are not alkalis.

Base	Formula	Common Name or Use
Sodium hydroxide	NaOH	Caustic soda: soap making
Calcium hydroxide	Ca(OH)₂	Slaked lime: whitewashing
Magnesium hydroxide	Mg(OH)₂	Milk of Magnesia: antacid
Ammonium hydroxide	NH₄OH	Cleaning agent
Calcium oxide	CaO	Quicklime: construction
Aluminium hydroxide	Al(OH)₃	Antacid

Milk of Magnesia is the common name for magnesium hydroxide, Mg(OH)₂. It is used as an antacid to treat stomach acidity. Its pH is approximately 10. [NDA 2024-I, NDA 2021-II]

3. Acids vs Bases

Property	Acid	Base
Taste	Sour	Bitter
Feel	—	Soapy, slippery
Litmus	Blue → Red	Red → Blue
pH	Less than 7	More than 7
Ion in water	H⁺	OH⁻
Examples	HCl, H₂SO₄, CH₃COOH	NaOH, Ca(OH)₂, Mg(OH)₂

4. Strong and Weak Acids and Bases

Some acids break apart completely in water. These are called strong acids. HCl, H₂SO₄, and HNO₃ are strong acids.

Some acids break apart only partially. These are called weak acids. Acetic acid (vinegar), citric acid, and lactic acid are weak acids.

The same idea applies to bases. NaOH and KOH are strong bases. They break apart completely in water. Mg(OH)₂ is a weak base. It breaks apart only partially.

This difference matters for antacids. A strong base like NaOH would burn and damage the stomach lining. Only mild, weak bases are safe to use as antacids. That is why Mg(OH)₂, Al(OH)₃, and NaHCO₃ are used, not NaOH.

NaOH is the wrong choice for an antacid. It is too strong and too corrosive. [NDA 2012-I]

5. The pH Scale

pH tells us how acidic or basic a solution is. The pH scale goes from 0 to 14.

• pH less than 7 = acidic
• pH equal to 7 = neutral
• pH more than 7 = basic

Substance	pH
Pure water	7 (neutral)
Human blood	7.35 to 7.45
Stomach acid	1 to 2
Milk of Magnesia	10
NaOH solution	13 to 14

There is one very important trap in NDA questions. When H⁺ concentration goes up, pH goes DOWN, not up. More acid means lower pH. This seems backwards but it is correct. NDA 2025-I directly tested this. The statement “higher H⁺ concentration means higher pH” is false. [NDA 2025-I]

Tooth decay starts when the pH in the mouth falls below 5.5. The acid dissolves the tooth enamel. Toothpaste is a mild base. It neutralises the acid in the mouth and protects the teeth.

6. Indicators

An indicator is a substance that changes colour when placed in an acid or a base. It tells us whether a solution is acidic, basic, or neutral.

Indicator	In Acid	In Base	In Neutral	Note
Litmus	Red	Blue	Purple	Made from lichens
Phenolphthalein	Colourless	Pink	Colourless	Used in titration
Methyl orange	Red	Yellow	Orange	Used in titration
Universal indicator	Red/Yellow	Blue/Violet	Green	Shows pH range
Turmeric	Yellow	Reddish-brown	Yellow	Natural indicator

Litmus is made from lichens. Lichens are organisms made of algae and fungi living together.

Turmeric as an indicator (very important for NDA): Turmeric is yellow in neutral and acidic conditions. When it touches a base like soap, it turns reddish-brown. When washed with water (which is neutral), it turns yellow again.

NDA asked this as a sequence question. The correct sequence is: Yellow → Reddish-brown → Yellow. [NDA 2020 I & II]

When you scrub soap on a turmeric stain on white clothes, the stain turns reddish-brown. This happens because soap is alkaline. When you wash with water, the stain turns yellow again.

Baking soda is alkaline. When you add baking soda to turmeric, the turmeric turns reddish-brown. It appears to fade or become colourless against white fabric. [NDA 2013-II]

Universal indicator is not used in titration. It does not give a sharp colour change at the endpoint. Phenolphthalein and methyl orange are used for titration instead.

7. Neutralisation

When an acid and a base react together, they produce a salt and water. This reaction is called neutralisation. It always releases heat. It is exothermic.

Acid + Base → Salt + Water

Examples:

• HCl + NaOH → NaCl + H₂O
• H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O

Situation	Problem	Solution
Stomach acidity	Too much HCl	Mg(OH)₂, Al(OH)₃, NaHCO₃
Bee sting (acidic)	Formic acid pain	Apply baking soda (base)
Wasp sting (alkaline)	Alkaline venom	Apply vinegar (acid)
Acidic soil	Poor crop growth	Add lime Ca(OH)₂
Tooth decay	Acid dissolves enamel	Toothpaste (mild base)

8. Salts

A salt is formed when the H⁺ of an acid is replaced by a metal ion. Salts are produced in neutralisation reactions.

Salt type	Formed from	pH	Example
Neutral salt	Strong acid + Strong base	7	NaCl
Acidic salt	Strong acid + Weak base	Less than 7	NH₄Cl
Basic salt	Weak acid + Strong base	More than 7	Na₂CO₃

9. Water of Crystallisation

Some salts have water molecules trapped inside their crystals. This water is called water of crystallisation. It gives the crystal its shape and colour.

When you heat these salts, they lose the water and change colour. For example, blue CuSO₄·5H₂O becomes white anhydrous CuSO₄ when heated. If you add water back, it turns blue again. This colour change is used to test for the presence of water.

[Image Suggestion: Blue hydrated copper sulphate crystals (CuSO₄·5H₂O) turning white anhydrous CuSO₄ on heating, and returning to blue when water is added back]

Salt	Common Name	Water molecules
CuSO₄·5H₂O	Blue vitriol / Blue copper sulphate	5
Na₂CO₃·10H₂O	Washing soda	10
CaSO₄·2H₂O	Gypsum	2
FeSO₄·7H₂O	Green vitriol / Ferrous sulphate	7
MgSO₄·7H₂O	Epsom salt	7
AlK(SO₄)₂·12H₂O	Alum	12

NDA asked: How many water molecules are in copper sulphate, sodium carbonate, and gypsum? The answer is 5, 10, and 2, respectively. [NDA 2020 I & II]

NDA asked: How many water molecules are in ferrous sulphate? The answer is 7. [NDA 2021-II]

10. Important Salts

Baking Soda (NaHCO₃)

Baking soda is sodium hydrogen carbonate. When heated to about 70°C, it breaks down and releases carbon dioxide gas. [NDA 2020 I & II]

2NaHCO₃ → Na₂CO₃ + H₂O + CO₂↑

The CO₂ gas creates bubbles in dough. This makes bread and cakes light and fluffy. That is why baking soda is used in baking.

Baking soda is also used as an antacid. It is a mild, safe base. It neutralises excess HCl in the stomach.

Baking soda is used in soda-acid fire extinguishers. It reacts with sulphuric acid to produce CO₂, which smothers the fire.

Baking soda is NOT corrosive. It is completely safe to eat.

Common salt (NaCl) is NOT used to make Plaster of Paris. NaCl is used to make NaOH, Na₂CO₃, NaHCO₃, and bleaching powder. But POP is made from gypsum, a calcium compound. NaCl has no role in making POP. [NDA 2021-I]

Washing Soda (Na₂CO₃·10H₂O)

Washing soda is sodium carbonate decahydrate. It has 10 water molecules.

It is used to soften hard water. It removes Ca²⁺ and Mg²⁺ ions from water by making them precipitate as insoluble carbonates. It is also used in laundry, glass making, and soap making.

Na₂CO₃ (without water) is called soda ash. It is the anhydrous form of washing soda.

Washing soda is used to soften hard water.

Property	Baking Soda	Washing Soda
Formula	NaHCO₃	Na₂CO₃·10H₂O
Nature	Mild base	Moderate-strong base
Safe to eat?	Yes	No
CO₂ on heating?	Yes	No
Main use	Baking, antacid	Water softening, cleaning

Bleaching Powder (CaOCl₂)

Bleaching powder is calcium oxychloride. It is made by passing chlorine gas over slaked lime.

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

It is used to bleach cotton and linen. It is used to disinfect drinking water. It is used to disinfect swimming pools.

Bleaching powder AND DDT both contain chlorine. This is an NDA trap. Bleaching powder (CaOCl₂) contains calcium AND chlorine. DDT is an organic compound that also contains chlorine, but it does NOT contain calcium. [NDA 2020 I & II]

Gypsum and Plaster of Paris (POP)

Gypsum is CaSO₄·2H₂O. It is a natural mineral. It has 2 water molecules.

Plaster of Paris (POP) is CaSO₄·½H₂O. It is made by heating gypsum at 120°C. Heating removes most of the water.

CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O (at 120°C)

When POP is mixed with water, the reverse reaction happens. POP absorbs water and turns back into gypsum. The mixture sets hard and expands slightly. This is why POP is used for fracture casts and moulds.

CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (sets hard)

NDA asked: What is the formula of Plaster of Paris? The answer is CaSO₄·½H₂O. [NDA 2021-II]

Property	Gypsum	Plaster of Paris
Formula	CaSO₄·2H₂O	CaSO₄·½H₂O
Water molecules	2	½
How made	Natural mineral	By heating gypsum at 120°C
On adding water	Stays as gypsum	Converts back to gypsum, sets hard
Use	Cement retarder, plasterboard	Fracture casts, moulds, statues

Gypsum is added to cement to slow down its setting time. Without gypsum, cement would harden too quickly. [NDA 2016-II]

Quicklime and Slaked Lime

Quicklime is calcium oxide, CaO. It is made by heating limestone.

CaCO₃ → CaO + CO₂ (on heating)

When water is added to quicklime, a vigorous reaction happens. A hissing sound is produced. A lot of heat is released. Slaked lime (Ca(OH)₂) is formed.

CaO + H₂O → Ca(OH)₂ + Heat

Calcium oxide dissolves in water and releases heat.

Slaked lime Ca(OH)₂ dissolved in water is called lime water. When CO₂ is passed through lime water, it turns milky. A white precipitate of calcium carbonate (CaCO₃) forms. This is the standard test for CO₂.

Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O

The white precipitate is CaCO₃. [NDA 2021-II]

Whitewashing sequence:

1. CaO + H₂O → Ca(OH)₂ (quicklime becomes slaked lime, hissing, heat)
2. Ca(OH)₂ applied to walls as a white liquid
3. Ca(OH)₂ + CO₂ → CaCO₃↓ (white shiny finish forms as CO₂ from air reacts)

Property	Quicklime	Slaked Lime
Formula	CaO	Ca(OH)₂
Made by	Heating limestone	Adding water to CaO
Reaction with water	Vigorous, exothermic, hissing	Already reacted
Use	Making slaked lime, cement	Whitewashing, neutralising acidic soil

Important Salts: Formula Table. NDA directly tests these formulas in match-list questions. Memorise all of them.

Common Name	Chemical Name	Formula
Baking soda	Sodium hydrogen carbonate	NaHCO₃
Washing soda	Sodium carbonate decahydrate	Na₂CO₃·10H₂O
Soda ash	Anhydrous sodium carbonate	Na₂CO₃
Bleaching powder	Calcium oxychloride	CaOCl₂
Plaster of Paris	Calcium sulphate hemihydrate	CaSO₄·½H₂O
Gypsum	Calcium sulphate dihydrate	CaSO₄·2H₂O
Common salt	Sodium chloride	NaCl
Quicklime	Calcium oxide	CaO
Slaked lime	Calcium hydroxide	Ca(OH)₂
Lime water	Calcium hydroxide solution	Ca(OH)₂ (aq)
Milk of Magnesia	Magnesium hydroxide	Mg(OH)₂

NDA 2025-I gave a match-list pairing washing soda, baking soda, bleaching powder, and gypsum with their formulas. [NDA 2025-I]

11. Soil pH and Agriculture

Crops grow best when soil pH is between 6 and 7, slightly acidic to neutral.

If soil is too acidic, lime (Ca(OH)₂) or calcium carbonate (CaCO₃) is added. These are basic. They neutralise the acid in the soil and raise the pH.

If soil is too alkaline, gypsum or sulphur is added to lower the pH.

Dolomite powder CaMg(CO₃)₂ is an alkaline substance. It is added to acidic soil to increase the pH.

12. Acid Rain

Normal rain is slightly acidic. Its pH is about 5.6. This is because CO₂ from the air dissolves in rain water and forms carbonic acid (H₂CO₃). This is natural and not harmful.

Acid rain is different. Its pH is below 5.6. It is caused by SO₂ and NO₂, not by CO₂.

Pollutant gas	Source	Acid formed
SO₂	Burning coal and oil, factories	H₂SO₄
NO₂	Vehicle exhaust, power plants	HNO₃

CO₂ alone does NOT cause acid rain. It only makes rain slightly acidic naturally.

Acid rain damages limestone and marble buildings. It damages aquatic life in lakes and rivers. It makes soil more acidic and reduces crop yield. It corrodes metal structures.