Acids and bases are everywhere — in your stomach, in soap, in a battery, even in the soil. For the NDA exam, Acids, Bases and the pH Scale is one of the most reliable scoring chapters, with direct questions on indicators, neutralisation and pH values almost every year. This page from The Cavalier makes every concept simple, exam-ready and easy to recall.
Why this topic matters for NDA
In the NDA General Ability Test, chemistry questions reward students who know the basics cold. Acids, Bases and the pH Scale is exactly that kind of high-yield, low-effort chapter — the questions are direct, factual and rarely tricky.
You will usually see one or two questions on indicator colours, the pH of a common substance, the chemical name of an everyday acid or base, or a neutralisation reaction. These are pure recall marks if your fundamentals are clear.
Do not over-read this topic. Memorise the indicator colour table, the pH scale, and a handful of everyday acids, bases and salts, and you can answer almost every NDA question here in seconds.
What are acids and bases?
An acid is a substance that tastes sour and produces hydrogen ions (H+) when dissolved in water. Common examples are lemon juice, vinegar and curd.
A base is a substance that tastes bitter, feels soapy and produces hydroxide ions (OH−) when dissolved in water. A base that dissolves in water is called an alkali, for example sodium hydroxide and potassium hydroxide.
According to the Arrhenius theory, acids give H+ ions in water and bases give OH− ions in water. According to the Bronsted-Lowry theory, an acid is a proton (H+) donor and a base is a proton acceptor.
In practice, the H+ ion does not float around alone in water. It joins a water molecule to form the hydronium ion (H3O+). So when we write that acids release H+, we really mean they release hydronium ions in solution. This is why acids show their properties — sour taste, reaction with metals, colour change in indicators — only when dissolved in water. A dry acid such as solid citric acid crystals will not turn dry litmus paper red.
Acids release H+; bases release OH−. An alkali is simply a water-soluble base. Never taste laboratory chemicals to test them — use indicators instead.
Strong and weak acids and bases
The strength of an acid or base depends on how completely it splits into ions (ionises) in water, not on how concentrated it is.
- Strong acids ionise almost completely: hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3).
- Weak acids ionise only partly: acetic acid (CH3COOH), carbonic acid (H2CO3), citric acid.
- Strong bases: sodium hydroxide (NaOH), potassium hydroxide (KOH).
- Weak bases: ammonium hydroxide (NH4OH), magnesium hydroxide.
Students confuse strong with concentrated. Strength is about degree of ionisation; concentration is about how much acid is dissolved in a given volume. A dilute HCl solution is still a strong acid.
Indicators: how to spot acids and bases
An indicator is a substance that changes colour to show whether a solution is acidic or basic. They are the most heavily tested part of this chapter, so learn the table by heart.
- Litmus: turns red in acid, blue in base. (Neutral litmus is purple.)
- Methyl orange: red in acid, yellow in base.
- Phenolphthalein: colourless in acid, pink in base.
- Turmeric: stays yellow in acid, turns red in base.
- Red onion / vanilla: these are olfactory indicators — their smell changes.
Memory hook: litmus → acid Red, base Blue. Phenolphthalein is Pink only in a base. Methyl orange goes Red in acid.
Natural indicators include litmus (from lichens), turmeric and the petals of the China rose (hydrangea), which turn pink in acid and green in base.
Two more useful examples worth remembering are red cabbage juice, which turns red in acid and greenish-yellow in base, and the petals of the Petunia flower. Indicators work because the molecule itself has a slightly different shape and colour depending on whether it has gained or lost a hydrogen ion, and that difference is what your eye sees as a colour change.
How acids and bases react
Knowing a few standard reactions helps you predict products and answer equation-based questions.
Acid + metal
An acid reacts with a metal to give a salt and hydrogen gas. Hydrogen is detected by the pop sound when a burning splinter is brought near it.
Acid + Metal → Salt + Hydrogen gas (H2↑)
Example: Zn + H2SO4 → ZnSO4 + H2
Acid + metal carbonate / bicarbonate
This gives a salt, water and carbon dioxide. CO2 turns lime water milky — a common test question.
Example: Na2CO3 + 2HCl → 2NaCl + H2O + CO2↑
Base + metal
Some metals like zinc and aluminium react with hot concentrated alkali to release hydrogen gas as well, which is why such metals are called amphoteric in behaviour.
Neutralisation reaction
When an acid reacts with a base, they cancel each other to form a salt and water. This is called neutralisation, and it is an exothermic reaction (heat is released).
Acid + Base → Salt + Water
Example: HCl + NaOH → NaCl + H2O
The real reaction at ionic level: H+ + OH− → H2O
Neutralisation has many everyday uses: antacids (a mild base like magnesium hydroxide) neutralise excess stomach acid; lime (a base) is added to acidic soil; baking soda on a bee sting neutralises the acidic venom; and toothpaste (basic) neutralises acids made by mouth bacteria.
A neutralisation reaction does not always give a neutral (pH 7) solution. Only a strong acid with a strong base gives pH 7; weak acid + strong base gives a basic salt solution.
The pH scale explained
The pH scale measures how acidic or basic a solution is. The letter p stands for potenz (German for power) and H stands for hydrogen ion. It was given by the scientist S.P.L. Sorensen.
The scale runs from 0 to 14:
- pH less than 7 → acidic (lower pH = stronger acid).
- pH equal to 7 → neutral (pure water).
- pH greater than 7 → basic / alkaline (higher pH = stronger base).
pH = −log10[H+]
where [H+] is the hydrogen ion concentration in moles per litre. As [H+] goes up, pH goes down.
A change of one pH unit means a tenfold change in hydrogen ion concentration. So a solution of pH 3 is ten times more acidic than one of pH 4, and a hundred times more acidic than pH 5.
pH of common substances
NDA loves to ask the approximate pH of an everyday substance, so commit these to memory.
- Gastric (stomach) acid: around 1 to 2 (strongly acidic).
- Lemon juice: around 2.
- Vinegar: around 3.
- Pure water / blood: 7 (blood is very close, 7.4).
- Baking soda solution: around 9.
- Milk of magnesia: around 10.
- Sodium hydroxide: around 13 to 14.
Human blood is kept at a steady pH of about 7.4. Our body cannot survive large changes in blood pH — this fact appears in both biology and chemistry questions.
Why pH matters in daily life
pH is not just theory — it controls many real processes that the exam links to this chapter.
- Living organisms survive only in a narrow pH range; our body keeps blood near 7.4.
- Soil pH affects crop growth; farmers add lime to acidic soil and gypsum to alkaline soil.
- Tooth decay begins when mouth pH falls below 5.5, because acid then dissolves tooth enamel.
- Acid rain has a pH below 5.6 and damages buildings, soil and aquatic life.
- The stomach makes hydrochloric acid to digest food; excess acid causes acidity, treated with antacids.
The stomach produces dilute hydrochloric acid to break down food and to kill harmful bacteria swallowed with it. When it makes too much acid we feel a burning sensation called acidity, treated with an antacid — a mild base such as magnesium hydroxide (milk of magnesia) or sodium bicarbonate that neutralises the excess. Tooth decay works the same way: bacteria turn leftover sugar into acid, and once the mouth pH drops below 5.5 this acid eats into the enamel, which is why a mildly basic toothpaste protects your teeth.
Acid rain is not pure acid falling from the sky. It is normal rain that has dissolved oxides of sulphur and nitrogen (SO2, NO2) to form weak sulphuric and nitric acids, lowering its pH.
When a bee stings, it injects an acidic liquid, so rubbing on a mild base like baking soda gives relief. A wasp sting is alkaline, so a mild acid such as vinegar helps instead.
Salts and their everyday uses
A salt is the compound formed when the hydrogen of an acid is replaced by a metal. Salts come from the parent acid and base of a neutralisation reaction.
- Common salt (NaCl): from HCl + NaOH; used in cooking and to make other chemicals.
- Washing soda (Na2CO3·10H2O): used to soften hard water and in cleaning.
- Baking soda (NaHCO3): used in cooking and as a mild antacid.
- Bleaching powder (CaOCl2): used to disinfect water and bleach fabric.
- Plaster of Paris (CaSO4·½H2O): used to set fractured bones and in moulds.
Salts can be acidic, basic or neutral. NaCl (strong acid + strong base) is neutral; sodium carbonate (weak acid + strong base) is basic; ammonium chloride (strong acid + weak base) is acidic.
Worked example: calculating pH
Let us see how to find pH from the hydrogen ion concentration, a calculation that sometimes appears in the exam.
A solution has a hydrogen ion concentration [H+] = 1 × 10−3 mol/L. Find its pH and state whether it is acidic or basic.
Notice that the power of 10 (with the sign flipped) directly gives the pH when the coefficient is 1. This shortcut saves time in the exam.
Previous-year style question
Here is a question in the exact style the NDA paper uses for this chapter.
Q. Which one of the following is correct about a solution of pH = 5? (a) It is basic (b) It is ten times more acidic than a solution of pH = 6 (c) It is neutral (d) It contains only OH− ions
Answer: (b). A solution of pH 5 is acidic, and since each pH unit is a tenfold change in H+ concentration, pH 5 is ten times more acidic than pH 6.
- Acids give H+, bases give OH−; an alkali is a soluble base.
- Litmus: red in acid, blue in base. Phenolphthalein: pink only in base.
- Acid + base → salt + water (neutralisation, exothermic).
- pH = −log[H+]; below 7 acidic, 7 neutral, above 7 basic.
- One pH unit = tenfold change in acidity; blood pH ≈ 7.4.
- Know everyday salts: common salt, washing soda, baking soda, bleaching powder, plaster of Paris.
Frequently asked questions
What is the difference between a base and an alkali?
Every alkali is a base, but not every base is an alkali. A base that dissolves in water, such as NaOH or KOH, is called an alkali. Insoluble bases like copper oxide are still bases but not alkalis.
Does strong always mean concentrated?
No. Strength refers to how completely an acid or base ionises in water, while concentration refers to how much is dissolved per litre. A dilute solution of HCl is still a strong acid because HCl ionises almost completely.
What does a pH of 7 mean?
A pH of 7 means the solution is neutral, with equal concentrations of H+ and OH- ions. Pure water at room temperature has a pH of 7. Below 7 is acidic and above 7 is basic.
Why is neutralisation important in daily life?
Neutralisation is used to treat acidity with antacids, to correct acidic soil with lime, to relieve bee and ant stings with baking soda, and in toothpaste to neutralise mouth acids that cause tooth decay.
Why is acid rain harmful?
Acid rain forms when rain dissolves oxides of sulphur and nitrogen to make weak acids, lowering its pH below 5.6. It corrodes buildings and statues, harms aquatic life, and damages soil fertility.
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