Acids, bases and the pH scale are among the most predictable scorers in the CDS Science paper. UPSC regularly asks about indicators, pH values of common substances, neutralisation, and the chemistry behind antacids. This Cavalier lesson, built from NCERT chemistry, turns the whole chapter into quick rules, ready facts and worked questions you can revise the night before the exam.
Why acids and bases matter in CDS
Every CDS Science paper carries a handful of direct questions on acids, bases and pH. They are easy marks because the facts rarely change: the pH of blood, the gas released when a metal meets an acid, the active ingredient in an antacid, and which indicator turns which colour.
The topic also connects to daily life — curd, lemon, soap, toothpaste, fertiliser and tea all appear in questions. Once you understand the underlying logic, you stop memorising blindly and start predicting answers. Examiners like this chapter because it tests both recall and reasoning at the same time, and a single page of revision can return three or four marks.
For OTA aspirants in particular, who face the same General Science standard, these are non-negotiable marks. The chapter rewards a candidate who can connect a definition to a real example — knowing that lemon is acidic is not enough; you should also know why it turns blue litmus red and roughly where it sits on the pH scale.
Acids and bases are about the transfer of hydrogen ions (H+). Almost every fact in this chapter traces back to whether a substance gives away H+ or accepts it.
What is an acid?
An acid is a substance that releases hydrogen ions (H+) when dissolved in water. These H+ ions are what make a solution acidic, sour to taste and capable of turning blue litmus red.
Key properties of acids
- Sour in taste (lemon, vinegar, tamarind).
- Turn blue litmus red; have no effect on red litmus.
- Conduct electricity in solution because they form ions.
- React with metals to release hydrogen gas.
- React with carbonates and bicarbonates to release carbon dioxide.
Strong vs weak acids
A strong acid ionises almost completely in water (HCl, H2SO4, HNO3). A weak acid ionises only partly (acetic acid in vinegar, carbonic acid, citric acid). Strength depends on how fully it releases H+, not on how concentrated it is.
Mineral vs organic acids
Acids are also grouped by source. Mineral (inorganic) acids such as hydrochloric, sulphuric and nitric acid are prepared from minerals and are usually strong and corrosive. Organic acids such as acetic, citric, lactic, tartaric and oxalic acid come from plants and animals and are usually weak and safe to consume. This is why we eat lemon and curd happily but never sip dilute sulphuric acid.
Acid + Metal → Salt + Hydrogen gas
Acid + Metal carbonate → Salt + Water + Carbon dioxide
The gas that puts out a burning splinter with a "pop" sound is hydrogen; the gas that turns lime water milky is CO2.
What is a base?
A base is a substance that releases hydroxide ions (OH−) in water, or accepts H+ ions. A base that dissolves in water is called an alkali — for example sodium hydroxide (NaOH) and potassium hydroxide (KOH).
Key properties of bases
- Bitter in taste and soapy to touch.
- Turn red litmus blue; no effect on blue litmus.
- Conduct electricity in solution.
- React with acids to form salt and water (neutralisation).
- Strong bases like NaOH are corrosive and can damage skin, just like strong acids.
Where bases show up
Common bases in everyday life include lime water, milk of magnesia, household ammonia, baking soda solution and soap. Their soapy, slippery feel comes from the way OH− ions react with the natural oils on your skin. This is also why cleaning agents and detergents are mildly basic — they cut through grease effectively.
All alkalis are bases, but not all bases are alkalis. A base must dissolve in water to be called an alkali. Copper oxide is a base but not an alkali because it is insoluble.
The pH scale explained
The pH scale measures how acidic or basic a solution is, on a range from 0 to 14. The letter "p" stands for the German potenz (power) and "H" for hydrogen ion concentration.
How to read it
- pH = 7 → neutral (pure water).
- pH less than 7 → acidic; the lower the value, the stronger the acid.
- pH greater than 7 → basic/alkaline; the higher the value, the stronger the base.
As the H+ ion concentration rises, pH falls. Each whole-number step on the scale means a tenfold change in acidity: a solution of pH 3 is ten times more acidic than one of pH 4.
pH = −log[H+]. Pure water has [H+] = 10−7 mol/L, so its pH = 7. Lower pH → more H+ → more acidic.
Memorise anchor values: gastric juice in the stomach is about pH 1.5–2, lemon juice about 2, human blood a tight 7.35–7.45, milk about 6.5, and household ammonia around 11.
pH of common substances
UPSC loves direct recall of pH values. Learn this short ladder from most acidic to most basic. Even if a question phrases the substance differently, the relative ordering will help you eliminate wrong options quickly.
- Gastric (stomach) juice — about 1.5 to 2
- Lemon juice, vinegar — about 2 to 3
- Tomato, soft drinks — about 4
- Black coffee — about 5
- Milk — about 6.5 (slightly acidic)
- Pure water, blood — about 7 to 7.4 (near neutral)
- Baking soda solution — about 9
- Milk of magnesia (antacid) — about 10
- Household ammonia — about 11 to 12
- Sodium hydroxide — close to 14
When fresh milk turns into curd, lactic acid forms, so its pH falls below 6.5. This "milk to curd" pH question is a CDS favourite.
Indicators and their colours
An indicator is a substance that signals whether a solution is acidic or basic by changing colour or smell. They are the quickest way to test acidity without instruments.
Common indicators
- Litmus: red in acid, blue in base. Neutral litmus is purple.
- Methyl orange: red/pink in acid, yellow in base.
- Phenolphthalein: colourless in acid, pink in base.
- Turmeric: yellow in acid, turns red/brown in base.
Olfactory indicators
Some substances change smell in acids or bases — onion and vanilla lose their characteristic odour in a basic solution. These are called olfactory indicators and are useful for visually impaired students.
A clean trick to remember phenolphthalein: "Phenolphthalein is Pink in the Presence of a base." In acid it stays colourless.
Neutralisation and salts
Neutralisation is the reaction between an acid and a base to form a salt and water. The H+ of the acid combines with the OH− of the base to give neutral water.
Acid + Base → Salt + Water
HCl + NaOH → NaCl + H2O
This is an exothermic reaction — it releases heat.
Nature of salts
- Strong acid + strong base → neutral salt (NaCl, pH 7).
- Strong acid + weak base → acidic salt (NH4Cl).
- Weak acid + strong base → basic salt (sodium carbonate, sodium acetate).
Useful salts
- Common salt (NaCl) — raw material for NaOH, baking soda and washing soda.
- Baking soda (NaHCO3) — mild antacid and used in fire extinguishers.
- Washing soda (Na2CO3·10H2O) — removes hardness of water.
- Bleaching powder (CaOCl2) — disinfects water and bleaches cloth.
Antacids and stomach chemistry
Your stomach secretes hydrochloric acid (HCl) to digest food and kill germs. When too much acid is produced, it causes acidity, heartburn and pain. An antacid is a mild base that neutralises this excess acid.
How antacids work
Antacids are weak bases — using a strong base like NaOH would be dangerous. They raise the stomach pH gently through neutralisation, turning excess HCl into harmless salt and water.
- Milk of magnesia — magnesium hydroxide, Mg(OH)2.
- Baking soda — sodium bicarbonate, NaHCO3.
- Antacid tablets — often contain aluminium hydroxide or calcium carbonate.
The same neutralisation logic explains farming: acidic soil is treated with quicklime (CaO) or slaked lime [Ca(OH)2], and a bee sting (acidic) is soothed with baking soda while a wasp sting (basic) is treated with a mild acid like vinegar.
Worked example
A solution has a hydrogen ion concentration [H+] = 10−3 mol/L. Find its pH and state whether it is acidic, neutral or basic. How does its acidity compare with a solution of pH 6?
This shows why a tiny pH change matters: dropping just three pH units makes a solution one thousand times more acidic. The same logic explains why a small fall in the pH of blood or soil can have large biological effects, and why the body works hard to keep blood pH within the narrow 7.35–7.45 band.
Common mistakes to avoid
Students often confuse concentration with strength. A dilute strong acid (like dilute HCl) is still a strong acid; a concentrated weak acid (like concentrated acetic acid) is still weak. Strength is about how fully it ionises.
- Higher pH does not mean more acidic — it means more basic. Acidity rises as pH falls.
- Phenolphthalein is colourless in acid, not red — that is methyl orange.
- Acids do not always react with metals to give hydrogen — reactions with nitric acid usually give nitrogen oxides instead.
- Curdling lowers the pH of milk; it does not raise it.
Previous-year style question
Q. Which one of the following is the most likely pH value of a solution of a strong base such as sodium hydroxide, and which gas is released when dilute sulphuric acid reacts with zinc metal?
Answer: A strong base like NaOH has a pH close to 14 (well above 7). When dilute sulphuric acid reacts with zinc, the gas released is hydrogen (H2), which burns with a characteristic "pop" sound. The reaction is Zn + H2SO4 → ZnSO4 + H2.
If a question pairs a metal with an acid and asks for the gas, the default answer is hydrogen — unless the acid is nitric acid, which behaves differently.
Quick revision
- Acids release H+, taste sour, turn blue litmus red; bases release OH−, taste bitter, turn red litmus blue.
- pH runs 0–14: below 7 acidic, 7 neutral, above 7 basic; each step is a tenfold change.
- Litmus, methyl orange, phenolphthalein and turmeric are the standard indicators — phenolphthalein is pink only in base.
- Acid + base → salt + water (neutralisation), an exothermic reaction.
- Antacids are mild bases (Mg(OH)2, NaHCO3) that neutralise excess stomach HCl.
- Strength (ionisation) is not the same as concentration.
Revise the pH ladder and the four indicators the night before your exam — together they cover most of the marks this chapter offers.
Frequently asked questions
What is the pH of pure water and why?
Pure water has a pH of 7, which is neutral. This is because it contains equal concentrations of H+ and OH- ions, each at 10^-7 mol/L at 25 degrees Celsius.
Why are antacids basic and not acidic?
Acidity is caused by excess hydrochloric acid in the stomach. An antacid must be a mild base so it can neutralise that acid through a neutralisation reaction, forming harmless salt and water without harming the stomach lining.
What is the difference between a strong acid and a concentrated acid?
Strength refers to how completely an acid ionises in water, while concentration refers to how much acid is dissolved per unit volume. A strong acid can be dilute, and a weak acid can be concentrated.
Which indicator is used to detect a base, and what colour does it show?
Phenolphthalein is commonly used. It remains colourless in acidic solutions and turns pink in basic solutions, making it ideal for spotting alkalinity.
What happens to the pH of milk when it turns into curd?
When milk turns into curd, lactic acid is produced. This increases the hydrogen ion concentration, so the pH falls below its original value of about 6.5, making the curd more acidic than the milk.
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