Acid, Base and Salts
Introduction
If you've ever bitten into a lemon and instantly puckered up, or mixed baking soda with vinegar just to watch it fizz over the kitchen counter, you've already done chemistry. Acids, bases, and salts aren't locked away in a lab somewhere — they're in your morning tea, your toothpaste, the antacid you popped after a heavy lunch, and the soap you used to wash your hands.
But if you're prepping for UPSC, SSC, JEE, NEET, or your Class 10 boards, you already know this chapter shows up everywhere. The problem is, most notes on this topic either oversimplify it into a list of definitions to memorize, or throw so much jargon at you that the actual logic gets lost. This article tries to do neither. We'll build the concept from the ground up, add the theory that most notes skip entirely, and work through the kind of numerical problems that actually show up in exams.
What
Are Acids, Bases and Salts?
Acids taste sour and turn blue litmus paper red. Think of lemon juice,
vinegar, or the acid in your stomach. Chemically, an acid is a substance that
releases hydrogen ions (H⁺)
when dissolved in water. Hydrochloric acid (HCl), for instance, splits into H⁺ and Cl⁻ ions in solution.
Bases taste bitter, feel slippery (think soap), and turn red litmus paper blue. A base releases hydroxide ions (OH⁻) in water. Sodium hydroxide (NaOH) is a classic example — it's what's used in soap-making.
Salts are what you get when an acid and a base cancel each other out. Mix hydrochloric acid with sodium hydroxide, and you get sodium chloride — plain table salt — plus water. This is called a neutralization reaction, and we'll come back to it in detail shortly.
Here's
a quick way to keep these straight:
|
Property |
Acid |
Base |
Salt |
|
Taste |
Sour |
Bitter |
Varies (often neither) |
|
Litmus test |
Blue → Red |
Red → Blue |
No color change (if neutral) |
|
Example |
Vinegar, lemon juice |
Soap, ammonia |
Table salt, baking soda |
|
Ion released in water |
H⁺ |
OH⁻ |
Depends on parent acid/base |
Acid-Base Theories — Arrhenius, Brønsted-Lowry & Lewis
Here's where most notes stop at the surface. The definitions above — acids release H⁺, bases release OH⁻ — come from the Arrhenius theory, proposed by Svante Arrhenius in 1884. It's a great starting point, but it has a problem: it can't explain why ammonia (NH₃), which contains no OH⁻ at all, still behaves like a base.
That gap is exactly why two more theories exist, and if you're aiming for JEE, NEET, or even the deeper conceptual questions in UPSC, you need to know all three.
Brønsted-Lowry theory redefines things in terms of protons. An acid is a proton (H⁺) donor, and a base is a proton acceptor. This immediately fixes the ammonia problem: when NH₃ reacts with water, it accepts a proton from water to form NH₄⁺ and OH⁻. NH₃ never needed to contain hydroxide — it just needed to grab a proton.
Lewis theory goes even broader. An acid is an electron pair acceptor, and a base is an electron pair donor. This is the definition that explains reactions with no protons involved at all — like boron trifluoride (BF₃) acting as an acid because it accepts an electron pair, even though there's no H⁺ in sight.
Here's how
the three stack up:
|
Theory |
Acid Defined As |
Base Defined As |
Limitation |
|
Arrhenius |
H⁺ releaser in water |
OH⁻ releaser in water |
Only works in aqueous solutions |
|
Brønsted-Lowry |
Proton donor |
Proton acceptor |
Still needs a proton to be involved |
|
Lewis |
Electron pair acceptor |
Electron pair donor |
Most general, but harder to apply quickly |
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Chemical Properties & Reactions of Acids and Bases
This is the part examiners test the most, because it's rule-based and easy to frame as a "predict the product" question. Let's go through each reaction with an actual example, not just the general equation.
Acid +
Metal → Salt + Hydrogen gas Drop a piece of zinc into dilute
hydrochloric acid, and you'll see bubbles forming — that's hydrogen gas
escaping.
Zn + 2HCl → ZnCl₂ + H₂↑
Metal
carbonate/bicarbonate + Acid → Salt + Carbon dioxide + Water This is the classic
"fizzing" reaction you get when vinegar meets baking soda (sodium
bicarbonate).
NaHCO₃ + HCl →
NaCl + CO₂↑ + H₂O
Acid + Base → Salt + Water (Neutralization) The reaction we mentioned earlier. This is also the chemistry behind antacid tablets neutralizing excess stomach acid. HCl + NaOH → NaCl + H₂O
Metal
oxide + Acid → Salt + Water Metal oxides are basic in nature, so
they behave just like a base when they meet an acid.
CuO + 2HCl → CuCl₂ + H₂O
Non-metal
oxide + Base → Salt + Water Non-metal oxides are acidic in nature
— this is actually why rising CO₂
levels make rainwater slightly acidic.
CO₂ +
Ca(OH)₂ →
CaCO₃ + H₂O
A
common mistake students make: forgetting to balance the equation after
identifying the products. Getting the reaction type right earns you half the
marks — balancing correctly earns you the rest. Always count atoms on both
sides before you finalize your answer, especially with reactions involving
carbonates, where CO₂
and H₂O both
show up as products and it's easy to lose track of oxygen atoms.
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pH Scale — Concept, Calculation & Importance
The pH scale is how we measure exactly how acidic or basic something is, on a scale from 0 to 14. A pH of 7 is neutral (pure water). Below 7 is acidic — the lower the number, the stronger the acid. Above 7 is basic — the higher the number, the stronger the base.
The formula is:
pH = -log[H⁺]
where [H⁺] is the concentration of hydrogen ions in moles per liter. There's also pOH, which follows the same logic for hydroxide ion concentration, and the two are related by:
pH + pOH = 14 (at 25°C)
Let's actually work through a few problems, because this is where most notes fall short — they give you the formula and leave you to figure out the rest.
Example 1: What is the pH of a solution with [H⁺] = 10⁻³ M? pH = -log(10⁻³) = 3 This is a fairly strong acid — think stomach acid territory.
Example 2: A solution has a pH of 12. What is its [OH⁻] concentration? Since pH + pOH = 14, pOH = 14 - 12 = 2 [OH⁻] = 10⁻² M This is a moderately strong base.
Example 3: If [H⁺] = 4 × 10⁻⁵ M, find the pH. pH = -log(4 × 10⁻⁵) = -(log 4 + log 10⁻⁵) = -(0.602 - 5) = 4.398 This kind of question tests whether you can handle non-round numbers, which is common in JEE-level papers.
Now, an important distinction: strong vs. weak acids and bases isn't about concentration — it's about how completely they ionize in water. HCl is a strong acid because it almost completely splits into H⁺ and Cl⁻. Acetic acid (found in vinegar) is weak because only a small fraction of its molecules ionize, even though you could make a very concentrated vinegar solution.
|
|
Strong |
Weak |
|
Acid Example |
HCl, H₂SO₄, HNO₃ |
Acetic acid, carbonic acid, formic acid |
|
Base Example |
NaOH, KOH |
Ammonium hydroxide (NH₄OH) |
|
Ionization |
Nearly complete |
Partial |
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Buffer Solutions
A buffer is a solution that resists changes in pH when small amounts of acid or base are added to it. It does this by containing a mix of a weak acid and its conjugate base (or a weak base and its conjugate acid), which can absorb extra H⁺ or OH⁻ ions without dramatically shifting the pH.
There are two types:
· Acidic buffer — a weak acid + its salt (e.g., acetic acid + sodium acetate). Maintains pH below 7.
· Basic buffer — a weak base + its salt (e.g., ammonium hydroxide + ammonium chloride). Maintains pH above 7.
The most important real-world example, and one that ties directly into NEET biology, is the bicarbonate buffer system in your blood. Your blood contains carbonic acid (H₂CO₃) and bicarbonate ions (HCO₃⁻) working together to keep blood pH locked between 7.35 and 7.45, even as your body constantly produces CO₂ and metabolic acids. Without this buffer, something as simple as intense exercise could throw your blood pH dangerously off balance.
The relationship between buffer components and pH is captured by the Henderson-Hasselbalch equation:
pH = pKa + log([conjugate base]/[weak acid])
You
don't necessarily need to memorize this for Class 10 or SSC-level exams, but
for JEE and NEET, it's fair game, especially in questions asking you to
calculate the pH of a buffer given the concentrations of its components.
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Salts — Types, Formation & Family of Salts
Salts form when an acid and a base neutralize each other, but not all salts are neutral. The pH of the resulting salt depends entirely on the strength of the parent acid and base:
|
Combination |
Resulting Salt pH |
Example |
|
Strong acid + Strong base |
Neutral (pH = 7) |
NaCl (from HCl + NaOH) |
|
Strong acid + Weak base |
Acidic (pH < 7) |
NH₄Cl (from HCl + NH₄OH) |
|
Weak acid + Strong base |
Basic (pH > 7) |
CH₃COONa (from CH₃COOH + NaOH) |
This trips up a lot of students because it feels counterintuitive — how can a "salt" be acidic? The answer lies in what happens when the salt dissolves in water. Take ammonium chloride: the ammonium ion (NH₄⁺) is a weak acid in its own right, and it partially reacts with water to release extra H⁺ ions, nudging the solution's pH below 7.
Salts
also belong to families based on shared ions. NaCl and Na₂SO₄ both belong to the "sodium
salt" family because they share the sodium cation. NaCl and KCl belong to
the "chloride salt" family because they share the chloride anion.
This classification matters when exams ask you to group or identify salts based
on shared properties.
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Chemicals from Common Salt
Sodium
chloride — the salt on your dinner table — is also the starting point for a
surprising number of everyday chemicals. Here's how:
Sodium hydroxide is produced by passing electricity through a concentrated solution of NaCl (called brine). This process is called the chlor-alkali process, and it also produces chlorine gas and hydrogen gas as byproducts.
Bleaching powder comes from passing that chlorine gas over dry slaked lime. You'll find it used to disinfect drinking water, bleach cotton and paper pulp, and as an oxidizing agent in various industries.
Baking soda (sodium bicarbonate) is actually a byproduct of the chlor-alkali process. Beyond making your cakes rise, it's used as an antacid, in fire extinguishers (it releases CO₂ when heated), and even to clean tarnished silverware.
Washing soda (sodium carbonate) is made by recrystallizing baking soda. It's a key ingredient in glass, soap, and paper manufacturing, and it's also used to remove the permanent hardness of water — something that matters a lot in industrial water treatment.
|
Chemical |
Made From |
Key Uses |
|
Sodium Hydroxide |
Electrolysis of brine (chlor-alkali process) |
Soap-making, paper industry |
|
Bleaching Powder |
Chlorine + slaked lime |
Water disinfection, textile bleaching |
|
Baking Soda |
Byproduct of chlor-alkali process |
Antacid, baking, fire extinguishers |
|
Washing Soda |
Recrystallized baking soda |
Glass/soap manufacturing, water softening |
Water of Crystallisation & Plaster of Paris
Some salts, when they crystallize, trap a fixed number of water molecules within their crystal structure. This water is called the water of crystallisation, and it's different from a salt simply being "wet" — it's chemically bound in a specific ratio.
Copper sulphate crystals (CuSO₄·5H₂O) hold exactly five water molecules per formula unit — which is also why blue copper sulphate turns white when heated (it loses this water) and turns blue again when water is added back.
Gypsum (CaSO₄·2H₂O) holds two water molecules. Heat gypsum to around 373 K, and it loses three-quarters of that water to become calcium sulphate hemihydrate — better known as Plaster of Paris. Mix Plaster of Paris with water, and it sets back into a hard, solid mass of gypsum. This is exactly why doctors use it to make casts for fractured bones — it's applied as a paste and hardens into a rigid support.
One
point that often trips students up in exams: water of crystallisation is not
the same as water of hydration in the general sense — it refers specifically to
water molecules chemically bonded within a crystal lattice in a fixed, definite
proportion.
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MCQs
1. The acid present in an ant's sting is:
A. Acetic acid
B. Citric acid
C. Formic acid
D. Oxalic acid
Answer: C — Formic acid. Ants inject formic (methanoic) acid
through their stinger, which is why the sting burns.
2.
Which gas is
released when dilute HCl reacts with sodium bicarbonate?
A. Oxygen
B. Hydrogen
C. Carbon dioxide
D. Chlorine
Answer: C — Carbon dioxide. This is the same fizzing reaction
you see with vinegar and baking soda.
3.
Which of the
following best explains why rainwater is classified as "acid rain"?
A. pH below 5.6 due to dissolved CO₂,
SO₂, and
NOx
B. pH above 7 due to dust particles
C. Presence of dissolved oxygen
D. High salinity
Answer: A. Natural rainwater is mildly acidic (~pH 5.6) due to
dissolved CO₂;
industrial SO₂/NOx
push it lower, causing acid rain.
4.
Chemical name of
baking soda is:
A. Sodium carbonate
B. Sodium bicarbonate
C. Calcium carbonate
D. Sodium hydroxide
Answer: B — Sodium bicarbonate (NaHCO₃).
5.
Plaster of Paris
is chemically known as:
A. Calcium sulphate dihydrate
B. Calcium sulphate hemihydrate
C. Calcium carbonate
D. Calcium oxide
Answer: B — Calcium sulphate hemihydrate, made by heating
gypsum to remove part of its water of crystallisation.
6. Consider the following statements about pH:
1. A solution with pH 3 is more acidic than one with pH 5.
2. Strong acids are always more
concentrated than weak acids.
Which is/are correct?
A. 1 only
B. 2 only
C. Both
D. Neither
Answer: A only. Strength depends on degree of ionization, not
concentration — statement 2 is a common trap.
7.
Which of the following
is used in the treatment of drinking water for disinfection?
A. Washing soda
B. Bleaching powder
C. Plaster of Paris
D. Baking soda
Answer: B — Bleaching powder, due to its oxidizing/germicidal
action.
8.
The chlor-alkali
process yields which of the following as by-products along with sodium
hydroxide?
A. Chlorine and hydrogen gas
B. Oxygen and nitrogen gas
C. Carbon dioxide and water
D. Sulphur dioxide
Answer: A. Electrolysis of brine gives NaOH at the cathode,
with Cl₂ and H₂ as by-products.
9.
Which salt is
used to remove permanent hardness of water?
A. Sodium chloride
B. Sodium bicarbonate
C. Sodium carbonate (washing soda)
D. Calcium sulphate
Answer: C — Washing soda (Na₂CO₃), commonly used in water softening.
10.
A salt formed
from a strong acid and a weak base will have:
A. pH = 7
B. pH < 7
C. pH > 7
D. Cannot be determined
Answer: B — pH < 7 (acidic). Example: Ammonium chloride (NH₄Cl), formed from HCl and NH₄OH.
Conclusion:
The real skill here isn't memorizing every reaction — it's recognizing the pattern. Once you understand that acids donate protons, bases accept them, and salts inherit their character from whichever parent was stronger, you can work out the answer to almost any question in this chapter, even one you haven't seen before. That's exactly why UPSC and JEE-level papers love twisting familiar facts into unfamiliar questions — they're testing whether you understood the logic or just memorized the list.
So here's the practical next step: don't just re-read this article. Pull out a blank sheet, try writing the reaction equations from memory, work through two or three pH numericals on your own, and attempt the PYQs above without looking at the answers first. That's where this chapter actually sticks — not in the reading, but in the retrying.
And the next time someone hands you a fizzy antacid, a pickle jar, or even a lump of Plaster of Paris, you'll know exactly what's going on at the molecular level — which, honestly, is a pretty good party trick too.
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