What Are Acids, Bases and Salts?
Every time you squeeze a lemon into your dal or take an antacid tablet after a heavy meal, you’re working with acids and bases. These aren’t abstract chemistry concepts — they’re the chemistry running your kitchen, your body, and half the industrial world.
The simplest way to think about it: acids donate hydrogen ions (H⁺) when dissolved in water. Bases accept those hydrogen ions — or donate hydroxide ions (OH⁻). When an acid and a base react, they cancel each other out and form a salt plus water. This reaction is called neutralisation, and it’s one of the most important reaction types you’ll see from Class 7 all the way to JEE.
Class 10 students need to go deeper — the Arrhenius definition, strength of acids/bases, pH scale, and the chemistry of common salts. Class 7 students, we’ll build the foundation first and the detail follows naturally.
Key Terms and Definitions
Acid — A substance that releases H⁺ ions (protons) in aqueous solution. Example: HCl → H⁺ + Cl⁻ in water.
Base — A substance that releases OH⁻ ions in aqueous solution. Soluble bases are called alkalis. Example: NaOH → Na⁺ + OH⁻.
Salt — An ionic compound formed when the H⁺ of an acid is replaced by a metal ion. Table salt (NaCl) is the product of HCl + NaOH.
Neutralisation — The reaction between an acid and a base to form salt and water. Net ionic equation: H⁺ + OH⁻ → H₂O.
pH — A scale from 0 to 14 measuring how acidic or basic a solution is. pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic.
Indicator — A substance that changes colour depending on pH. Litmus, phenolphthalein, and methyl orange are the three you must know cold.
Natural Indicators You Should Know
| Indicator | Acidic Medium | Basic Medium |
|---|---|---|
| Litmus | Red | Blue |
| Turmeric | Yellow | Red-brown |
| China rose | Magenta/Pink | Green |
| Phenolphthalein | Colourless | Pink |
Turmeric is a brilliant example for Class 7 — rub haldi on a white shirt, then touch it with soap (a base). It turns red-brown. That’s your indicator in action.
Types of Acids and Bases
Strong vs. Weak Acids (Class 10)
Strong acids dissociate completely in water. Every molecule breaks apart.
Examples: HCl (hydrochloric acid), H₂SO₄ (sulphuric acid), HNO₃ (nitric acid).
Weak acids dissociate partially — only some molecules release H⁺.
Examples: CH₃COOH (acetic acid/vinegar), H₂CO₃ (carbonic acid in soft drinks), citric acid.
Strong vs. Weak Bases
Strong bases: NaOH, KOH, Ca(OH)₂ — fully dissociate.
Weak bases: NH₄OH (ammonium hydroxide) — partial dissociation.
CBSE Class 10 board exams consistently ask: “What happens when you dilute a strong acid?” The answer: diluting decreases H⁺ ion concentration, so pH increases (moves closer to 7). The acid doesn’t stop being an acid — it just becomes less acidic.
The pH Scale
Each unit on the pH scale is a 10× change in H⁺ concentration. pH 4 is 10 times more acidic than pH 5. pH 3 is 100 times more acidic than pH 5.
pH of Common Substances
| Substance | Approximate pH |
|---|---|
| Gastric juice | 1.5–2.0 |
| Lemon juice | 2.5 |
| Vinegar | 3.0 |
| Pure water | 7.0 |
| Blood | 7.4 |
| Baking soda solution | 8.3 |
| Milk of magnesia | 10.5 |
| Drain cleaner (NaOH) | 13–14 |
The pH of blood is tightly controlled between 7.35–7.45. Even a shift to 7.0 or 7.7 causes medical emergencies. Your kidneys and lungs work constantly to maintain this.
Key Reactions — Step by Step
1. Acid + Metal → Salt + Hydrogen Gas
Test for H₂: bring a burning splint near the mouth of the test tube. A “pop” sound confirms hydrogen gas.
Copper and silver do NOT react with dilute acids to produce hydrogen. These metals are below hydrogen in the activity series. If an exam question asks which metal won’t react with HCl, the answer is Cu or Ag.
2. Acid + Metal Carbonate → Salt + Water + CO₂
Test for CO₂: pass the gas through lime water (Ca(OH)₂ solution). It turns milky because:
3. Acid + Base → Salt + Water (Neutralisation)
This is the reaction in antacid tablets. Excess HCl in your stomach is neutralised by Mg(OH)₂ or Al(OH)₃.
4. Acid + Metal Oxide → Salt + Water
Metal oxides are basic oxides. When they react with acids:
Blue copper sulphate solution forms — a clean way to remember that CuO is a base.
Chemistry of Important Salts
Sodium Chloride (Common Salt, NaCl)
Not just food. NaCl is the raw material for making NaOH, Cl₂, and Na — all via electrolysis of brine (concentrated NaCl solution). This process is called the Chlor-alkali process.
Sodium Hydroxide (Caustic Soda, NaOH)
Used in: soap making (saponification), paper industry, de-greasing metals.
Baking Soda (NaHCO₃)
Sodium hydrogen carbonate. When heated:
This CO₂ makes cakes rise. Baking soda is also used in fire extinguishers (releases CO₂ which smothers flames) and as an antacid.
Washing Soda (Na₂CO₃·10H₂O)
Used for removing permanent hardness of water. The carbonate ions precipitate Ca²⁺ and Mg²⁺ ions:
Bleaching Powder (CaOCl₂)
Made by passing Cl₂ over dry slaked lime Ca(OH)₂. Used for disinfecting water, bleaching cloth and paper.
Plaster of Paris (CaSO₄·½H₂O)
Made by heating gypsum (CaSO₄·2H₂O) to 100°C:
When mixed with water, it sets hard — used in surgical casts, moulds, and blackboard chalk.
Plaster of Paris + water → Gypsum is a highly scoring Class 10 board question. The reaction is the reverse of its formation. Remember: PoP absorbs water and expands slightly as it sets, which is why it fits casts perfectly.
Solved Examples
Example 1 — Class 7 / CBSE Basic
Q: A student tested three solutions with litmus paper. Solution A turned red litmus blue. Solution B turned blue litmus red. Solution C showed no change in either litmus paper. Classify them.
Solution A → Turns red litmus blue → Basic Solution B → Turns blue litmus red → Acidic Solution C → No change → Neutral (pH = 7)
This is the standard indicator test. Three solutions, three categories. No calculation needed.
Example 2 — CBSE Class 10
Q: What happens when SO₃ dissolves in water? Write the equation. Is the resulting solution acidic, basic, or neutral?
Sulphuric acid forms. H₂SO₄ is a strong acid, so the solution is acidic (pH < 7).
Non-metal oxides like SO₃, CO₂, NO₂ form acids with water. Metal oxides form bases. This pattern appears in Board exams every 2–3 years.
Example 3 — Class 10 / CBSE Higher Order
Q: Fresh milk has a pH of 6. When it turns sour (curd forms), does the pH increase or decrease? Why?
The pH decreases (becomes more acidic). During souring, bacteria convert lactose to lactic acid (CH₃CH(OH)COOH). More acid means more H⁺ ions, so pH drops below 6.
This is a 3-mark question that tests understanding, not memorisation. The examiner wants you to connect pH change to the chemical process happening.
Example 4 — JEE Main Level
Q: 100 mL of 0.1 M HCl is mixed with 100 mL of 0.1 M NaOH. What is the pH of the resulting solution?
Moles of HCl = 0.1 × 0.1 = 0.01 mol Moles of NaOH = 0.1 × 0.1 = 0.01 mol
Equal moles — complete neutralisation. The solution is NaCl in water, which is neutral.
pH = 7
Many students try to calculate residual H⁺ here and get confused. When moles are exactly equal, stop there. The result is a neutral salt solution, pH = 7. Don’t overthink it.
Exam-Specific Tips
CBSE Class 10 Board Pattern
- Acids, Bases and Salts typically carries 8–10 marks in the board exam (Section B + C questions).
- High-frequency topics: Chlor-alkali process, properties of NaHCO₃ and Na₂CO₃, pH scale, indicators.
- Diagram questions: electrolysis of brine (label anode, cathode, products at each electrode).
- 5-mark questions often combine two concepts — e.g., “What is pH? How does pH change when you add water to an acid?”
CBSE Class 7 Pattern
- Expect 1-mark MCQs on identifying acids/bases from indicators.
- 2-mark questions on examples of everyday acids and bases.
- Neutralisation reaction questions — always write the equation with state symbols.
JEE Main Relevance
Acids and Bases feeds directly into Ionic Equilibrium (a JEE Main chapter with high weightage — appeared in 7 of the last 10 JEE Main sessions). The conceptual foundation here — what makes a strong acid strong, how pH behaves on dilution — must be airtight before you tackle Ka, Kb, and buffer solutions.
JEE Main 2024 Shift 1 (January) had a direct question on the pH of a buffer solution — but buffer chemistry becomes impossible without a solid grip on acid-base dissociation. Invest time here; it pays compound interest.
Common Mistakes to Avoid
Mistake 1: Confusing alkali with base. All alkalis are bases, but not all bases are alkalis. A base is any proton acceptor. An alkali is a base that dissolves in water. CuO is a base but NOT an alkali (it doesn’t dissolve in water).
Mistake 2: “Neutral salt = pH 7.” This is wrong. Salts of strong acid + weak base (like NH₄Cl) are acidic (pH < 7). Salts of weak acid + strong base (like CH₃COONa) are basic (pH > 7). Only salts of strong acid + strong base (like NaCl) give pH = 7. Class 10 board exams test exactly this.
Mistake 3: Writing the PoP reaction incorrectly. Plaster of Paris is CaSO₄·½H₂O, not CaSO₄·H₂O. That half water molecule trips up more students than any other formula in the chapter.
Mistake 4: pH decreases when acid is diluted. Dilution adds water, which decreases H⁺ concentration. pH increases (moves toward 7). You’re making it less acidic, not more. Remember: more water = higher pH for an acid.
Mistake 5: Hydrogen gas test = blue litmus turns red. No — H₂ is a neutral gas, it has no effect on litmus. The test for hydrogen is the pop sound with a burning splint. Students mix up the tests for H₂, CO₂, and O₂ in board exams regularly.
Practice Questions
Q1. Name the acid present in (a) vinegar, (b) lemon, (c) tamarind, (d) ant’s sting.
(a) Acetic acid (CH₃COOH) (b) Citric acid (c) Tartaric acid (d) Formic acid (methanoic acid, HCOOH)
These are standard one-mark answers. Commit them to memory — they appear in Class 7 and Class 10 exams alike.
Q2. What gas is produced when zinc reacts with dilute sulphuric acid? How would you test for it?
Hydrogen gas (H₂) is produced.
Test: Bring a burning splint near the mouth of the test tube. A “pop” sound confirms H₂.
Q3. A solution has a pH of 3. Another has a pH of 11. Which is acidic? If you mix equal volumes of both, what roughly happens to the pH?
pH 3 is acidic (H⁺ concentration = 10⁻³ mol/L). pH 11 is basic (OH⁻ concentration = 10⁻³ mol/L).
Equal volumes of pH 3 acid and pH 11 base: the H⁺ and OH⁻ concentrations are equal (both 10⁻³ mol/L), so they neutralise completely. The resulting solution has pH ≈ 7.
This is a thinking question — no formula required if you recognise the symmetry around pH 7.
Q4. Why does Plaster of Paris harden when mixed with water?
Plaster of Paris (CaSO₄·½H₂O) reacts with water to reform gypsum (CaSO₄·2H₂O):
As gypsum crystals form and interlock, the mixture becomes rigid. This is a physical setting process driven by crystal growth, not evaporation of water.
Q5. In the Chlor-alkali process, what is produced at (a) the cathode and (b) the anode?
Electrolysis of brine (NaCl solution):
(a) Cathode (negative electrode): Hydrogen gas (H₂) At cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻
(b) Anode (positive electrode): Chlorine gas (Cl₂) At anode: 2Cl⁻ → Cl₂ + 2e⁻
NaOH accumulates in solution near the cathode chamber.
Memory trick: Oil Rig — Oxidation Is Loss (at anode), Reduction Is Gain (at cathode). Cl⁻ loses electrons at anode → Cl₂.
Q6. A student accidentally spills concentrated HCl on their hand. Should they immediately apply NaOH solution to neutralise it? Why or why not?
No — this is dangerous. NaOH is also highly corrosive (caustic). Applying one corrosive substance to counter another can cause more damage, and the neutralisation reaction itself is exothermic (releases heat).
Correct first aid: flush immediately with large amounts of running water for 15–20 minutes. Water dilutes the acid rapidly and safely. Seek medical attention after.
This question tests real-world application of chemistry — a common 3-mark HOTS question in CBSE.
Q7. Why does a bee sting cause pain while a wasp sting requires a different treatment?
Bee sting injects formic acid (HCOOH) — acidic. Treatment: apply baking soda (NaHCO₃, a mild base) to neutralise.
Wasp sting injects an alkaline substance. Treatment: apply vinegar (acetic acid) to neutralise.
This is classic neutralisation applied to biology. Class 7 students — this exact question appears in NCERT. Class 10 students — the underlying chemistry is the same neutralisation principle.
Q8. Tooth enamel (hydroxyapatite) is destroyed at pH below 5.5. How do toothpastes protect teeth?
When bacteria in the mouth ferment sugars, they produce acids that lower the pH of saliva below 5.5, which dissolves enamel.
Toothpastes are mildly basic (pH 7–8). They neutralise the acids produced by bacteria, bringing mouth pH back above 5.5 and stopping enamel erosion.
Fluoride toothpastes also help re-mineralise early enamel damage — but that’s biology, not chemistry. For exam purposes: toothpaste works because it neutralises mouth acids.
FAQs
Q: What is the difference between an acid and a base in simple terms?
An acid is a “proton donor” — it gives away H⁺ ions in water. A base is a “proton acceptor” — it takes those H⁺ ions (or gives OH⁻ ions). The simplest test: acids turn blue litmus red; bases turn red litmus blue.
Q: Why is pH measured on a scale of 0 to 14?
Because water dissociates to give H⁺ and OH⁻ concentrations of 10⁻⁷ mol/L each (at 25°C). The pH scale is based on this range — concentrated strong acids approach pH 0 (H⁺ ≈ 1 mol/L) and concentrated strong bases approach pH 14. Beyond these limits is theoretically possible but chemically unusual under normal conditions.
Q: What are amphoteric substances?
Amphoteric substances can act as both acid and base depending on what they’re reacting with. Water is the best example — it donates H⁺ to a base and accepts H⁺ from an acid. Amino acids (biology) and Al₂O₃ (chemistry) are also amphoteric. Class 10 students: this concept appears in the Chemical Reactions chapter too.
Q: Why do acids conduct electricity?
They conduct because they dissociate into ions in water. Ions are mobile charged particles — when a potential difference is applied, positive ions (H⁺) move to the cathode and negative ions (Cl⁻, SO₄²⁻ etc.) move to the anode. Pure (anhydrous) acids don’t conduct — it’s the water that enables dissociation.
Q: Is distilled water an acid, base, or neutral?
Neutral — pH = 7. But rainwater is slightly acidic (pH ≈ 5.6) because CO₂ from the atmosphere dissolves in it to form carbonic acid (H₂CO₃). Acid rain has pH < 5 due to dissolved SO₂ and NOₓ from industrial emissions.
Q: What is the difference between baking soda and baking powder?
Baking soda is pure NaHCO₃. Baking powder is baking soda mixed with a mild acid (cream of tartar / tartaric acid). When moistened, they react to produce CO₂ immediately. Baking powder is used when the recipe doesn’t already have an acidic ingredient; baking soda is used when the recipe includes buttermilk, vinegar, or lemon juice.
Q: Why does NaHCO₃ solution turn litmus blue if it contains Na, H, C, and O?
The H in NaHCO₃ doesn’t make it acidic. The bicarbonate ion (HCO₃⁻) actually acts as a weak base in water — it accepts H⁺ from water molecules, releasing OH⁻ ions. So NaHCO₃ solution is mildly basic (pH ≈ 8.3). This surprises students who assume anything with “H” in the formula is acidic.
Q: How do antacids work chemically?
Antacids contain mild bases — most commonly Mg(OH)₂ (milk of magnesia), Al(OH)₃, or NaHCO₃. They neutralise excess HCl in the stomach:
The salt formed (MgCl₂) is soluble and harmless. Antacids must be mild bases — using a strong base like NaOH would damage the stomach lining.