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Salts and Their Uses

🎓 Class 10 Science CBSE Theory Ch 2 — Acids, Bases and Salts ⏱ ~22 min
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This MCQ module is based on: Salts and Their Uses

[myaischool_lt_science_assessment grade_level="class_10" science_domain="chemistry" difficulty="intermediate"]

2.4 More About Salts

In the previous sections, we saw that salts are formed when acids react with bases. But there is much more to know about salts — they form families, have different pH values, and many of them have crucial industrial and domestic uses.

2.4.1 Family of Salts

Activity 2.13 — Identifying Salt Families L2 Understand
Predict first: What do NaCl, KCl, and CaCl2 have in common? Can you group salts into families based on a shared feature?
  1. Write the chemical formulae of sodium sulphate (Na2SO4), potassium sulphate (K2SO4), and calcium sulphate (CaSO4).
  2. Now write the formulae of sodium chloride (NaCl), potassium chloride (KCl), and calcium chloride (CaCl2).
  3. Identify the common part in each group.
Observations:
Sulphate family (from H2SO4)Chloride family (from HCl)
Na2SO4NaCl
K2SO4KCl
CaSO4CaCl2

Salts that share the same acid origin (same anion) belong to the same family. The sulphate family comes from H2SO4, the chloride family from HCl, the nitrate family from HNO3, and so on.

2.4.2 pH of Salts

Not all salts are neutral. The pH of a salt depends on the strengths of the acid and base that formed it:

Acid StrengthBase StrengthSalt pHExample
Strong (HCl)Strong (NaOH)pH = 7 (Neutral)NaCl
Strong (HCl)Weak (NH4OH)pH < 7 (Acidic)NH4Cl
Weak (CH3COOH)Strong (NaOH)pH > 7 (Basic)CH3COONa

Some common natural sources of acids:

Natural SourceAcid Present
VinegarAcetic acid (CH3COOH)
Orange / LemonCitric acid
Tamarind (imli)Tartaric acid
TomatoOxalic acid
Curd / Sour milkLactic acid
Ant / Bee stingMethanoic acid (formic acid)

2.4.3 Common Salt (NaCl) — The Starting Material

Common salt (NaCl) is obtained by evaporation of sea water or mined from underground rock salt deposits. It is one of the most important raw materials in the chemical industry, providing the starting point for manufacturing several key chemicals.

2.4.4 Sodium Hydroxide (NaOH) — The Chlor-Alkali Process

When electricity is passed through a concentrated solution of sodium chloride (brine), it decomposes to give three useful products:

\(2\text{NaCl}(aq) + 2\text{H}_2\text{O}(l) \xrightarrow{\text{electrolysis}} 2\text{NaOH}(aq) + \text{Cl}_2(g) + \text{H}_2(g)\)

This process is called the chlor-alkali process because of the two main products — chlor(ine) and (alk)ali (NaOH).

Electrolysis Cell Brine (conc. NaCl solution) + Anode - Cathode Battery Cl₂ gas at anode H₂ gas at cathode NaOH (aq) near cathode 2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g) Electrolysis of brine — the chlor-alkali process
The chlor-alkali process: electrolysis of brine produces Cl2 at the anode, H2 at the cathode, and NaOH solution near the cathode.

Uses of the three products:

ProductUses
NaOHSoap and detergent making, paper manufacturing, de-greasing metals, petroleum refining
Cl2Water purification, PVC plastics, disinfectants, CFC compounds, bleaching powder
H2Fuels, margarine production, ammonia synthesis (for fertilisers)

2.4.5 Bleaching Powder (CaOCl2)

Bleaching powder is produced when chlorine gas reacts with dry slaked lime:

\(\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}\)

Uses: (i) Bleaching cotton, linen, and wood pulp in the textile and paper industry. (ii) Disinfecting drinking water to make it germ-free. (iii) Oxidising agent in many chemical industries.

2.4.6 Baking Soda (NaHCO3)

Baking soda (sodium hydrogen carbonate, NaHCO3) is produced using common salt as the starting material:

\(\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2 + \text{NH}_3 \rightarrow \text{NH}_4\text{Cl} + \text{NaHCO}_3\)

When heated, baking soda decomposes:

\(2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\)
🍞
Baking
Ingredient in baking powder. Releases CO2 when heated, making dough rise.
💊
Antacid
Neutralises excess stomach acid (HCl) to relieve indigestion.
🔥
Fire Extinguisher
Soda-acid extinguishers use NaHCO3 + acid to generate CO2.
🍳
Cooking
Makes food like dhokla/idli soft and spongy by releasing CO2.
Baking Powder vs Baking Soda: Baking powder is a mixture of baking soda (NaHCO3) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed with water, the acid reacts with NaHCO3 to release CO2, which causes bread/cake to rise and become soft and fluffy.

2.4.7 Washing Soda (Na2CO3·10H2O)

Washing soda is obtained by recrystallising sodium carbonate with 10 molecules of water:

\(\text{Na}_2\text{CO}_3 + 10\text{H}_2\text{O} \rightarrow \text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O}\)

The Na2CO3 itself is obtained by heating baking soda:

\(2\text{NaHCO}_3 \xrightarrow{\Delta} \text{Na}_2\text{CO}_3 + \text{H}_2\text{O} + \text{CO}_2\)

Uses of washing soda: (i) Cleaning agent — removes grease and dirt. (ii) Softening hard water by removing Ca2+ and Mg2+ ions. (iii) Manufacturing of glass, soap, and paper. (iv) Manufacture of sodium compounds like borax.

2.4.8 Plaster of Paris (CaSO4·½H2O)

Plaster of Paris is obtained by heating gypsum (CaSO4·2H2O) at 373 K (100°C):

\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\,\text{K}} \text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O} + \tfrac{3}{2}\text{H}_2\text{O}\)

When Plaster of Paris is mixed with water, it sets into a hard mass by reverting back to gypsum:

\(\text{CaSO}_4 \cdot \tfrac{1}{2}\text{H}_2\text{O} + \tfrac{3}{2}\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)

Uses: (i) Setting fractured bones as plaster casts. (ii) Making decorative items, toys, and statues. (iii) Making smooth surfaces (wall finishing). (iv) Making moulds in pottery and dentistry. (v) Making chalk for writing on blackboards.

Storage Precaution: Plaster of Paris must be stored in a moisture-proof (air-tight) container. If it comes in contact with moisture, it will slowly absorb water and set into a hard mass (gypsum), becoming useless.

Water of Crystallisation

Water of crystallisation is the fixed number of water molecules present in one formula unit of a salt in its crystalline form. For example, CuSO4·5H2O has 5 water molecules, Na2CO3·10H2O has 10, and CaSO4·2H2O has 2.

Activity 2.14/2.15 — Heating Copper Sulphate Crystals L3 Apply
Predict first: Blue copper sulphate crystals contain water molecules. What will happen to their colour if we heat them strongly?
  1. Take a few crystals of copper sulphate (CuSO4·5H2O) in a dry boiling tube. Note the blue colour.
  2. Heat the crystals gently over a flame. Observe the colour change and any moisture on the tube walls.
  3. After the crystals become completely white, let the tube cool.
  4. Add 2-3 drops of water to the white powder and observe.
Observations:
On heating, the blue CuSO4·5H2O crystals gradually turn white. Water droplets appear on the cooler parts of the boiling tube. When water is added back to the white powder, it turns blue again.

\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O} \xrightarrow{\Delta} \text{CuSO}_4 + 5\text{H}_2\text{O}\)

(blue) → (white) + (water vapour)

This experiment confirms that the blue colour of copper sulphate crystals is due to the water of crystallisation. When these water molecules are removed, the substance loses its blue colour and turns white.

Before Heating Blue crystals CuSO₄·5H₂O Heat After Heating White powder CuSO₄ (anhydrous) +Water Blue again!
Fig 2.9: Heating CuSO4·5H2O removes water of crystallisation (blue → white). Adding water restores the blue colour.

Competency-Based Questions

Riya's mother uses baking soda while making cakes, and her father uses washing soda to clean oily kitchen surfaces. In the factory nearby, common salt is processed using the chlor-alkali process to obtain NaOH, Cl2, and H2.

Q1. L1 Remember What is the chemical formula of Plaster of Paris?

  • A. CaSO4·2H2O
  • B. CaSO4·½H2O
  • C. Ca(OH)2
  • D. CaCO3
Answer: B. Plaster of Paris is calcium sulphate hemihydrate — CaSO4·½H2O. Option A (CaSO4·2H2O) is gypsum, from which Plaster of Paris is made by heating.

Q2. L2 Understand Explain why baking soda is used as an ingredient in antacid tablets. (2 marks)

Answer: Baking soda (NaHCO3) is a mild, non-corrosive base. When consumed, it reacts with the excess hydrochloric acid (HCl) in the stomach and neutralises it: NaHCO3 + HCl → NaCl + H2O + CO2. This neutralisation reduces acidity and provides relief from indigestion and heartburn.

Q3. L3 Apply Write the balanced equation for the preparation of bleaching powder. Name the raw materials needed. (2 marks)

Answer:
\(\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}\)
Raw materials needed: Dry slaked lime — Ca(OH)2, and chlorine gas — Cl2 (obtained from the chlor-alkali process).

Q4. L4 Analyse In the chlor-alkali process, why are the products (NaOH, Cl2, H2) formed at different electrodes? What would happen if Cl2 and NaOH came into contact? (3 marks)

Answer: The products form at different electrodes because of the nature of electrolysis: Cl- ions migrate to the anode (+) where they lose electrons and become Cl2 gas. H+ ions (from water) migrate to the cathode (-) where they gain electrons and become H2 gas. NaOH accumulates near the cathode as Na+ and OH- ions remain in solution. If Cl2 and NaOH came into contact, they would react to form bleach (sodium hypochlorite): 2NaOH + Cl2 → NaCl + NaOCl + H2O.

Q5. L5 Evaluate A student says, "Since washing soda and baking soda are both sodium compounds, they can be used interchangeably." Evaluate this claim. (3 marks)

Answer: This claim is incorrect. Although both are sodium compounds, they have very different properties and uses. Baking soda (NaHCO3) is a mild base that is safe to consume — it is used in cooking and as an antacid. Washing soda (Na2CO3·10H2O) is a much stronger base and is corrosive — it is used for heavy-duty cleaning and industrial purposes but is not safe for consumption. Using washing soda as an antacid would be harmful, and using baking soda for industrial cleaning would be ineffective.

Assertion-Reason Questions

Assertion (A): Plaster of Paris should be stored in moisture-proof containers.

Reason (R): Plaster of Paris absorbs water (moisture) and slowly sets into hard gypsum, making it useless.

  • A. Both A and R are true, and R is the correct explanation of A.
  • B. Both A and R are true, but R is NOT the correct explanation of A.
  • C. A is true, but R is false.
  • D. A is false, but R is true.
Answer: A. Both are true and R correctly explains A. Plaster of Paris (CaSO4·½H2O) readily absorbs moisture from the air and reacts with it to form gypsum (CaSO4·2H2O), which is a hard solid. Once this happens, the Plaster of Paris can no longer be used for casting or moulding.

Assertion (A): The blue colour of copper sulphate crystals disappears on strong heating.

Reason (R): Copper sulphate pentahydrate loses its water of crystallisation on heating, leaving behind white anhydrous copper sulphate.

  • A. Both A and R are true, and R is the correct explanation of A.
  • B. Both A and R are true, but R is NOT the correct explanation of A.
  • C. A is true, but R is false.
  • D. A is false, but R is true.
Answer: A. Both are true and R is the correct explanation. CuSO4·5H2O is blue due to the 5 water molecules in its crystal structure. When heated, these water molecules escape as steam, leaving white anhydrous CuSO4. The blue colour returns when water is added back.

Assertion (A): A salt formed from a strong acid and a weak base will have a pH less than 7.

Reason (R): Such salts are acidic in nature because the strong acid dominates.

  • A. Both A and R are true, and R is the correct explanation of A.
  • B. Both A and R are true, but R is NOT the correct explanation of A.
  • C. A is true, but R is false.
  • D. A is false, but R is true.
Answer: A. Both statements are true and R correctly explains A. When a strong acid reacts with a weak base, the resulting salt is acidic (pH < 7) because the strong acid component contributes more H+ ions in solution. Example: NH4Cl (from HCl + NH4OH) has pH < 7.

Did You Know?

Frequently Asked Questions — Salts and Their Uses

What is salts and their uses in Class 10 Science (CBSE board)?

Salts and Their Uses is a key topic in NCERT Class 10 Science Chapter 2 — Acids, Bases and Salts. It explains types of salts, their ph, and uses of common salts like nacl, naoh, nahco3, na2co3, bleaching powder and pop. Core ideas covered include salt family, pH of salts, common salt, sodium hydroxide. Mastering this subtopic is essential for scoring well in the CBSE Class 10 Science board exam because board papers repeatedly test these concepts through MCQs, short answers and long-answer questions. This part gives a complete, exam-ready explanation with activities, diagrams and competency-based practice aligned to NCERT.

Why is salt family important in NCERT Class 10 Science?

Salt family is important in NCERT Class 10 Science because it forms the foundation for understanding salts and their uses in Chapter 2 — Acids, Bases and Salts. Without a clear idea of salt family, students cannot answer higher-order CBSE board questions involving pH of salts, common salt, sodium hydroxide. Board papers regularly include 2-mark and 3-mark questions on this concept, and competency-based questions often link salt family to real-life situations. Building clarity here pays off directly in board marks.

How is salts and their uses tested in the Class 10 Science CBSE board exam?

The CBSE Class 10 Science board exam tests salts and their uses through a mix of 1-mark MCQs, 2-mark short answers, 3-mark explanations with examples, 5-mark descriptive questions (often with diagrams or balanced equations) and 4-mark competency-based questions. Expect direct questions on salt family, pH of salts, common salt and application-based questions drawn from NCERT activities. Students who follow NCERT thoroughly and practice this chapter's questions consistently score in the 90%+ range.

What are the key terms to remember for salts and their uses in Class 10 Science?

The key terms to remember for salts and their uses in NCERT Class 10 Science Chapter 2 are: salt family, pH of salts, common salt, sodium hydroxide, bleaching powder, baking soda. Each of these concepts carries exam weightage and regularly appears in the CBSE board paper. Write clear one-line definitions of every term in your revision notes and revisit them before the exam. Linking these terms visually through a flowchart or concept map makes recall easier during the Class 10 Science board exam.

Is Salts and Their Uses included in the Class 10 Science syllabus for 2025–26 CBSE board exam?

Yes, Salts and Their Uses is a part of the NCERT Class 10 Science syllabus (2025–26) prescribed by CBSE. It falls under Chapter 2 — Acids, Bases and Salts — and is examined in the annual board paper. The current syllabus retains the full treatment of salt family, pH of salts, common salt as per the NCERT textbook. Because CBSE bases every board question on NCERT, studying this part thoroughly ensures complete syllabus coverage and guarantees marks from this chapter.

How should I prepare salts and their uses for the CBSE Class 10 Science board exam?

Prepare salts and their uses for the CBSE Class 10 Science board exam in three steps. First, read this NCERT part carefully, highlighting definitions and diagrams of salt family, pH of salts, common salt. Second, solve every in-text question and end-of-chapter exercise — CBSE questions often come directly from NCERT. Third, practice competency-based and assertion-reason questions to sharpen reasoning. Write answers in the exam-style format (point-wise with diagrams) and time yourself. This method delivers confidence and full marks in the board exam.

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