CDS Chemistry · Acids, Bases and Salts
Common Salts and Carbonates
The trade names and formulas of everyday salts, how many water molecules their crystals carry, and the carbonates behind marble, pearl and the fire extinguisher.
Why this matters
Ten CDS questions. Four are match-the-list or spot-the-wrong-pair on trade names and formulas, so one table answers them all. The rest ask about calcium carbonate, baking soda and water of crystallisation.
Concept 1 of 3: Trade names and formulas of common salts
CDS loves to list four trade names against four formulas. The trick is to know each name's formula exactly, including the water it carries, because the wrong options swap one or two pairs.
Definition
The names to know:
- Baking soda / cooking soda: sodium hydrogen carbonate, NaHCO₃.
- Washing soda: Na₂CO₃·10H₂O. Soda ash is anhydrous Na₂CO₃.
- Bleaching powder: calcium oxychloride, CaOCl₂.
- Plaster of Paris: CaSO₄·½H₂O, also written 2CaSO₄·H₂O. Mixed with water it sets hard into gypsum, which is why it holds fractured bones in place.
- Gypsum: CaSO₄·2H₂O. Slaked lime: Ca(OH)₂. Quicklime: CaO.
- Baking powder is baking soda mixed with a mild acid such as tartaric acid.
| Trade name | Chemical name | Formula |
|---|---|---|
| Baking soda (cooking soda) | Sodium hydrogen carbonate | NaHCO₃Q CDS 2026 (II): cooking soda is NaHCO₃, not Na₂CO₃. |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O |
| Soda ash | Sodium carbonate (anhydrous) | Na₂CO₃ |
| Bleaching powder | Calcium oxychloride | CaOCl₂ |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O (2CaSO₄·H₂O)Q CDS 2017 (II): its paste holds a fractured bone in place. |
| Gypsum | Calcium sulphate dihydrate | CaSO₄·2H₂O |
| Slaked lime | Calcium hydroxide | Ca(OH)₂ |
| Quicklime | Calcium oxide | CaO |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2024 · CDS (II) 2024 — General Knowledge · Q17Easy
Example 1 · Acids, Bases and Salts · Common Salts and Carbonates
Match List-I with List-II and select the correct answer using the code given below the Lists :
Code :
| List-I (Trade Name) | List-II (Chemical Name) |
|---|---|
| A. Bleaching powder | 1. Sodium bicarbonate |
| B. Baking soda | 2. Sodium carbonate, 10 water |
| C. Washing soda | 3. Calcium oxychloride |
| D. Plaster of Paris | 4. Calcium sulphate, water |
Baking soda and washing soda are different salts
Baking soda is NaHCO₃ (a hydrogen carbonate). Washing soda is Na₂CO₃·10H₂O (a carbonate with ten waters). Most wrong options swap these two.
Plaster of Paris has half a water, not two
Plaster of Paris is CaSO₄·½H₂O. Gypsum, which it turns into when it sets, is CaSO₄·2H₂O.
Concept 2 of 3: Water of crystallisation
Some salts lock a fixed number of water molecules into their crystals. The water is part of the formula, and heating drives it off. Blue copper sulphate turns white when it loses its water.
Definition
What it is:
- Water of crystallisation is the fixed number of water molecules in one formula unit of a crystalline salt.
- Copper sulphate crystals (blue vitriol) are CuSO₄·5H₂O: five waters. Heated, they lose them and turn white; adding water turns them blue again.
- Washing soda carries 10, gypsum 2, plaster of Paris ½.
| Salt | Formula | Waters per formula unit |
|---|---|---|
| Copper sulphate (blue vitriol) | CuSO₄·5H₂O | 5Q CDS 2020 (I): copper sulphate crystals carry five water molecules. |
| Washing soda | Na₂CO₃·10H₂O | 10 |
| Gypsum | CaSO₄·2H₂O | 2 |
| Plaster of Paris | CaSO₄·½H₂O | ½ |
Practice this concept3 quick reps
The same idea in a real exam question:
CDS · 2020 · CDS (I) 2020 — General Knowledge · Q22Easy
Example 2 · Acids, Bases and Salts · Common Salts and Carbonates
The number of water molecules associated with copper sulphate molecule to form crystals is
Blue vitriol has five waters
Copper sulphate crystals are CuSO₄·5H₂O. Options of 2, 4 or 6 are distractors.
Concept 3 of 3: Calcium carbonate and hydrogen carbonates
Calcium carbonate is everywhere: marble, chalk, limestone, eggshell and pearl are all CaCO₃. Hydrogen carbonates are the other half of this page: baking soda puts out fires and raises cakes because it gives off carbon dioxide.
Definition
The facts:
- Marble, chalk and limestone are all forms of calcium carbonate, CaCO₃.
- A pearl is mainly calcium carbonate (nacre), laid down by a mollusc.
- A soda-acid fire extinguisher holds sodium hydrogen carbonate solution and a bottle of sulphuric acid. Mixing them gives CO₂, which smothers the fire.
- Heating baking soda gives CO₂: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂. That gas makes cakes rise.
- Lithium forms no solid hydrogen carbonate; LiHCO₃ exists only in solution. Sodium, potassium and caesium all give solid ones.
| Material or use | Compound |
|---|---|
| Marble, chalk, limestone | Calcium carbonate, CaCO₃Q CDS 2026 (II): all three are forms of calcium carbonate. |
| Pearl | Calcium carbonate (nacre)Q |
| Soda-acid fire extinguisher | Sodium hydrogen carbonate with sulphuric acidQ |
| No solid hydrogen carbonate | LithiumQ CDS 2016 (II), HARD: LiHCO₃ exists only in solution. |
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
CDS · 2026 · CDS (II) 2026 — General Knowledge · Q66Easy
Example 3 · Acids, Bases and Salts · Common Salts and Carbonates
Which of the following are the forms of calcium carbonate ?
1. Marble
2. Chalk
3. Limestone
Select the answer using the code given below :
The extinguisher uses the hydrogen carbonate
A soda-acid extinguisher uses sodium hydrogen carbonate (NaHCO₃), not sodium carbonate or sodium chloride.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Reference tables (3)
Trade names and formulas of common salts8 rows
| Trade name | Chemical name | Formula |
|---|---|---|
| Baking soda (cooking soda) | Sodium hydrogen carbonate | NaHCO₃Q CDS 2026 (II): cooking soda is NaHCO₃, not Na₂CO₃. |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O |
| Soda ash | Sodium carbonate (anhydrous) | Na₂CO₃ |
| Bleaching powder | Calcium oxychloride | CaOCl₂ |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O (2CaSO₄·H₂O)Q CDS 2017 (II): its paste holds a fractured bone in place. |
| Gypsum | Calcium sulphate dihydrate | CaSO₄·2H₂O |
| Slaked lime | Calcium hydroxide | Ca(OH)₂ |
| Quicklime | Calcium oxide | CaO |
Water of crystallisation4 rows
| Salt | Formula | Waters per formula unit |
|---|---|---|
| Copper sulphate (blue vitriol) | CuSO₄·5H₂O | 5Q CDS 2020 (I): copper sulphate crystals carry five water molecules. |
| Washing soda | Na₂CO₃·10H₂O | 10 |
| Gypsum | CaSO₄·2H₂O | 2 |
| Plaster of Paris | CaSO₄·½H₂O | ½ |
Calcium carbonate and hydrogen carbonates4 rows
| Material or use | Compound |
|---|---|
| Marble, chalk, limestone | Calcium carbonate, CaCO₃Q CDS 2026 (II): all three are forms of calcium carbonate. |
| Pearl | Calcium carbonate (nacre)Q |
| Soda-acid fire extinguisher | Sodium hydrogen carbonate with sulphuric acidQ |
| No solid hydrogen carbonate | LithiumQ CDS 2016 (II), HARD: LiHCO₃ exists only in solution. |
Watch out for (4)
- Baking soda and washing soda are different salts→ Trade names and formulas of common salts
- Plaster of Paris has half a water, not two→ Trade names and formulas of common salts
- Blue vitriol has five waters→ Water of crystallisation
- The extinguisher uses the hydrogen carbonate→ Calcium carbonate and hydrogen carbonates
Test yourself on Acids, Bases and Salts
15 past CDS questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.