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Acids Bases and Salts

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Overview

Acids release H⁺ ions, bases release OH⁻ ions, and they neutralise each other to form salt and water.

Key points

PH < 7 acidic, 7 neutral, > 7 basic.
Litmus: acids turn blue litmus red; bases turn red litmus blue.
Phenolphthalein turns pink in bases and stays colourless in acids.
Neutralisation: acid + base → salt + water.
Common acids: hydrochloric acid in the stomach, citric acid in lemon, acetic acid in vinegar, formic acid in ant stings.

Explanation

Baking soda: NaHCO₃; washing soda: Na₂CO₃·10H₂O; bleaching powder: CaOCl₂; plaster of Paris: CaSO₄·½H₂O.
Antacids (bases) relieve acidity.
Acid rain has pH below 5.6.
Toothpaste is basic to neutralise mouth acids.

Step by step

Identify the concept or law involved.
Recall the definition, unit or formula.
Check units and the direction of the effect before choosing.

Examples

Phenolphthalein in NaOH solution → pink.
An ant sting (formic acid) is soothed with baking soda.
Vinegar contains acetic acid.

Traps and advanced points

Trap: Mixing baking and washing soda.
Trap: Wrong litmus colours.
Trap: Calling pH 8 acidic.

Quick revision

PH 7 neutral.
Phenolphthalein pink in base.
Salt formulas table.

Practice MCQs

Lemon juice is —
1. acidic
2. basic
3. neutral
4. salty
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Answer: (a) acidic. It contains citric acid.
The pH of pure water is —
1. 0
2. 5
3. 7
4. 14
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Answer: (c) 7. pH 7 is neutral.
Acids turn blue litmus —
1. red
2. green
3. yellow
4. unchanged
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Answer: (a) red. Acids turn blue litmus red.
A solution with pH 7 is —
1. acidic
2. neutral
3. basic
4. oxidising
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Answer: (b) neutral. pH 7 is neutral.
Acid + base gives —
1. only gas
2. metal + oxygen
3. nothing
4. salt + water
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Answer: (d) salt + water. Neutralisation forms salt and water.
The pH of human blood is about —
1. 2
2. 5
3. 7.4
4. 11
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Answer: (c) 7.4. Blood is slightly basic (about 7.4).
Baking soda is —
1. sodium chloride
2. sodium carbonate
3. sodium bicarbonate
4. calcium carbonate
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Answer: (c) sodium bicarbonate. Baking soda = NaHCO₃.
Antacids treat acidity because they are —
1. salts of strong acids
2. neutral
3. acidic
4. mild bases
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Answer: (d) mild bases. They neutralise excess stomach acid.
Vinegar contains —
1. acetic acid
2. sulphuric acid
3. nitric acid
4. citric acid
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Answer: (a) acetic acid. Vinegar is dilute acetic (ethanoic) acid.
Plaster of Paris is made by heating —
1. washing soda
2. gypsum
3. baking soda
4. common salt
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Answer: (b) gypsum. Gypsum heated to about 373 K gives Plaster of Paris.

Acids Bases and Salts

Overview

Key points

Explanation

Step by step

Examples

Traps and advanced points

Quick revision

Practice MCQs

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