📘 Chapter 4: Acids, Bases and Salts
🌱 1. Introduction
Substances around us taste differently: sour, bitter, salty, sweet.
These tastes are linked with their chemical nature:
- Sour → Acidic (e.g., lemon juice, curd, vinegar).
- Bitter & soapy → Basic (e.g., baking soda, lime water, soap).
- Salty → Salts (e.g., common salt).
- Neutral → Neither acidic nor basic (e.g., distilled water, sugar).
🔹 2. Acids
Substances that taste sour and contain acid are called acids.
Word acid comes from Latin acere = sour.
Examples: Lemon juice, vinegar, orange juice, tamarind, curd.
Acids found in plants/food are called natural acids.
Common Natural Acids and Their Sources:
| Name of Acid | Source |
|---|---|
| Acetic acid | Vinegar |
| Formic acid | Ant sting |
| Citric acid | Citrus fruits (orange, lemon) |
| Lactic acid | Curd |
| Oxalic acid | Spinach |
| Ascorbic acid (Vit C) | Amla, citrus fruits |
| Tartaric acid | Tamarind, grapes, unripe mango |
🔹 3. Bases
Substances that are bitter and soapy to touch are bases.
They are opposite in nature to acids.
Examples: Baking soda, lime water, soap solution, milk of magnesia.
Common Bases and Their Sources:
| Name of Base | Source/Use |
|---|---|
| Calcium hydroxide | Lime water |
| Magnesium hydroxide | Milk of magnesia |
| Sodium hydroxide | Soap |
| Ammonium hydroxide | Window cleaner |
🔹 4. Indicators
Substances that show different colours in acids and bases are called indicators.
- Litmus – Extracted from lichens. Purple in distilled water. Turns red in acids, blue in bases.
- Turmeric – Yellow in colour. Turns red-brown with bases, remains yellow with acids.
- China Rose – Extract from petals. In acids → dark pink/magenta. In bases → green.
🔹 5. Neutral Substances
Substances which are neither acidic nor basic. They do not change colour of red/blue litmus.
Examples: Distilled water, sugar, common salt.
🔹 6. Neutralisation Reaction
When an acid reacts with a base, both neutralise each other. They form salt, water and heat.
General Equation:
Acid + Base → Salt + Water (+ Heat)
Example: HCl + NaOH → NaCl + H₂O
(Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water)
🔹 7. Neutralisation in Everyday Life
- Indigestion: Too much acid in stomach → antacid (milk of magnesia) neutralises it.
- Ant bite: Formic acid injected → neutralised by baking soda or calamine.
- Soil treatment: Acidic soil → lime; Basic soil → compost.
- Factory waste: Acidic wastes neutralised with bases before release.
🌍 8. Acid Rain
Rain becomes acidic due to dissolved gases:
- CO₂ → Carbonic acid
- SO₂ → Sulphuric acid
- NO₂ → Nitric acid
Damages monuments, buildings, plants, and aquatic life.
🔑 Keywords
- Acid: Sour-tasting, turns blue litmus red.
- Base: Bitter, soapy, turns red litmus blue.
- Indicator: Detects acidic/basic nature by colour change.
- Neutralisation: Acid + base → salt + water (+ heat).
- Salt: Product of neutralisation.
📝 Exercise – Questions and Answers
Q1. State differences between acids and bases.
| Acids | Bases |
|---|---|
| Sour taste | Bitter taste |
| Turns blue litmus red | Turns red litmus blue |
| Examples: Lemon juice, vinegar | Examples: Soap, lime water |
Q2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Ammonia is basic in nature.
Q3. Name the source from which litmus solution is obtained. What is the use of this solution?
Litmus is obtained from lichens. It is used as an indicator to test whether a substance is acidic or basic.
Q4. Is distilled water acidic/basic/neutral? How would you verify it?
Distilled water is neutral. It does not change the colour of either red or blue litmus paper.
Q5. Describe the process of neutralisation with the help of an example.
When an acid reacts with a base, they cancel each other’s effect. The reaction produces salt, water, and heat.
Example:
HCl + NaOH → NaCl + H₂O
(Hydrochloric acid + Sodium hydroxide → Common salt + Water)
Example:
HCl + NaOH → NaCl + H₂O
(Hydrochloric acid + Sodium hydroxide → Common salt + Water)
Q6. Mark ‘T’ if the statement is true and ‘F’ if it is false:
(i) Nitric acid turns red litmus blue. → F
(ii) Sodium hydroxide turns blue litmus red. → F
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. → T
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. → T
(v) Tooth decay is caused by the presence of a base. → F (Caused by acids)
(ii) Sodium hydroxide turns blue litmus red. → F
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. → T
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. → T
(v) Tooth decay is caused by the presence of a base. → F (Caused by acids)
Q7. Dorji has a few bottles of soft drink in his restaurant. But unfortunately, these are not labelled. He has to serve the drinks: one acidic, one basic, one neutral. How will Dorji decide?
Dorji should use indicators:
Acidic drink → turns blue litmus red.
Basic drink → turns red litmus blue.
Neutral drink → no effect on litmus.
Acidic drink → turns blue litmus red.
Basic drink → turns red litmus blue.
Neutral drink → no effect on litmus.
Q8. Explain why:
(a) An antacid tablet is taken when you suffer from acidity. → Because antacids (like milk of magnesia) are basic and they neutralise excess acid in the stomach.
(b) Calamine solution is applied on the skin when an ant bites. → Ant injects formic acid. Calamine contains zinc carbonate (a base) that neutralises the acid.
(c) Factory waste is neutralised before disposing it into the water bodies. → Factory wastes may contain acids which can kill fish and aquatic organisms. Neutralisation makes it safe.
(b) Calamine solution is applied on the skin when an ant bites. → Ant injects formic acid. Calamine contains zinc carbonate (a base) that neutralises the acid.
(c) Factory waste is neutralised before disposing it into the water bodies. → Factory wastes may contain acids which can kill fish and aquatic organisms. Neutralisation makes it safe.
Q9. Three liquids are given to you: hydrochloric acid, sodium hydroxide, sugar solution. How will you identify them using turmeric indicator?
Hydrochloric acid → no change in turmeric colour (stays yellow).
Sodium hydroxide → turmeric turns red-brown.
Sugar solution → no change (remains neutral).
Sodium hydroxide → turmeric turns red-brown.
Sugar solution → no change (remains neutral).
Q10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
The solution is either basic or neutral. Because acids turn blue litmus red. Since no change occurred, it is not acidic.
Q11. Consider the following statements:
(a) Both acids and bases change colour of all indicators.
(b) If an indicator gives a colour change with an acid, it does not give a change with a base.
(c) If an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Correct Answer: (iii) b, c and d.
(b) If an indicator gives a colour change with an acid, it does not give a change with a base.
(c) If an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Correct Answer: (iii) b, c and d.