Chapter Overview and Weightage
Acids, Bases and Indicators is a foundational chapter in Class 7 Science that introduces students to the concept of acidity and basicity. This chapter typically carries 5–8 marks in annual exams and forms the conceptual base for pH, neutralisation reactions, and salts in higher classes (Class 10 and beyond).
This chapter appears in both unit tests and annual exams. Expect at least one 1-mark definition question, one 2-mark comparison question, and one 3-mark application/experiment question. Indicators and neutralisation reactions are the most tested topics.
| Question Type | Typical Marks |
|---|---|
| Define acids/bases/indicators | 1 mark |
| Name indicator and colour change | 1–2 marks |
| Difference between acid and base | 3 marks |
| Application (which substance is acidic/basic) | 2 marks |
| Neutralisation reaction with equation | 3–4 marks |
Key Concepts You Must Know
What are Acids?
Acids are substances that have a sour taste and turn blue litmus paper red. They react with certain metals to produce hydrogen gas. The word “acid” comes from the Latin “acidus” meaning sour.
Examples of acids:
- Hydrochloric acid (HCl) — found in our stomach
- Sulphuric acid (H₂SO₄) — used in car batteries
- Citric acid — found in lemon and orange (sour taste)
- Acetic acid — found in vinegar
- Formic acid — found in ant sting/bee sting
What are Bases?
Bases are substances that feel soapy/slippery to touch, taste bitter, and turn red litmus paper blue. They neutralise acids.
Examples of bases:
- Sodium hydroxide (NaOH) — caustic soda, used in making soap
- Calcium hydroxide Ca(OH)₂ — lime water, used in whitewashing
- Ammonium hydroxide NH₄OH — used in cleaning liquids
- Magnesium hydroxide Mg(OH)₂ — milk of magnesia, used in antacids
- Sodium bicarbonate (baking soda) — used in cooking
Alkali: A base that is soluble in water is called an alkali (e.g., NaOH, KOH). All alkalis are bases, but not all bases are alkalis.
Natural Indicators
Indicators are substances that show different colours in acidic and basic solutions. They help us identify whether a substance is acidic, basic, or neutral.
| Indicator | In Acid | In Base | In Neutral |
|---|---|---|---|
| Litmus | Red | Blue | Purple |
| Turmeric | Yellow | Red/Brown | Yellow |
| China rose | Magenta/Pink | Green | No change |
| Red cabbage juice | Red | Green/Yellow | Purple |
| Methyl orange | Red | Yellow | Orange |
| Phenolphthalein | Colourless | Pink | Colourless |
Litmus (extracted from lichens like Rocella tinctoria) is the most commonly used natural indicator. The blue litmus paper turns red with acids; red litmus turns blue with bases.
Turmeric as an indicator: Turmeric powder turns red/brownish-red in basic solutions. This is why turmeric stains turn reddish when washed with soap (soap is basic).
Neutral Substances
Some substances are neither acidic nor basic — they are neutral. Examples: pure water, common salt (NaCl) solution, sugar solution.
Neutral solutions do not change the colour of litmus paper.
Neutralisation Reaction
When an acid and a base are mixed, they cancel each other out — this is called neutralisation. The products are a salt and water.
Example:
(Hydrochloric acid + Sodium hydroxide → Common salt + Water)
Real-life applications of neutralisation:
-
Ant bite: An ant injects formic acid. Apply baking soda (basic) to neutralise the acid and reduce the burning sensation.
-
Stomach acidity: Excess HCl in stomach causes heartburn. Antacids (e.g., milk of magnesia, which is basic Mg(OH)₂) neutralise the excess acid.
-
Soil treatment: Acidic soil is treated with slaked lime Ca(OH)₂ or calcium carbonate (basic) to neutralise the acid and make the soil suitable for farming.
-
Factory waste: Industrial acid waste is neutralised with bases before release into water bodies (and vice versa).
-
Bee sting vs wasp sting: Bee sting = acidic (apply baking soda to neutralise). Wasp sting = alkaline (apply vinegar to neutralise).
Solved Examples
Question 1: A student tested a solution with blue litmus and red litmus. The blue litmus turned red but red litmus did not change. What is the nature of the solution?
Answer: Blue litmus turned red → the solution is acidic. Red litmus remains red in acid (no change). So the solution is acidic.
Question 2: Name three acids and their sources found in everyday life.
Answer:
- Citric acid — found in lemon, orange (sour taste)
- Acetic acid — found in vinegar
- Lactic acid — found in curd/sour milk
Question 3: A farmer notices his crop is not growing well. The soil test shows it is highly acidic. What should the farmer do?
Answer: The farmer should add lime (calcium hydroxide, Ca(OH)₂) or calcium carbonate (CaCO₃) to the soil. These are basic substances that will neutralise the excess acid in the soil, making it suitable for agriculture. This process is called liming of soil.
Question 4: Why does turmeric stain on white clothes turn red when washed with soap?
Answer: Soap solution is basic (alkaline). Turmeric is a natural indicator that turns red/brownish-red in basic solutions. When the turmeric stain is washed with soap water, the basic soap solution causes the yellow turmeric pigment to turn red. When rinsed with plain water (neutral), the colour may return to yellow or remain slightly reddish.
Difficulty Distribution
| Level | Typical Questions |
|---|---|
| Easy (40%) | Name an indicator; state colour of litmus in acid |
| Medium (40%) | Compare acids and bases; explain neutralisation |
| Hard (20%) | Apply neutralisation to real-life situations; explain turmeric/ant sting |
Expert Strategy
For Class 7 exams, this chapter is highly memorisation + application based. Focus on:
- Litmus colour changes — this appears in almost every exam: blue → red (acid); red → blue (base)
- Three real-life examples of neutralisation — particularly bee/wasp sting, antacids, and soil treatment
- Five examples each of common acids and bases with their sources
Remember the phrase: “Acids are sour, bases are bitter and slippery.” This helps recall the basic sensory properties. Also remember: we never taste or touch concentrated acids/bases in the lab — these are safety rules that can also appear as exam questions.
Common Traps
Trap 1: Students write that all bases are alkalis. Not true — an alkali is specifically a base that dissolves in water. Calcium carbonate (CaCO₃) is a base but NOT an alkali (it doesn’t dissolve well in water). Always distinguish between base (general) and alkali (water-soluble base).
Trap 2: Confusing bee sting (acidic) with wasp sting (alkaline). For bee sting, apply baking soda (basic to neutralise acid). For wasp sting, apply vinegar or lemon juice (acidic to neutralise alkali). Getting these reversed costs marks.
Trap 3: Students say that neutral solutions “have no litmus paper reaction.” More precisely: neutral solutions do NOT change the colour of litmus. Blue litmus remains blue; red litmus remains red. The solution is neither acidic nor basic.
Trap 4: Writing that salt formed in neutralisation is “always common salt (NaCl).” Different acid+base combinations give different salts: HCl + NaOH → NaCl (common salt); H₂SO₄ + NaOH → Na₂SO₄ (sodium sulphate); HCl + Ca(OH)₂ → CaCl₂ (calcium chloride). The only thing common to all neutralisation reactions is the formation of water.