Summary
Chapter 4 of the Class 7 Science NCERT textbook, "Acids, Bases and Salts", explains that acids taste sour and turn blue litmus red while bases taste bitter, feel soapy, and turn red litmus blue — and that an acid reacting with a base neutralises to form a salt and water, releasing heat.
- Telling acids and bases apart — Acids like citric and acetic acid are sour, while bases such as sodium hydroxide are bitter and soapy. Their names carry history — 'acid' comes from the Latin acere, meaning sour.
- Indicators reveal the hidden nature — Natural indicators change colour to show whether a solution is acidic or basic. Litmus from lichens turns red or blue, China rose gives magenta or green, and turmeric darkens in bases — while neutral distilled water shifts neither.
- Neutralisation in everyday life — When an acid and base combine in the right amounts they cancel out, forming salt and water and giving off heat. This idea treats indigestion, soothes ant bites, corrects acidic soil, and cleans factory waste before it reaches rivers.
Key points & formulas
- 01Acids are sour in taste; the word acid comes from the Latin word acere, meaning sour.
- 02Bases are bitter in taste and feel soapy on touching; examples include sodium hydroxide (soap) and magnesium hydroxide (milk of magnesia).
- 03Litmus, extracted from lichens, is the most common natural indicator — mauve (purple) in distilled water, red in acidic solutions, blue in basic solutions.
- 04China rose (Gudhal) indicator turns acidic solutions dark pink (magenta) and basic solutions green.
- 05Turmeric is a natural indicator that turns red/dark in basic (alkaline) solutions.
- 06Neutral substances neither turn red litmus blue nor blue litmus red; distilled water is a neutral solution.
- 07Neutralisation reaction: Acid + Base → Salt + Water, with heat always evolved; example: HCl + NaOH → NaCl + H₂O.
- 08Natural acids include citric acid (lemons/oranges), lactic acid (curd), acetic acid (vinegar), tartaric acid (tamarind), formic acid (ant's sting), oxalic acid (spinach), and ascorbic acid/Vitamin C (amla).
Frequently asked questions
01What are acids and where are they found naturally?
Acids are substances that taste sour and are acidic in nature. Natural acids include citric acid in lemons and oranges, lactic acid in curd, acetic acid in vinegar, tartaric acid in tamarind and grapes, oxalic acid in spinach, and ascorbic acid (Vitamin C) in amla and citrus fruits.
02What are bases? Give examples.
Bases are substances that taste bitter and feel soapy on touching. Examples include calcium hydroxide (lime water), ammonium hydroxide (window cleaner), sodium hydroxide and potassium hydroxide (soap), and magnesium hydroxide (milk of magnesia).
03What is an indicator? Name some natural indicators.
An indicator is a substance that changes its colour when added to an acidic or basic solution. Naturally occurring indicators include turmeric, litmus, and China rose petals (Gudhal).
04From where is litmus obtained, and what colour changes does it show?
Litmus is extracted from lichens. It is mauve (purple) in distilled water. It turns red when added to an acidic solution and blue when added to a basic solution.
05What colour changes does China rose indicator show?
China rose (Gudhal) indicator turns acidic solutions dark pink (magenta) and basic solutions green.
06What is neutralisation? Write the equation with an example.
The reaction between an acid and a base is called neutralisation. Salt and water are produced and heat is evolved. Example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O).
07What acid does an ant inject when it bites, and how is the effect neutralised?
When an ant bites, it injects formic acid into the skin. The effect can be neutralised by rubbing moist baking soda (sodium hydrogencarbonate) or calamine solution, which contains zinc carbonate.
08How is indigestion caused by excess stomach acid treated?
Our stomach contains hydrochloric acid that helps digest food. Too much acid causes indigestion. An antacid such as milk of magnesia, which contains magnesium hydroxide, is taken to neutralise the excess acid.
09How is acidic soil treated to help plants grow better?
When soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is too basic, organic matter (compost) is added, as it releases acids that neutralise the basic nature of the soil.
10What causes acid rain and what does it damage?
Acid rain forms when pollutants like carbon dioxide, sulphur dioxide, and nitrogen dioxide dissolve in raindrops to form carbonic acid, sulphuric acid, and nitric acid respectively. Acid rain can damage buildings, historical monuments, plants, and animals.
11What is phenolphthalein and what colour does it give in acidic and basic solutions?
Phenolphthalein is an indicator used in the laboratory. It gives a pink colour in basic solutions and remains colourless in acidic solutions.
12Why must factory wastes be neutralised before flowing into water bodies?
The wastes of many factories contain acids. If allowed to flow into water bodies, the acids will kill fish and other organisms. Factory wastes are therefore neutralised by adding basic substances before disposal.
13Is the NCERT PDF for Class 7 Science Chapter 4 free to read?
Yes — the NCERT textbook PDF for Class 7 Science Chapter 4 Acids, Bases and Salts is available free with no sign-up on cbseprepmaster.com.
More chapters in Science
Read Chapter 4 of Science — the Class 7 Science NCERT textbook (2026-27 edition) — online for free: the complete chapter as published by NCERT with every diagram, solved example and exercise, with step-by-step solutions, answers and revision notes. Open the NCERT PDF above, or browse all NCERT Class 7 textbooks.
Read offline with notes, solutions & mock tests
CBSE Prepmaster — free on iOS & Android