🧪 Chemistry · Topic 4 of 10
Start with the study notes on Acids, Bases & Salts (definitions, examples and a quick revision), then work through 60 practice questions, each with the correct answer and a clear explanation of why it is right. When you are ready, take this topic as an interactive quiz to test yourself.
Topic overview
Acids are substances that taste sour and turn blue litmus red. The word comes from the Latin word “acidus”, meaning sour.
Example: Lemon juice, vinegar, tamarind and curd are acidic.
Mineral acids such as sulphuric, hydrochloric and nitric acid are strong acids used in laboratories and industry.
Bases are substances that taste bitter, feel soapy and turn red litmus blue.
Examples:
Remember: Never taste or touch laboratory acids and bases. Many are corrosive and burn the skin and eyes. Always test them with indicators.
An indicator is a substance that changes colour to show whether something is acidic or basic.
Example: A turmeric stain on a shirt turns red when you rub soap on it, because soap is a base.
A neutral substance is neither acidic nor basic. It does not change the colour of litmus.
Example: Distilled water, sugar solution and common salt solution.
The pH scale shows how acidic or basic a solution is, from 0 to 14.
pH below 7 = acidic · pH 7 = neutral · pH above 7 = basic
Neutralisation is the reaction between an acid and a base. They cancel each other's effects and form a salt and water. Heat is given out.
Acid + Base → Salt + Water (+ heat)
Example: Hydrochloric acid + sodium hydroxide → sodium chloride (common salt) + water.
A salt is the substance formed, along with water, when an acid neutralises a base. The metal comes from the base, and the rest comes from the acid.
Examples:
Remember: To dilute an acid, always add the acid slowly to water, never water to acid. A lot of heat is released, and the mixture can splash.
| Misconception | Correct understanding |
|---|---|
| Tasting is a good way to test for acids. | Never taste chemicals; use indicators such as litmus. |
| All acids are dangerous. | Many acids in food are weak and safe, like citric acid in lemons; strong mineral acids are dangerous. |
| pH 2 is a weaker acid than pH 5. | The lower the pH, the stronger the acid; pH 2 is stronger. |
| Neutralisation destroys both substances and leaves nothing. | It produces a salt and water, and gives out heat. |
| Water can be poured into concentrated acid. | Always add acid to water slowly; the reverse can splash dangerously. |
What colour does blue litmus turn in an acid?
Red.
Name the acid in curd and the acid in vinegar.
Lactic acid in curd; acetic acid in vinegar.
Why does a turmeric stain turn red when soap is applied?
Soap is basic, and turmeric turns red in a base.
What is the pH of a neutral solution?
7.
What do you get when an acid reacts with a base?
A salt and water (and heat).
How does milk of magnesia relieve acidity?
It is a base that neutralises the extra stomach acid.
Why do farmers add lime to some soils?
Lime is basic and neutralises soil that is too acidic.
Which gas is given off when an acid reacts with zinc?
Hydrogen, which burns with a pop sound.
Key takeaway: Acids turn blue litmus red, bases turn red litmus blue, and together they neutralise each other to form a salt and water.
Notes written by Velnov for Class 6–8 students, following the topics of the NCERT science syllabus.
Q1. easy
Substances that taste SOUR and turn blue litmus red are called:
Answer: A — Acids
Acids are sour in taste and turn blue litmus paper red. Lemon juice, vinegar and tamarind are common acidic substances. (Never taste laboratory chemicals.)
Q2. easy
Substances that taste BITTER, feel soapy and turn red litmus blue are called:
Answer: A — Bases
Bases are bitter, soapy to touch and turn red litmus blue. Baking soda solution, lime water and soap solution are basic.
Q3. medium
The word ACID comes from a Latin word meaning:
Answer: A — Sour
'Acid' comes from the Latin word acidus/acere, meaning sour - reflecting the characteristic sour taste of acidic substances.
Q4. medium
A substance used to test whether a solution is acidic or basic is called:
Answer: A — An indicator
Indicators change colour in the presence of acids or bases. Litmus, turmeric, china rose and phenolphthalein are common indicators.
Q5. medium
Litmus is a natural indicator obtained from:
Answer: A — Lichens
Litmus is extracted from lichens, a plant that belongs to the division Thallophyta. In its natural state litmus solution is purple/mauve.
Q6. medium
What colour is litmus solution in a NEUTRAL (distilled water) solution?
Answer: A — Purple / mauve
Litmus solution is purple when neutral. It turns red in acids and blue in bases, which is why it can detect both.
Q9. medium
China rose (gudhal) indicator turns which colour in an ACID?
Answer: A — Dark pink / magenta
China rose solution turns dark pink (magenta) in acids and green in bases, making it another useful natural indicator.
Q10. medium
China rose indicator turns which colour in a BASE?
Answer: A — Green
In a basic solution, china rose petals' extract turns green. In acid it turns magenta - a clear two-colour test.
Q12. medium
Methyl orange turns which colour in an acid?
Answer: A — Red / pink
Methyl orange is red in acidic solutions and yellow in basic ones. Like phenolphthalein, it is a synthetic indicator.
Q13. medium
The acid present in LEMON and orange juice is:
Answer: A — Citric acid
Citrus fruits like lemon, orange and sweet lime contain citric acid, which gives them their sour taste.
Q14. medium
The acid present in VINEGAR is:
Answer: A — Acetic acid
Vinegar is a dilute solution of acetic acid (about 5-8%). It is used in cooking and as a preservative.
Q18. medium
The acid found in TOMATOES is:
Answer: A — Oxalic acid
Tomatoes contain oxalic acid, which contributes to their tangy taste. Spinach also contains oxalic acid.
Q19. medium
The acid present in our STOMACH that helps in digestion is:
Answer: A — Hydrochloric acid
The stomach secretes dilute hydrochloric acid, which kills germs in food and helps enzymes break down proteins. Excess of it causes acidity or indigestion.
Q21. medium
The base present in soap and used in making detergents is:
Answer: A — Sodium hydroxide
Sodium hydroxide (caustic soda) is a strong base used to make soaps and detergents. It feels soapy and is corrosive, so it must be handled carefully.
Q22. medium
Lime water is a solution of:
Answer: A — Calcium hydroxide
Lime water is calcium hydroxide dissolved in water. It is a base and is used to test for carbon dioxide, which turns it milky.
Q24. medium
Milk of magnesia is used to treat acidity because it is:
Answer: A — A base (magnesium hydroxide) that neutralises excess stomach acid
Milk of magnesia contains magnesium hydroxide, a mild base. It neutralises the excess hydrochloric acid in the stomach, relieving the burning feeling of acidity.
Q25. medium
A medicine taken to relieve acidity is called an:
Answer: A — Antacid
Antacids are mild bases such as magnesium hydroxide or baking soda that neutralise the excess acid produced in the stomach.
Q27. medium
The reaction between an acid and a base to give a salt and water is called:
Acid + Base → Salt + Water (+ heat)Answer: A — Neutralisation
In neutralisation, the acid and base cancel each other's effects, producing salt and water and usually releasing heat. It is a chemical change.
Q30. medium
Farmers add lime or quicklime to soil that is too acidic because lime:
Answer: A — Is basic and neutralises the excess acid in the soil
Quicklime (calcium oxide) or slaked lime (calcium hydroxide) is basic, so it neutralises acidic soil and brings the pH back to a range suitable for crops.
Q31. medium
If soil is too BASIC (alkaline), farmers treat it by adding:
Answer: A — Organic matter or compost, which releases acids
Decaying organic matter releases acids that neutralise excess alkalinity. Adding compost or manure is the usual remedy for basic soil.
Q33. medium
Rubbing baking soda on an ant sting relieves the pain because baking soda:
Answer: A — Is a mild base and neutralises the formic acid injected by the ant
The ant injects formic acid. Baking soda (sodium hydrogen carbonate) is basic and neutralises the acid, reducing the burning sensation.
Q35. medium
Which is used in fire extinguishers along with an acid to produce carbon dioxide?
Answer: A — Baking soda
In a soda-acid fire extinguisher, sulphuric acid mixes with sodium hydrogen carbonate solution, releasing carbon dioxide that smothers the fire.
Q37. medium
The salt formed when hydrochloric acid neutralises sodium hydroxide is used in daily life as:
Answer: A — Common table salt
Sodium chloride (NaCl) is common table salt. It seasons and preserves food, and is the raw material for making washing soda, baking soda and caustic soda.
Q39. medium
A solution with pH 7 is:
Answer: A — Neutral
pH 7 is the neutral point. Pure water has pH 7 - neither acidic nor basic.
Q41. medium
A solution with pH 12 is:
Answer: A — Strongly basic
Values above 7 are basic, and the higher the value the stronger the base. pH 12 is strongly basic - like a concentrated soap or lime solution.
Q43. medium
Acid rain is harmful because it:
Answer: A — Damages buildings, statues, soil and aquatic life
Acid rain forms when sulphur and nitrogen oxides from burning fuels dissolve in rainwater. It corrodes marble monuments, acidifies soil and lakes, and harms crops and fish.
Q45. medium
Toothpastes are usually mildly BASIC because they:
Answer: A — Neutralise the acid produced by bacteria in the mouth
Bacteria in the mouth produce acids that attack tooth enamel. A mildly basic toothpaste neutralises this acid and helps prevent tooth decay.
Q47. medium
Which acid is produced by bacteria in the mouth from leftover sugar?
Answer: A — Lactic acid
Mouth bacteria break down sugar and produce lactic acid, which lowers the pH and attacks enamel, leading to cavities.
Q50. medium
Why should we NEVER taste or touch laboratory acids and bases?
Answer: A — They are corrosive and can burn skin, eyes and the mouth badly
Concentrated acids and bases are corrosive and cause severe burns. Always use indicators to test them, wear safety goggles, and never taste any laboratory chemical.
Q52. medium
Which of these solutions would turn red litmus BLUE?
Answer: A — Soap solution
Only bases turn red litmus blue. Soap solution is basic; lemon juice, vinegar and curd are all acidic and would turn blue litmus red instead.
Q53. medium
Which of these is NEUTRAL?
Answer: A — Distilled water
Distilled water has pH 7 and does not change the colour of litmus. Vinegar is acidic; lime water and baking soda solution are basic.
Q55. medium
Nettle leaves cause a burning sting. Traditionally, people rub the affected area with:
Answer: A — Dock plant leaves, which contain a base
Nettle hairs inject formic acid. The leaves of the dock plant, which often grows nearby, contain a base that neutralises the acid and relieves the sting.
Q57. medium
A salt formed from a strong acid and a strong base is:
Answer: A — Neutral
When a strong acid and a strong base react completely, the resulting salt solution is neutral - sodium chloride solution is a common example.
Q59. medium
Which overall statement about acids, bases and salts is CORRECT?
Answer: A — Acids turn blue litmus red, bases turn red litmus blue, and together they neutralise to form a salt and water
This summarises the topic: acids are sour and turn blue litmus red; bases are bitter, soapy and turn red litmus blue; indicators detect them; and neutralisation of an acid by a base gives salt, water and heat.
Q60. medium
Why is a natural indicator like china rose useful in a school laboratory?
Answer: A — It is easily made from flower petals and clearly distinguishes acids from bases
Soaking china rose petals in warm water gives a light pink extract that turns magenta in acids and green in bases. It costs nothing and gives two clear, distinct colours.
Q7. hard
Turmeric indicator turns which colour in a BASIC solution?
Answer: A — Red / reddish-brown
Yellow turmeric turns reddish-brown in a base but stays yellow in an acid. This is why a turmeric stain on cloth turns red when soap (a base) is applied.
Q8. hard
A turmeric stain on a white shirt turns red when soap is rubbed on it because:
Answer: A — Soap is basic and turmeric turns red in a base
Soap solution is basic, and turmeric acts as an indicator that turns reddish-brown in bases. Washing with plenty of water removes the base and the stain fades back to yellow.
Q11. hard
Phenolphthalein is colourless in acid and turns which colour in a base?
Answer: A — Pink
Phenolphthalein is a synthetic indicator: colourless in acidic and neutral solutions, and pink in basic ones. It is widely used in titrations.
Q15. hard
The acid present in CURD and sour milk is:
Answer: A — Lactic acid
Bacteria convert the milk sugar lactose into lactic acid, which sours the milk and sets it into curd.
Q16. hard
The acid present in TAMARIND and grapes is:
Answer: A — Tartaric acid
Tamarind, grapes and unripe mangoes contain tartaric acid, which gives them their characteristic sourness.
Q17. hard
The acid injected when an ANT bites or a nettle plant stings is:
Answer: A — Formic acid
Ant stings and nettle leaf hairs inject formic acid (methanoic acid), causing pain and burning. Rubbing with baking soda or dock leaves (both basic) neutralises it.
Q20. hard
Which of these is a MINERAL (strong) acid used in laboratories?
Answer: A — Sulphuric acid
Sulphuric acid, hydrochloric acid and nitric acid are strong mineral acids made in industry. Citric, lactic and tartaric acids are weak organic acids found in food.
Q23. hard
The base found in window cleaners and used for cleaning is:
Answer: A — Ammonium hydroxide
Ammonium hydroxide (ammonia solution) is a base commonly used in glass and window cleaners because it dissolves grease and evaporates without streaks.
Q26. hard
A substance that is neither acidic nor basic is called:
Answer: A — Neutral
Neutral substances such as distilled water, common salt solution and sugar solution have no effect on litmus - it stays purple.
Q28. hard
Neutralisation reactions are usually:
Answer: A — Exothermic - heat is released
When an acid reacts with a base, heat is given out, so the mixture becomes warmer. This is why neutralisation is described as an exothermic reaction.
Q29. hard
Hydrochloric acid reacting with sodium hydroxide gives:
HCl + NaOH → NaCl + H2OAnswer: A — Sodium chloride and water
This is the classic neutralisation: HCl + NaOH → NaCl + H2O. The salt formed is common salt, and the solution becomes neutral.
Q32. hard
Factory waste is often neutralised before being released into water bodies because:
Answer: A — Acidic or basic waste can kill fish and other aquatic life
Industrial effluents are often strongly acidic or basic and would harm aquatic organisms. Neutralising them with a suitable base or acid before discharge protects the ecosystem.
Q34. hard
The chemical name of baking soda is:
Answer: A — Sodium hydrogen carbonate (sodium bicarbonate)
Baking soda is sodium hydrogen carbonate, NaHCO3. It is a mild base, used in cooking, as an antacid and in fire extinguishers.
Q36. hard
The chemical name of washing soda is:
Answer: A — Sodium carbonate (Na2CO3·10H2O)
Washing soda is hydrated sodium carbonate. It is basic, removes hardness from water and is used in laundries and glass manufacture.
Q38. hard
The pH scale measures:
Answer: A — How acidic or basic a solution is, on a scale from 0 to 14
pH runs from 0 to 14. Values below 7 are acidic, exactly 7 is neutral, and above 7 is basic. The lower the pH, the stronger the acid.
Q40. hard
A solution with pH 2 is:
Answer: A — Strongly acidic
The lower the pH below 7, the stronger the acid. pH 2 is strongly acidic - similar to lemon juice or stomach acid.
Q42. hard
Universal indicator is preferred over litmus because it:
Answer: A — Shows the strength of an acid or base through a range of colours, not just which it is
Universal indicator is a mixture of dyes that gives a different colour at each pH value, so it tells us not just whether a solution is acidic or basic but how strongly so.
Q44. hard
The Taj Mahal is damaged by acid rain because marble is:
Answer: A — Calcium carbonate, which reacts with acids
Marble is calcium carbonate. Acids in polluted rain react with it, corroding and discolouring the surface - a problem often called 'marble cancer'.
Q46. hard
Tooth decay begins when the pH of the mouth falls below about:
Answer: A — 5.5
Below pH 5.5 the acid is strong enough to dissolve tooth enamel, which is made of calcium phosphate. Brushing after eating sugary food helps keep the pH higher.
Q48. hard
Acids react with active metals like zinc to produce:
Zn + 2 HCl → ZnCl2 + H2↑Answer: A — A salt and hydrogen gas
Dilute acids react with reactive metals to release hydrogen: Zn + 2HCl → ZnCl2 + H2. The hydrogen burns with a pop sound.
Q49. hard
Acids react with metal carbonates (like baking soda or marble) to produce:
CaCO3 + 2 HCl → CaCl2 + H2O + CO2↑Answer: A — A salt, water and carbon dioxide gas
Acid plus carbonate gives salt, water and carbon dioxide, which is why marble chips fizz in acid. The gas turns lime water milky.
Q51. hard
When diluting a concentrated acid, the correct and safe practice is:
Answer: A — Add acid slowly to water with constant stirring, never water to acid
Dilution releases a lot of heat. Adding acid to a large volume of water spreads the heat safely. Adding water to concentrated acid can make it boil and splash dangerously.
Q54. hard
Bee stings cause pain because they inject an acid. The best remedy is to apply:
Answer: A — A mild base such as baking soda
A bee sting injects an acidic liquid. Baking soda, being a mild base, neutralises it and eases the pain. Applying another acid would make it worse.
Q56. hard
The salt formed depends on the acid and base used. Sulphuric acid with sodium hydroxide gives:
H2SO4 + 2 NaOH → Na2SO4 + 2 H2OAnswer: A — Sodium sulphate and water
The metal comes from the base and the rest from the acid: H2SO4 + 2NaOH → Na2SO4 + 2H2O. So sulphuric acid gives sulphates, hydrochloric acid gives chlorides, and nitric acid gives nitrates.
Q58. hard
Why is a lemon or tamarind used to clean tarnished copper and brass vessels?
Answer: A — The acid in them dissolves the dull oxide/carbonate coating
The green or dull coating on copper and brass is basic copper carbonate. The citric or tartaric acid in lemon and tamarind reacts with and dissolves it, restoring the shine.