Exploring Substances: Acidic, Basic, and Neutral – MCQs with Answers and Explanations
CBSE Board Examinations – Multiple Choice Questions (MCQs)
These 50 topic-wise MCQs with Answers and Explanations for Class 7 Science – Chapter 2: Exploring Substances: Acidic, Basic, and Neutral are designed strictly as per the NCERT syllabus and are ideal for:
- CBSE Class 7 Annual / Term-End Examinations
- Class Tests, Periodic Tests and Internal Assessments
- Objective and application-based practice
- Quick revision and concept strengthening
Practice these carefully framed Multiple Choice Questions to master the concepts of acids, bases, neutral substances, indicators, neutralisation, acid rain, pH and uses & safety of acids and bases explained in this chapter.
Topic 1: Basic Concepts – Acids, Bases and Neutral Substances
Q1. Which of the following is most likely to be an acid?
- Soap solution
- Lemon juice
- Baking soda solution
- Lime water
Correct Answer: (b) Lemon juice
Explanation: Substances with a sour taste are usually acids. Lemon juice is sour and contains citric acid. Soap solution, baking soda and lime water are basic in nature.
Q2. Which statement about bases is correct?
- They always taste sweet.
- They turn blue litmus red.
- They feel soapy and turn red litmus blue.
- They have no effect on litmus.
Correct Answer: (c) They feel soapy and turn red litmus blue.
Explanation: Bases generally taste bitter, feel soapy and turn red litmus blue. Acids turn blue litmus red. Neutral substances do not change litmus colour.
Q3. Which of the following is a neutral substance?
- Vinegar
- Pure water
- Soap solution
- Lemon juice
Correct Answer: (b) Pure water
Explanation: Pure water is neutral – it is neither acidic nor basic. Vinegar and lemon juice are acidic, while soap solution is basic.
Q4. Which property is common to all acids?
- They taste bitter.
- They feel slippery.
- They change red litmus to blue.
- They change blue litmus to red.
Correct Answer: (d) They change blue litmus to red.
Explanation: A key property of acids is that they turn blue litmus paper red. Bitter taste and slippery feel are properties of bases.
Q5. Which of the following pairs is correctly matched?
- Lemon – base
- Soap – acid
- Curd – acid
- Baking soda – acid
Correct Answer: (c) Curd – acid
Explanation: Curd is sour due to the presence of lactic acid, hence acidic. Lemon is also acidic; soap and baking soda are basic, not acidic.
Q6. Which substance is most likely to be a base used at home?
- Vinegar
- Sugar solution
- Baking soda
- Orange juice
Correct Answer: (c) Baking soda
Explanation: Baking soda (sodium hydrogen carbonate) is a mild base. Vinegar and orange juice are acidic; sugar solution is neutral.
Q7. Why should we never taste laboratory acids and bases?
- They do not have any taste.
- They may be corrosive and harmful.
- They always taste very bitter.
- They may turn colourless.
Correct Answer: (b) They may be corrosive and harmful.
Explanation: Many lab acids and bases are corrosive. Tasting them can burn the mouth or cause poisoning. We identify them using indicators, not by tasting.
Q8. Which of these is most likely to be a weak acid found in food?
- Hydrochloric acid
- Sulphuric acid
- Nitric acid
- Acetic acid
Correct Answer: (d) Acetic acid
Explanation: Acetic acid is a weak organic acid present in vinegar and used in food. Hydrochloric, sulphuric and nitric acids are strong mineral acids used in labs and industries.
Q9. Which statement is TRUE about neutral substances?
- They turn blue litmus red.
- They turn red litmus blue.
- They do not change the colour of litmus paper.
- They are always colourless.
Correct Answer: (c) They do not change the colour of litmus paper.
Explanation: Neutral substances do not affect red or blue litmus. They may be coloured or colourless; colour is not the deciding factor.
Q10. Which of the following sets contains only basic substances?
- Lemon juice, vinegar, curd
- Soap solution, baking soda, lime water
- Sugar solution, salt solution, water
- Orange juice, tamarind juice, tomato juice
Correct Answer: (b) Soap solution, baking soda, lime water
Explanation: Soap solution, baking soda solution and lime water are all basic. Options (a) and (d) list acids; (c) lists neutral solutions.
Topic 2: Indicators – Litmus, Turmeric and China Rose
Q11. What is an indicator?
- A substance used only for taste
- A substance that changes colour in acids and bases
- A substance that changes temperature
- A substance that reacts only with metals
Correct Answer: (b) A substance that changes colour in acids and bases
Explanation: An indicator shows different colours in acidic and basic solutions, helping us identify their nature without tasting or touching them.
Q12. From which natural source is litmus obtained?
- Turmeric plant
- China rose petals
- Lichens
- Neem leaves
Correct Answer: (c) Lichens
Explanation: Litmus is a natural indicator obtained from lichens, a plant-like organism. Turmeric and china rose also give natural indicators, but they are different.
Q13. Blue litmus paper turns red in which type of solution?
- Basic solution
- Acidic solution
- Neutral solution
- Sugar solution only
Correct Answer: (b) Acidic solution
Explanation: A characteristic test for acids is that they turn blue litmus paper red. Bases do the opposite, while neutral solutions do not change litmus colour.
Q14. Red litmus paper turns blue when dipped in:
- Lemon juice
- Vinegar
- Soap solution
- Distilled water
Correct Answer: (c) Soap solution
Explanation: Bases like soap solution turn red litmus paper blue. Lemon juice and vinegar are acids; distilled water is neutral and does not change litmus colour.
Q15. Turmeric indicator turns reddish-brown when:
- Acid is added
- Base is added
- Neutral solution is added
- Only water is added
Correct Answer: (b) Base is added
Explanation: Turmeric remains yellow in acidic and neutral solutions but turns reddish-brown in the presence of bases such as soap solution or baking soda solution.
Q16. China rose (hibiscus) indicator usually turns:
- Red in acids and green in bases
- Green in acids and red in bases
- Yellow in both acids and bases
- Colourless in acids and bases
Correct Answer: (a) Red in acids and green in bases
Explanation: China rose extract often becomes bright red in acidic solutions and greenish in basic solutions, helping us distinguish them.
Q17. A solution does not change the colour of either red or blue litmus. This solution is:
- Strongly acidic
- Strongly basic
- Neutral
- Corrosive
Correct Answer: (c) Neutral
Explanation: Neutral solutions do not affect the colour of red or blue litmus paper. Strong acids and bases would show clear changes in colour.
Q18. Which indicator is most easily available in the kitchen?
- Litmus solution
- Phenolphthalein
- Turmeric
- Methyl orange
Correct Answer: (c) Turmeric
Explanation: Turmeric is a common kitchen spice and works as a natural indicator. Litmus, phenolphthalein and methyl orange are usually found in laboratories.
Q19. Which of these is the safest way to identify an unknown solution?
- Taste it carefully.
- Smell it deeply.
- Check its temperature.
- Use suitable indicators like litmus.
Correct Answer: (d) Use suitable indicators like litmus.
Explanation: Tasting or smelling unknown chemicals is dangerous. Using indicators like litmus, turmeric or china rose is the safe and correct scientific method.
Q20. A student adds soap solution to a strip of turmeric paper. The colour turns reddish-brown. What can be concluded?
- Soap solution is acidic.
- Soap solution is basic.
- Soap solution is neutral.
- Turmeric is neutral.
Correct Answer: (b) Soap solution is basic.
Explanation: Turmeric changes from yellow to reddish-brown in the presence of a base. Since this happens with soap solution, it is basic in nature.
Topic 3: Neutralisation – Reaction Between Acids and Bases
Q21. Neutralisation is a reaction between:
- Two acids
- Two bases
- An acid and a base
- Salt and water
Correct Answer: (c) An acid and a base
Explanation: Neutralisation occurs when an acid reacts with a base to form salt and water, reducing the effect of both acid and base.
Q22. Which products are formed in a neutralisation reaction?
- Acid and metal
- Base and hydrogen gas
- Salt and water
- Only gas
Correct Answer: (c) Salt and water
Explanation: In neutralisation, an acid + base → salt + water. Sometimes heat is also produced, but salt and water are the main products.
Q23. Which of the following is an example of neutralisation in daily life?
- Melting of ice
- Boiling of water
- Using antacid for acidity
- Evaporation of water
Correct Answer: (c) Using antacid for acidity
Explanation: Antacids are basic medicines that neutralise excess acid in the stomach – a clear example of neutralisation. Melting, boiling and evaporation are physical changes.
Q24. When hydrochloric acid reacts with sodium hydroxide, the salt formed is:
- Sodium chloride
- Sodium carbonate
- Calcium chloride
- Magnesium chloride
Correct Answer: (a) Sodium chloride
Explanation: The reaction is:
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water.
Sodium chloride is common salt.
Q25. Why does the temperature of the mixture often rise during neutralisation?
- Because it is an exothermic reaction
- Because it is an endothermic reaction
- Because the solution freezes
- Because gases escape out
Correct Answer: (a) Because it is an exothermic reaction
Explanation: Neutralisation usually releases heat, so it is an exothermic reaction. This heat warms the solution and raises its temperature.
Q26. Which of the following pairs shows a correct neutralisation example?
- Acidic soil – add vinegar
- Acidic soil – add lime
- Acidic soil – add more fertiliser
- Acidic soil – add sugar
Correct Answer: (b) Acidic soil – add lime
Explanation: Lime (quicklime or slaked lime) is basic and is added to acidic soil to neutralise it and improve soil quality for plant growth.
Q27. Bee sting injects an acidic liquid into the skin. Which is the best first-aid?
- Apply lemon juice
- Apply vinegar
- Apply baking soda paste
- Apply more acid
Correct Answer: (c) Apply baking soda paste
Explanation: Bee sting is acidic; a mild base like baking soda paste is applied to neutralise the acid and reduce pain. Using more acid would worsen the condition.
Q28. Which is true for a wasp sting which is basic in nature?
- It should be treated with a base.
- It should be treated with an acid like vinegar.
- It should be treated only with water.
- It should be treated with more alkali.
Correct Answer: (b) It should be treated with an acid like vinegar.
Explanation: Wasp stings are usually basic. They are treated with a weak acid such as vinegar or lemon juice to neutralise the base and reduce pain.
Q29. Factory waste often contains acids. Before releasing it into the river, it is treated with:
- More acids
- Bases
- Only water
- Salt
Correct Answer: (b) Bases
Explanation: Acidic factory waste is neutralised using basic substances before release so that it does not harm aquatic life in rivers and lakes.
Q30. Which word equation correctly represents neutralisation?
- Acid + Metal → Salt + Hydrogen
- Acid + Base → Salt + Water
- Base + Metal → Salt + Water
- Acid + Salt → Base + Water
Correct Answer: (b) Acid + Base → Salt + Water
Explanation: The general word equation for neutralisation is: Acid + Base → Salt + Water. Other equations show different types of reactions.
Topic 4: Acid Rain and Environmental Impact
Q31. Acid rain is caused when rainwater mixes mainly with:
- Oxygen and nitrogen
- Sulphur dioxide and nitrogen oxides
- Hydrogen and helium
- Carbon monoxide and methane
Correct Answer: (b) Sulphur dioxide and nitrogen oxides
Explanation: Gases like sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) from vehicles and factories dissolve in rainwater to form acids, causing acid rain.
Q32. Which of the following is NOT an effect of acid rain?
- Damage to leaves and crops
- Increase in fish population
- Corrosion of buildings and monuments
- Acidification of lakes and ponds
Correct Answer: (b) Increase in fish population
Explanation: Acid rain harms aquatic life. It reduces fish population, damages plants, and corrodes monuments. It does not increase fish population.
Q33. Marble monuments like the Taj Mahal are damaged by acid rain because marble contains:
- Calcium carbonate
- Sodium chloride
- Iron oxide
- Aluminium sulphate
Correct Answer: (a) Calcium carbonate
Explanation: Marble is mainly calcium carbonate, which reacts with acids in acid rain and slowly dissolves, causing damage to the monument’s surface.
Q34. How can we reduce the problem of acid rain?
- By increasing the number of vehicles
- By burning more coal
- By reducing air pollution from factories and vehicles
- By cutting more trees
Correct Answer: (c) By reducing air pollution from factories and vehicles
Explanation: Acid rain is mainly caused by industrial and vehicle emissions. Reducing these pollutants through cleaner fuels and pollution control devices helps reduce acid rain.
Q35. Acid rain makes lake water harmful to aquatic life because:
- It increases pH too much.
- It decreases pH too much.
- It removes all salts.
- It makes water colourless.
Correct Answer: (b) It decreases pH too much.
Explanation: Acid rain makes water strongly acidic (low pH). Many aquatic organisms cannot survive in such conditions, so they die.
Q36. Which action by students can help reduce acid rain?
- Using bicycles or walking for short distances
- Keeping vehicles running unnecessarily
- Bursting more crackers
- Wasting electricity at home
Correct Answer: (a) Using bicycles or walking for short distances
Explanation: Using cycles or walking reduces fuel use and air pollution. Less pollution means fewer acidic gases, which helps reduce acid rain.
Q37. In an area with frequent acid rain, crops fail because:
- Soil becomes too basic
- Soil becomes too acidic
- Soil loses all water
- Soil becomes very cold
Correct Answer: (b) Soil becomes too acidic
Explanation: Acid rain increases soil acidity. Highly acidic soil is not suitable for most crops and reduces their growth and yield.
Q38. Planting more trees helps reduce acid rain because:
- Trees absorb sulphur dioxide and other pollutants
- Trees produce more acids
- Trees give out sulphur dioxide
- Trees stop clouds from forming
Correct Answer: (a) Trees absorb sulphur dioxide and other pollutants
Explanation: Trees absorb many air pollutants including SO₂ and CO₂. This helps reduce the formation of acid rain and improves air quality.
Topic 5: Simple Idea of pH and Body Fluids
Q39. The pH scale is used to measure:
- Temperature of a solution
- Colour of a solution
- How acidic or basic a solution is
- Weight of a solution
Correct Answer: (c) How acidic or basic a solution is
Explanation: The pH scale tells us the degree of acidity or basicity of a solution. Lower pH means more acidic, higher pH means more basic.
Q40. A solution with pH 7 is:
- Acidic
- Basic
- Neutral
- Highly corrosive
Correct Answer: (c) Neutral
Explanation: On the pH scale, pH 7 represents a neutral solution, like pure water. Values below 7 are acidic, above 7 are basic.
Q41. A solution has pH 3. It is:
- Strongly acidic
- Weakly acidic
- Neutral
- Strongly basic
Correct Answer: (a) Strongly acidic
Explanation: A pH value much less than 7, like 3, indicates a strongly acidic solution. The smaller the pH, the stronger the acid.
Q42. Why must the pH of blood remain almost constant?
- To change food taste
- To keep body reactions working properly
- To make blood colourful
- To increase body weight
Correct Answer: (b) To keep body reactions working properly
Explanation: Many body processes work only in a narrow pH range. If blood pH changes too much, important reactions are disturbed and we can become seriously ill.
Q43. Excess acid in the stomach causes acidity. The pH inside the stomach becomes:
- Too high (more basic)
- Too low (more acidic)
- Exactly 7
- Colourless
Correct Answer: (b) Too low (more acidic)
Explanation: When too much hydrochloric acid is produced, the stomach pH becomes very low (highly acidic), causing pain and burning sensation.
Q44. Antacids are useful because they:
- Increase acidity in the stomach
- Neutralise extra acid in the stomach
- Produce more gas
- Stop digestion completely
Correct Answer: (b) Neutralise extra acid in the stomach
Explanation: Antacids are mild bases like milk of magnesia. They neutralise extra acid and bring the pH closer to normal, giving relief from acidity.
Topic 6: Uses and Safety of Acids and Bases
Q45. Which acid is present in our stomach to help digestion?
- Acetic acid
- Citric acid
- Hydrochloric acid
- Carbonic acid
Correct Answer: (c) Hydrochloric acid
Explanation: Our stomach contains hydrochloric acid (HCl). It helps in digestion and kills harmful germs in the food we eat.
Q46. Which base is used in whitewashing walls?
- Sodium hydroxide
- Calcium hydroxide
- Magnesium hydroxide
- Ammonium hydroxide
Correct Answer: (b) Calcium hydroxide
Explanation: In whitewashing, quicklime (calcium oxide) is mixed with water to form calcium hydroxide (slaked lime), which is applied to walls.
Q47. Toothpaste is usually made:
- Strongly acidic
- Strongly basic
- Slightly basic
- Completely neutral
Correct Answer: (c) Slightly basic
Explanation: Toothpaste is made slightly basic so that it can neutralise acids produced by bacteria in the mouth, protecting teeth from decay.
Q48. Which of the following is a correct safety rule in the laboratory?
- Taste chemicals to identify them.
- Add water to acid while diluting.
- Add acid slowly to water while diluting.
- Carry acid bottles with one hand.
Correct Answer: (c) Add acid slowly to water while diluting.
Explanation: The correct rule is “Always add acid to water, never water to acid”, to avoid dangerous splashing. Tasting or careless handling of chemicals is unsafe.
Q49. If a small amount of acid accidentally falls on your hand in the lab, you should first:
- Rub it with dry cloth
- Wash it immediately with plenty of water
- Ignore it
- Smell it to identify it
Correct Answer: (b) Wash it immediately with plenty of water
Explanation: The correct first-aid is to wash immediately with plenty of clean water and inform the teacher. Rubbing or ignoring can worsen the burn.
Q50. Studying acids, bases and neutral substances helps students to:
- Believe more in superstitions
- Understand everyday phenomena scientifically
- Avoid all experiments
- Use chemicals carelessly
Correct Answer: (b) Understand everyday phenomena scientifically
Explanation: This chapter helps students explain indigestion, soil treatment, tooth decay, acid rain and many other events using scientific ideas, which builds a scientific attitude.