Exploring Substances: Acidic, Basic, and Neutral – Short Answer Type Questions

Q1. Define an acid. Give two everyday examples of acids.

Ans. An acid is a substance that usually tastes sour and turns blue litmus paper red. It may also react with metals to produce hydrogen gas. Lemon juice (citric acid) and vinegar (acetic acid) are common examples of acids found in our kitchen.

Q2. Define a base. Mention two characteristic properties of bases.

Ans. A base is a substance that tastes bitter, feels soapy when touched and turns red litmus paper blue. Bases often form hydroxide ions in water. They can neutralise acids to form salt and water.

Q3. What are neutral substances? Give any two examples.

Ans. Substances which are neither acidic nor basic in nature are called neutral substances. They do not change the colour of litmus paper. Pure water and sugar solution are common examples of neutral substances.

Q4. How do acids differ from bases in terms of taste and effect on litmus?

Ans. Acids generally have a sour taste and turn blue litmus red. Bases have a bitter taste, feel soapy and turn red litmus blue. Thus, taste and litmus test clearly distinguish acids from bases.

Q5. Why should we never taste unknown substances in the science laboratory to test whether they are acidic or basic?

Ans. Many laboratory acids and bases are strong and corrosive. They can burn our tongue, mouth or internal organs and may be poisonous. Hence, tasting chemicals is very unsafe and we always use indicators instead of tasting them.

Q6. Distinguish between mineral acids and organic acids with one example of each.

Ans. Mineral acids are man-made acids obtained from minerals in the earth, such as hydrochloric acid (HCl) and sulphuric acid (H₂SO₄). Organic acids occur naturally in plants and animals, for example, citric acid in lemon and lactic acid in curd. Mineral acids are usually stronger than most organic acids.

Q7. Name any three basic substances commonly found at home and write one use for each.

Ans. (i) Baking soda – used in making fluffy cakes and as a mild cleaning agent.
(ii) Soap solution – used for bathing and washing clothes due to its cleaning action.
(iii) Toothpaste – used to clean teeth and neutralise acids produced by bacteria in the mouth.

Q8. Why are strong acids and strong bases called corrosive? Give one example of a corrosive acid.

Ans. Strong acids and bases can eat away or damage metals, skin, cloth and other materials, so they are called corrosive. When they fall on skin, they may cause burns. An example of a corrosive acid is concentrated sulphuric acid (H₂SO₄).

Q9. Explain why all substances with sour taste are not safe to eat, even if they are acidic.

Ans. Mild natural acids present in fruits and curd are safe to eat in small amounts. However, strong acids like hydrochloric acid or sulphuric acid are very corrosive and unsafe. Some sour substances may contain harmful or concentrated acids, so we should never taste them without proper knowledge.

Q10. How can you say that acids, bases and neutral substances are important in everyday life?

Ans. Acids give sour taste to many fruits and help in digestion in our stomach. Bases help in cleaning (soaps, detergents, toothpastes) and in treating acidic soils. Neutral substances like water and salts maintain balance in our body and environment. Thus all three types of substances are closely linked to our daily life.

Q11. What is an indicator? Why are indicators useful in chemistry?

Ans. An indicator is a substance that shows a different colour in acidic and basic solutions. Indicators are useful because they help us identify the nature (acidic, basic or neutral) of a given solution safely, without tasting or touching it.

Q12. Describe how litmus paper is used to test whether a solution is acidic or basic.

Ans. To test a solution, we dip a small strip of blue and red litmus paper in it separately. If blue litmus turns red, the solution is acidic. If red litmus turns blue, the solution is basic. If neither paper changes colour, the solution is neutral.

Q13. What are natural indicators? Give two examples and state one use of each.

Ans. Natural indicators are obtained from natural substances like plants. Litmus (from lichens) and turmeric are examples. Litmus is used in the form of blue and red paper to identify acids and bases, while turmeric is used to test for basic solutions like soap, where it turns reddish-brown.

Q14. How does turmeric act as an indicator? Explain with an example.

Ans. Turmeric is yellow in colour. When a base like soap solution or baking soda is added, it turns reddish-brown. For example, if a drop of soap water falls on a cloth stained with turmeric, the spot appears reddish-brown, showing the basic nature of soap water.

Q15. What is china rose indicator? How does it help in identifying acids and bases?

Ans. China rose indicator is prepared from the petals of the hibiscus (china rose) flower. The coloured extract generally turns bright red in acidic solutions and greenish in basic solutions. This colour change helps us to distinguish between acids and bases.

Q16. Can a single indicator give complete information about the strength of an acid or base? Why or why not?

Ans. No. Simple indicators like litmus and turmeric only tell whether a solution is acidic or basic, not how strong it is. To know the strength, more advanced methods and special indicators or pH papers are needed, which you will study in higher classes.

Q17. A solution does not change the colour of blue litmus or red litmus. What can you say about its nature? Give one example of such a solution.

Ans. If neither blue nor red litmus paper changes colour, the solution is neutral in nature. An example of a neutral solution is distilled water or sugar solution.

Q18. Why are indicators especially important while working with colourless solutions?

Ans. Colourless solutions look similar and we cannot judge their nature just by seeing them. Indicators show different colours in acidic and basic colourless solutions, allowing us to identify them easily and safely.

Q19. What is neutralisation? Write its general word equation.

Ans. Neutralisation is the chemical reaction in which an acid reacts with a base to form salt and water, and the effects of both are reduced. General word equation:
Acid + Base → Salt + Water (+ heat).

Q20. Explain with an example how neutralisation occurs in a test tube experiment.

Ans. If we take some dilute hydrochloric acid in a test tube and add a few drops of indicator, then slowly add dilute sodium hydroxide solution, the indicator colour changes showing neutralisation. The reaction is:
Hydrochloric acid + Sodium hydroxide → Sodium chloride (salt) + Water.

Q21. Why does the temperature of the mixture usually rise during neutralisation?

Ans. During neutralisation, energy is released in the form of heat. This heat warms up the reaction mixture, so its temperature rises. Such reactions are called exothermic reactions.

Q22. How does neutralisation help in treating indigestion in our body?

Ans. Our stomach produces hydrochloric acid for digestion. Sometimes excess acid causes indigestion and pain. Doctors give antacids such as milk of magnesia, which is basic and neutralises the extra acid, giving relief.

Q23. What type of substances are antacids? Give one example and its use.

Ans. Antacids are mild basic substances that neutralise excess acid in the stomach. An example is milk of magnesia (magnesium hydroxide). It is taken when we suffer from acidity or heartburn, and it neutralises the extra acid produced.

Q24. Explain why farmers add quicklime or slaked lime to acidic soil.

Ans. Some soils become too acidic due to overuse of fertilisers or acid rain, which is harmful for crops. Quicklime (calcium oxide) or slaked lime (calcium hydroxide) are basic substances. They neutralise the excess acid in the soil and make it suitable for plant growth.

Q25. How does neutralisation help in the case of bee stings and wasp stings?

Ans. The fluid injected by a bee sting is acidic, so a mild base like baking soda solution can be applied to neutralise it. Wasp stings are usually basic, so a mild acid like vinegar or lemon juice is applied to reduce the effect by neutralising the base.

Q26. A science teacher mixes vinegar with baking soda solution. Fizzing occurs. Is this neutralisation? Explain briefly.

Ans. Vinegar (acetic acid) reacts with baking soda (a basic substance) to form salt, water and carbon dioxide gas. The fizzing is due to carbon dioxide. Since an acid and a base are reacting to form salt and water, it is an example of neutralisation, though a gas is also produced.

Q27. State two everyday situations (other than indigestion and soil treatment) where neutralisation is useful.

Ans. (i) Factory waste containing acids is neutralised with basic substances before being released into rivers to protect aquatic life.
(ii) Chemical treatments used in water purification plants often involve neutralisation to make water safe for use.

Q28. Why is neutralisation important in our daily life? Give a brief explanation.

Ans. Neutralisation helps to reduce the harmful effects of strong acids or bases in many situations like stomach acidity, acidic soil, insect stings and industrial waste. By balancing the acidity or basicity, it protects living organisms and the environment, making it very important in everyday life.

Q29. What is acid rain? How is it formed?

Ans. Rainwater that contains more acids than normal is called acid rain. It is formed when gases like sulphur dioxide and nitrogen oxides released from factories and vehicles dissolve in rainwater to form acids such as sulphuric and nitric acid. This makes the rainwater strongly acidic.

Q30. Mention any three harmful effects of acid rain on the environment.

Ans. Acid rain damages the leaves and roots of plants, reducing crop yield. It makes lakes and ponds too acidic for fish and other aquatic life. It also slowly corrodes buildings and monuments, especially those made of marble and limestone.

Q31. How does acid rain affect soil quality and crop production?

Ans. Acid rain increases the acidity of soil and can dissolve important nutrients needed by plants. As a result, plants do not get enough nutrients and their growth is reduced. This lowers crop production and may harm forests as well.

Q32. Why are monuments like the Taj Mahal getting affected by acid rain?

Ans. The Taj Mahal is made of marble, which contains calcium carbonate. Acids present in acid rain react with calcium carbonate and gradually dissolve it, making the marble surface dull and rough. This chemical corrosion slowly damages the monument.

Q33. Suggest two steps to reduce acid rain in cities.

Ans. (i) Use cleaner fuels and maintain vehicles properly to reduce emission of sulphur dioxide and nitrogen oxides.
(ii) Install pollution control devices in factory chimneys and encourage the use of public transport, cycling and walking to reduce air pollution.

Q34. How is acid rain related to the topic of acids and bases studied in this chapter?

Ans. Acid rain is an application of acidic substances in the environment. The chapter helps us understand that certain gases form acids in water, making rain acidic. Knowing properties of acids and bases helps us understand the harmful effects of acid rain and ways to control it.

Q35. What is meant by pH of a solution? (Conceptual understanding)

Ans. pH is a scale used to measure how acidic or basic a solution is. It usually ranges from 0 to 14. For Class 7, we simply remember that pH below 7 indicates an acidic solution, pH 7 indicates a neutral solution, and pH above 7 indicates a basic solution.

Q36. What is the pH value of a neutral solution? What does it indicate?

Ans. A neutral solution has a pH value of 7. This indicates that the solution is neither acidic nor basic and has balanced acidic and basic properties, like pure water.

Q37. If a solution has a pH of 3, what can you say about it? How will it affect litmus?

Ans. A solution with pH 3 is strongly acidic. It will turn blue litmus paper red but will not affect red litmus paper. Such acidic solutions should be handled carefully as they may be corrosive.

Q38. Why is it important for our blood to maintain a nearly constant pH?

Ans. Many chemical reactions in our body, like digestion and respiration, work properly only within a narrow pH range. If the pH of blood changes too much, it can disturb the working of cells and organs, leading to serious health problems. So, the body maintains the blood pH nearly constant.

Q39. Excess acid production in the stomach is harmful. How is this related to the pH concept you learnt?

Ans. Excess acid in the stomach lowers the pH too much, making the stomach contents highly acidic. This can cause pain and discomfort. Antacids are basic; they increase the pH closer to neutral by neutralising the extra acid and provide relief.

Q40. Give four important uses of acids in our daily life or industry.

Ans. (i) Hydrochloric acid in the stomach helps in digestion of food.
(ii) Sulphuric acid is used in car batteries and fertiliser industries.
(iii) Acetic acid in vinegar is used as a food preservative and in cooking.
(iv) Citric acid in lemon and orange adds sour taste and vitamin C to food.

Q41. List four important uses of bases in daily life and industry.

Ans. (i) Sodium hydroxide is used in making soaps and detergents.
(ii) Calcium hydroxide (lime) is used in whitewashing walls and treating acidic soil.
(iii) Magnesium hydroxide is used as an antacid in medicines.
(iv) Ammonium hydroxide is used in cleaning solutions and in fertiliser production.

Q42. Why is toothpaste made slightly basic? How does it protect our teeth?

Ans. When we eat, bacteria in the mouth produce acids from food particles. These acids can damage tooth enamel. Toothpaste is slightly basic and neutralises these acids, preventing tooth decay and protecting our teeth.

Q43. Explain how acids and bases are involved in the process of whitewashing a house.

Ans. For whitewashing, quicklime (calcium oxide), a basic substance, is mixed with water to form slaked lime (calcium hydroxide). This basic solution is painted on walls. With time, it reacts with carbon dioxide in air to form calcium carbonate, giving a shiny, white finish to the walls.

Q44. How do acids and bases help in the field of agriculture?

Ans. If soil is too acidic, plant growth is affected. Farmers add basic substances like lime to neutralise the acid. In some cases, if soil is too basic, mild acids may be added or organic matter applied. Thus, using acids and bases correctly helps maintain soil pH suitable for crops.

Q45. Why must industrial acidic wastes be treated with bases before being released into water bodies?

Ans. Industrial wastes may contain strong acids which can drastically lower the pH of lakes and rivers and kill fish and other organisms. Treating these wastes with bases neutralises the acids and brings the pH closer to neutral, protecting aquatic life and the environment.

Q46. Give four safety rules you must follow while handling acids and bases in the laboratory.

Ans. (i) Never taste or directly smell any chemical.
(ii) Wear safety goggles and follow teacher’s instructions.
(iii) Handle acid and base bottles carefully and keep them closed.
(iv) If acid or base spills, wash the area with plenty of water and inform the teacher.

Q47. Why should we always add acid to water and not water to acid while diluting an acid?

Ans. When water is added to a concentrated acid suddenly, a lot of heat is produced at once and the acid may splash out, causing burns. If acid is added slowly to a larger amount of water, the heat spreads in water gradually and no splashing occurs, making it safer.

Q48. A student labels two bottles “acid” and “base” but forgets which is which. Suggest a safe way to identify them.

Ans. The student can use litmus paper to test each solution. A few drops from each bottle are placed on red and blue litmus strips. The solution that turns blue litmus red is acid, and the one that turns red litmus blue is base. No tasting or touching is needed.

Q49. How does studying acids, bases and neutral substances help in developing scientific attitude in students?

Ans. This topic encourages students to observe carefully, use indicators instead of guessing, and think logically about causes and effects (like neutralisation or acid rain). They learn to solve daily-life problems using scientific knowledge rather than superstitions, which builds a scientific attitude.

Q50. Summarise in 3–4 lines why Chapter 2: “Exploring Substances: Acidic, Basic, and Neutral” is important for Class 7 students.

Ans. The chapter helps students understand that many substances around us are acidic, basic or neutral and can be identified using indicators. It explains neutralisation and its uses in health, agriculture and pollution control. It also teaches safe handling of chemicals and develops a strong basic foundation for further study of chemistry in higher classes.