Chapter 5- Acids, Bases and Salts-Class 7 Question and Answer (Long)

Chapter 5- Acids, Bases and Salts – Long Questions and Answers(Solved Worksheet)

Question- Can you list down some natural sources of acids and the specific acids they contain? 

• Answer- Yes, natural sources of acids and the specific acids they contain include-
  • Citrus fruits like oranges, lemons, grapefruits, and limes contain citric acid.
  • Vinegar contains acetic acid.
  • Ant stings contain formic acid as a defence mechanism.
  • Milk fermentation results in the formation of lactic acid, which is found in curd/yoghurt.

Question- How are some natural bases formed or where are they found? 

• Answer- Some natural bases and how they are formed or where they are found include-
  • Calcium hydroxide is naturally formed when limestone is heated.
  • Sodium bicarbonate is found in mineral springs and is also produced in certain biological processes.
  • Potassium hydroxide is found naturally in the ashes of burnt wood.

Also Check – Rapid Revision – Class 7 Science – Chapter 5-Acids, Bases and Salts

Question- Can you explain how indicators work in identifying acids and bases?

• Answer- Indicators are special substances used to identify whether something is acidic or basic. They react with acids and bases to produce a colour change. For example, in the presence of an acid or a base, substances like litmus or turmeric will change colour, helping in identifying the nature of the substance under observation.

Question- Why is taste not a reliable method for identifying all substances as acidic or basic?

•  Answer- Taste is not a reliable method for identifying all substances as acidic or basic due to safety and practical reasons. Tasting can be dangerous as some substances might be harmful, toxic, or corrosive. Hence, using indicators is considered a safe and effective way to test the nature of various substances.

Also Check – NCERT Exemplar Solutions- Class 7 Science Chapter – 5- Acids, Bases and Salts

Question- How does citric acid affect the taste of the fruits in which it is found?

• Answer- Citric acid is naturally present in citrus fruits like oranges and lemons, giving these fruits their characteristic tart taste. It is responsible for the sourness and zest in these fruits.

Question- Can you explain the role of ascorbic acid in the body and where it is commonly found? 

• Answer- Ascorbic acid, also known as Vitamin C, plays a crucial role in various body functions, including the maintenance of skin, blood vessels, bones, cartilage, and wound healing. It is commonly found in fruits such as amla and citrus fruits.

Question- How is calcium hydroxide used in water treatment and food preparation?

• Answer- Calcium hydroxide, present in lime water, is used in water treatment by helping to remove impurities and hardness from the water. In food preparation, it is used in processes like the nixtamalization of corn, where it helps to improve the nutritional quality and flavour of the corn.

Question- What roles do sodium hydroxide and potassium hydroxide play in the formulation of soaps and detergents? 

• Answer- Sodium hydroxide and potassium hydroxide are strong bases used in the manufacture of soaps and detergents. They play a crucial role in saponification, the process that produces soap, by reacting with fats and oils to create soap and glycerol, helping remove dirt and oils effectively.

Question- Can you describe the procedure for testing the acidic, basic, or neutral nature of various substances using litmus paper? 

• Answer-
  • Firstly, mix water with various substances like lemon juice, detergent, etc., in separate containers.
  • Secondly, use a dropper to put a drop of each mixed solution on red and blue litmus papers.
  • Finally, observe and record any colour changes to identify whether the substance is acidic, basic, or neutral based on the colour changes observed with the litmus papers.

Question- What observations and conclusions can be made regarding the nature of substances based on their interaction with litmus paper?

• Answer-
  • Acidic Substances- These turn blue litmus paper red, indicating their acidic nature.
  • Basic Substances- They turn red litmus paper blue, signifying their basic nature.
  • Neutral Substances- Neutral substances do not affect the colour of either red or blue litmus paper, indicating they are neither acidic nor basic.

Question- Can you explain the procedure to prepare and use the turmeric indicator?

• Answer-
  • To prepare the turmeric indicator, make a paste using turmeric powder and water. Apply this paste on blotting paper or filter paper and allow it to dry. Cut it into thin strips for use.
  • For testing, place a drop of different test solutions on the strips of turmeric paper and observe any changes, aiming to identify whether the substances are acidic, basic, or neutral.

Question- How can you distinguish between acidic, basic, and neutral substances using turmeric paper? 

• Answer-
  • Acidic substances like lemon juice, orange juice, and vinegar don’t show significant colour change on turmeric paper.
  • Basic substances like baking soda and lime water turn the turmeric paper red or brownish-red.
  • Neutral substances like sugar and common salt do not affect the colour of the turmeric paper.

Question- What is the objective of testing various solutions with the China Rose indicator and how is the testing carried out? 

• Answer- The objective of testing various solutions with the China Rose indicator is to observe the effect of the indicator on various solutions and determine whether they are acidic, basic, or neutral. The testing is carried out by adding five drops of the indicator to each test solution and observing the change in colour.

Question- Can you elaborate on the expected colour changes when the China Rose indicator is added to different types of solutions, like acidic, basic, and neutral?

• Answer- When China Rose indicator is added to acidic solutions, it turns dark pink or magenta. For basic solutions, the indicator turns green. Neutral solutions may not show significant colour changes when the indicator is added, likely remaining colourless or showing no significant change.

Question- Explain the effect of different indicators (Litmus Paper, Turmeric Paper, China Rose Solution) on acidic and basic solutions based on the observations.

• Answer-
  • Litmus Paper-
    • In the presence of acids such as hydrochloric acid, sulphuric acid, nitric acid, and acetic acid, it turns red, indicating an acidic nature.
    • For bases like sodium hydroxide, ammonium hydroxide, and calcium hydroxide, it turns blue, identifying the presence of a base.
  • Turmeric Paper-
    • Shows no significant change with acids but turns red in the presence of basic solutions, signifying the basic nature.
  • China Rose Solution-
    • Turns dark pink or magenta when exposed to acids, and green in the presence of bases, helping in distinguishing between acids and bases.

Question- Discuss the importance of indicators in distinguishing between acidic and basic solutions based on the observed results in the experiment.

•  Answer-
  • Indicators like litmus paper, turmeric paper, and China Rose solution are crucial in distinguishing between acidic and basic solutions.
  • Litmus paper offers a quick visual cue, turning red for acids and blue for bases.
  • Turmeric paper is specific to bases, turning red in their presence, and helps in excluding acidic or neutral nature.
  • The China Rose solution provides vivid colour changes—dark pink/magenta for acids and green for bases, making it easier to identify the nature of the solutions visually.

Question- Explain the observed changes when phenolphthalein is used as an indicator in the neutralisation reaction between hydrochloric acid and sodium hydroxide. 

• Answer-
  • In the experiment, dilute hydrochloric acid is first taken in a test tube, and phenolphthalein indicator is added, which remains colourless indicating the acidic nature of the solution.
  • Upon the gradual addition of sodium hydroxide (a base) to the mixture, a colour change is observed; the solution turns pink, reflecting the basic environment inside the test tube.
  • If more hydrochloric acid is added to the now pink solution, the colour reverts to colourless, showing the solution’s return to an acidic state.
  • This cycle of colour changes visually demonstrates the neutralisation process, where the acid and base counteract each other’s effects.

Question- Discuss the importance of using phenolphthalein as an indicator in understanding the concept of neutralisation. 

• Answer-
  • Phenolphthalein serves as a vital tool in visually interpreting and understanding the neutralisation process.
  • Its ability to change colours in acidic and basic environments helps in distinguishing between the two easily, providing a visual cue of the ongoing reaction.
  • Through the colour transitions (colourless in acidic, pink in basic), one can visually track the neutralisation process’s progress, offering a clear and comprehensible demonstration of the abstract concept of neutralisation in chemistry.

Question- Explain the general observations one can make during the neutralisation process.

•  Answer-
  • During neutralisation, when an acid and a base are mixed, they neutralise each other’s effects, resulting in a mixture that is neither acidic nor basic.
  • Heat is evolved during this chemical reaction, making the container warm to touch.
  • The end products of this reaction are always salt and water, where the nature of the salt can vary, being acidic, basic, or neutral.

Question- Discuss the experiment involving sulphuric acid and lime water in terms of neutralisation. What are the observations and conclusions that can be drawn?

• Answer-
  • In the experiment involving sulphuric acid and lime water, a neutralisation reaction occurs. Dilute sulphuric acid is added to lime water, and as with all neutralisation reactions, heat is evolved, making the reaction mixture hot.
  • This demonstrates the general characteristic of neutralisation reactions where acid and base react to neutralise each other’s effects, resulting in the formation of salt and water, and the release of heat.
  • Thus, it can be concluded that the experiment aligns with the fundamental principles of neutralisation reactions.

Question- Can you explain the role of neutralisation in managing factory wastes?

• Answer-
  • Factories often release wastes containing acidic substances that can be harmful to aquatic life if directly released into water bodies.
  • To manage this, basic substances are added to the acidic wastes to neutralise them before they are released into the environment.
  • Through this neutralisation process, the harmful effects of acidic wastes on aquatic organisms are mitigated, thus protecting aquatic life.

Question- How does neutralisation play a role in soil treatment for agriculture?

• Answer-
  • In agriculture, continuous use of chemical fertilisers can make the soil either too acidic or too basic, which is harmful for plant growth.
  • To treat this, if the soil is acidic, basic substances like quicklime or slaked lime are added to neutralise the acidity.
  • If the soil is too basic, slightly acidic organic matter is added to reduce the soil’s basic nature.
  • Neutralising the excessive acidity or basicity in the soil creates a conducive environment for plant growth.

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