Class 7 Science Chapter 2 Question Answers - Acids, Bases And Salts

Short Types Questions

Q.1. What are acidic substances?

Acidic substances are substances that have a pH value less than 7. They can donate hydrogen ions (H+) in aqueous solutions.

Q.2. Name two common uses of acids.

Acids have various uses in daily life. Two common uses of acids are:

(i) Dilute hydrochloric acid is used to clean household surfaces.

(ii) Acetic acid is used to make vinegar, which is used in cooking.

Q.3. Define basic substances.

Basic substances are substances that have a pH value greater than 7. They can accept hydrogen ions (H+) in aqueous solutions.

Q.4. What are indicators? Give an example.

Indicators are substances that change their colour in acidic or basic solutions. Litmus paper is a common example of an indicator. It turns red in acidic solutions and blue in basic solutions.

Q.5. What are salts? How are they formed?

Salts are ionic compounds that are formed when an acid reacts with a base. During the reaction, the hydrogen ions (H+) from the acid combine with the hydroxide ions (OH-) from the base to form water (H₂O). The remaining positive and negative ions combine to form a salt.

Long Types Questions

Q.1. Explain the concept of acidic substances. How is the strength of an acid determined and what are some common uses of acids in daily life?

Acids are substances that release hydrogen ions (H+) when dissolved in water. The strength of an acid is determined by its pH value, with lower pH indicating stronger acidity. Common uses of acids include cleaning agents, food preservation, and production of fertilizers.

Q.2. What are basic substances? Describe the differences between acids and bases. How do bases neutralize acids?

Bases are substances that release hydroxide ions (OH-) when dissolved in water. Acids and bases differ in their chemical properties, as acids have a pH less than 7 while bases have a pH greater than 7. When a base is added to an acid, it neutralizes the acid by accepting hydrogen ions and forming water.

Q.3. Explain the concept of indicators. What are some common indicators and how do they work? How are indicators used in testing the pH of a substance?

Indicators are substances that change color based on the pH of a solution. Common indicators include litmus paper, phenolphthalein, and methyl orange. These indicators work by reacting with the hydrogen ions or hydroxide ions present in the solution, resulting in a color change. Indicators are used to test the pH of a substance by adding a small amount of the substance to be tested to the indicator, and observing the resulting color change.

Q.4. What are salts? How are salts formed? Describe the properties of salts and give examples of common salts.

Salts are ionic compounds formed by the reaction of an acid and a base. When an acid and a base react, they neutralize each other and form a salt and water. Salts are characterized by their ionic structure, high melting and boiling points, and ability to conduct electricity when dissolved in water. Examples of common salts include sodium chloride (table salt), calcium carbonate (limestone), and potassium nitrate (saltpeter).

Q.5. Describe the preparation and uses of baking soda and washing soda. How do they differ from each other in their chemical properties and uses?

Baking soda (sodium bicarbonate) is prepared by reacting sodium chloride, ammonia, and carbon dioxide. It is commonly used in cooking and baking as a leavening agent, as well as a cleaning agent and deodorizer. Washing soda (sodium carbonate) is prepared by heating baking soda, and is used in laundry detergents, as a cleaning agent, and in glass and paper manufacturing. The two substances differ in their chemical properties, with washing soda being more alkaline than baking soda, and having a higher pH and melting point.