NCERT Solutions for Class 7 Science Chapter 4: Acids, Bases, and Salts

1. Acids ,Bases and Salts This chapter Acids Bases and Salts delves into the fascinating world of acids bases and salts, exploring their properties, interactions, and importance in our world. Acids: ● Taste: Sour or tangy. Imagine the pucker you get from lemon juice! ● Litmus Test: Acids turn blue litmus paper red. This is a simple way to identify an acid in a solution. ● Chemical Properties: Acids can react with metals to release hydrogen gas. They can also dissolve some metals. ● Examples: Common acids include hydrochloric acid (in stomach acid), sulfuric acid (used in car batteries), and citric acid (found in fruits like lemons and oranges). Bases: ● Taste: Bitter. Think of the taste of baking soda or soap. ● Litmus Test: Bases turn red litmus paper blue. ● Chemical Properties: Bases are slippery to the touch and can neutralize acids. They react with oils and grease, making them useful for cleaning. ● Examples: Everyday bases include sodium hydroxide (lye), calcium hydroxide (lime), and ammonia (used in some cleaning products). Salts: ● Taste: Neutral, not sour or bitter (like table salt). ● Litmus Test: Salts don’t affect litmus paper. ● Formation: Salts are formed when an acid and a base react in a process called neutralization. During neutralization, the acid and base cancel out each other’s properties, resulting in a salt and water. ● Examples: Familiar salts include sodium chloride (table salt), calcium carbonate (limestone), and sodium bicarbonate (baking soda).

2. Importance of Acids Bases and Salts: ● Biological Processes: Acids play a role in digestion (stomach acid) and some metabolic processes. Bases are important for maintaining a healthy pH balance in our bodies. ● Chemistry: Understanding acids and bases is crucial for many chemical reactions, from manufacturing processes to cleaning products. ● Everyday Life: Acids and bases are found in many household items like vinegar (acetic acid), baking soda (sodium bicarbonate), and citrus fruits (citric acid). Neutralization: ● This is a very important reaction between acids and bases. When an acid and a base come together in the right proportions, they neutralize each other, forming a salt and water. This reaction is helpful in many ways: ○ Stomach Acid Control: Our bodies naturally produce acids in the stomach to aid digestion. Sometimes, excess stomach acid can lead to discomfort. Antacids, which are basic substances, can neutralize this excess acid and provide relief. ○ Soil Treatment: Soil can become too acidic or too basic for optimal plant growth. Adding lime (a base) to acidic soil or adding sulfur (an acidic compound) to basic soil can help achieve a more balanced pH level for plants. NCERT Solutions for Class 7 Science Chapter 4 : Acids Bases and Salts Exercises 1. State the difference between acids and bases. Ans : Feature Acids Bases

3. Taste Sour Bitter Litmus Test Turns blue litmus paper red Turns red litmus paper blue Chemical Properties Reacts with metals to release hydrogen gas, can dissolve some metals Slippery to the touch, neutralizes acids, reacts with oils and grease (useful for cleaning) Examples Hydrochloric acid (stomach acid), sulfuric acid (car batteries), citric acid (lemons, oranges) Sodium hydroxide (lye), calcium hydroxide (lime), ammonia (cleaning products) 2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature? Ans :Ammonia is a base. 3.Name the source from which litmus solution is obtained. What is the use of this solution? Ans : Litmus solution is obtained from a natural dye extracted from lichens. These are complex organisms formed by a symbiotic relationship between algae and fungi. The primary use of litmus solution is as a pH indicator. It helps identify whether a solution is acidic or basic. 4.Is the distilled water acidic/basic/neutral? How would you verify it? Ans: Distilled water is neutral. Here’s how you can verify it: ● Litmus paper: Dip a piece of litmus paper (either red or blue) into the distilled water. If the litmus paper remains its original color (purple for

4. most universal litmus paper), it indicates that the water is neutral (pH 7). 5.Describe the process of neutralisation with the help of an example. Ans : Neutralization is a fascinating chemical reaction between an acid and a base. When they come together in the right proportions, they essentially “cancel out” each other’s properties, forming a new substance called a salt and water. Example: Heartburn? Antacids (bases) fight the stomach’s extra acid, creating water and a harmless salt, leaving you feeling better! 6.Mark ‘T’ if the statement is true and ‘F’ if it is false: (i) Nitric acid turns red litmus blue. (ii) Sodium hydroxide turns blue litmus red (iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (id) Indicator is a substance which shows different colours in acidic and basic solutions. . (v) Tooth decay is caused by the presence of a base. Ans : (i) False (ii) True (iii) True (iv) True

5. (v) False 7. Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom? Ans : Dorji’s in a pickle! No labels on his drinks! Customers want acidic, basic, or neutral drinks, but taste and looks aren’t reliable. 1.Be honest: Tell customers there are no labels and offer them to choose by taste or skip the special requests. 2.Hold off on guessing: It’s risky! Some drinks might be surprisingly acidic. 3.Get a pH tester (later): Invest in a tester that shows if a drink is acidic, basic, or neutral (perfect for the future!). 8.Explain why: (a) An antacid tablet is taken when you suffer from acidity. (b) Calamine solution is applied on the skin when an ant bites. (c) Factory waste is neutralised before disposing it into the water bodies. Ans : (a) Antacid tablet for acidity: ● Acidity: This happens when there’s excess acid in your stomach. ● Antacid tablet: These are basic substances.

6. ● Neutralization: When you take an antacid, the base in the tablet reacts with the excess acid in your stomach. This neutralization reduces the acidity, providing relief. (b) Calamine solution for ant bites: ● Ant bite: Ant stings contain formic acid, which irritates the skin. ● Calamine solution: This contains zinc carbonate, which is basic. ● Neutralization: When applied to the bite, the calamine solution neutralizes the formic acid, reducing the irritation and itchiness. (c) Neutralizing factory waste: ● Factory waste: This can contain harmful acids or bases. ● Water bodies: If dumped directly, these acids or bases can harm aquatic life. ● Neutralization: By treating the waste with appropriate chemicals before disposal, factories can neutralize the harmful components. This makes the waste less harmful to the environment. 9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator. Ans. 1.Prepare the turmeric indicator: Make a turmeric paste by mixing turmeric powder with a little water. 2.Test each liquid separately: Dip a clean stirrer or spoon into each liquid one at a time. Then, add a drop of the turmeric indicator solution to the liquid on the stirrer. Observe the color change. ● Result: ○ Hydrochloric acid (HCl): No color change. Turmeric remains yellow in acidic solutions. ○ Sodium hydroxide (NaOH): Turns red. Turmeric turns red in basic solutions. ○ Sugar solution: Remains yellow. Sugar solution is neutral and won’t affect the turmeric indicator’s color.

7. 10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain. Ans.When you dip blue litmus paper into a solution and it remains blue, there are two possibilities for the solution’s nature: 1.Neutral: The solution could be neutral, meaning it has a pH of 7. In this case, the solution doesn’t have enough acidic or basic properties to cause a color change in the litmus paper. 2.Basic (weak possibility): There’s a slight chance the solution could be a very weak base. Very weak bases might not cause a strong enough color change to turn the blue litmus paper red. 11. Consider the following statements: (a) Both acids and bases change colour of all indicators. (b) If an indicator gives a colour change with an acid, it does not give a change with a base. (c) If an indicator changes colour with a base, it does not change colour with an acid. (d) Change of colour in an acid and a base depends on the type of the indicator. Which of these statements are correct? (i) All four (ii) (a) and (d) (iii) (b) and (c) (iv) only (d) Ans : The correct answer is (iv) only (d).