Acids, Bases and Salts - Class 7 Science
Acids are sour in taste, bases are bitter in taste and soapy to touch.
Acids turns blue litmus red and bases turn blue litmus red.
Sustances which are neither acidic nor basic are called neutral.
Solutions of sustances that show different color in acidic, basic and neutral solutions are called indicators.
An acid and a base neutralize each other and form a salt. A salt may be acidic, basic or neutral in nature.
Exercises
Q1. State differences between acids and bases
Answer - The differences between acids and bases are:
| Acids | Bases |
|---|---|
| Acids are sour in taste | Bases are bitter in taste |
| Acids turn blue litmus red | Bases turn red litmus blue |
| Acids do not give soapy touch | Bases are soapy to touch |
Q2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Answer. As ammonia turns red litmus blue so it is basic in nature.
Q3. Name the source from which litmus solution is obtained. What is the use of this solution?
Answer. Litmus solution is extracted from lichens. It is used to determine the solution is neutral/acidic/basic. It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue.
Q4. Is the distilled water acidic/basic/neutral? How would you verify it?
Answer. We take two test tubes and add distilled water in those tubes. Once done we add blue litmus paper in one test tube and in the other we add red litmus paper, we observe that there is no change in color, so we can confirm that distilled water is neutral in nature.
Q5. Describe the process of neutralisation with the help of an example.
Answer. Neutralisation is a process where an acid and a base react to give salt and water. Let us take Hydrochloric acid(HCl) and a base Sodium hydroxide(NaOH) and react them, then we get HCl + NaOH - NaCl(Sodium Chloride)+H 2O(Water)
Q6. Mark ‘T’ if the statement is true and ‘F’ if it is false: (i) Nitric acid turn red litmus blue. (T/F) - F (ii) Sodium hydroxide turns blue litmus red. (T/F) - F (iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F) - T (iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F) - T (v) Tooth decay is caused by the presence of a base. (T/F) - F
Q7. 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?
Answer. Dorji can decide in two ways, first he can taste a little part of the drinks for the drink which is sour then it will be acidic, for the drink which is bitter then it will be basic and the neutral drink will have no taste. Secondly if he has a litmus paper with him then he can put the litmus paper in a drink if the blue litmus paper turns red then it is acidic, if the red litmus paper turns blue then it is basic and if the drink shows no change in color with litmus paper then it is neutral.
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.
Answer (a). An antacid contains bases which neutralizes the acidity and thus we feel relief from acidity. (b). When an ant bites it injects formic acid in skin and calamine solution is basic in nature so it neutralizes the effect. (c). Some factory waste contains acids and if they are disposed directly into water bodies then aquatic life will be harmed to prevent any harm from occuring to the aquatic life; factory waste is neutralized with a base.
