Acids, Bases and Salts Question Answers: NCERT Class 7 Science

Blue litmus paper is dipped in a solution, it remains blue; this means that the solution is either basic or neutral. Because basic and neutral solutions does not change the colour of the blue litmus. We can check it by putting a drop of the solution on red litmus paper. If the red litmus paper changes into blue then it shows the solution is basic and if it does not change the colour, then it is neutral by nature.  

(iv) only d

Change of the colour in an acid or the base depends on the type of the indicator. It is so because we know that a basic solution do not change the colour of the blue litmus paper whereas the acid does. The acidic solution do not change the colour of the red litmus paper whereas the basic solution does.

Ammonia, also known as NH₃, is a colourless gas with a distinct odour composed of nitrogen and hydrogen atoms. Ammonical smell comes from human excreta. It changes turmeric indicator into the red colour. 

Ammonia in Household Cleaning Products: Ammonium hydroxide (NH₄OH) - commonly known as household ammonia, is an ingredient in many household cleaning products used to clean a variety of surfaces including tubs, sinks, toilets, counter tops and tiles. 

Litmus solution is obtained from the lichens. It has purple colour in distilled water. It is found as two types: Red litmus and Blue litmus paper. When we add it in the acidic solution, it turns blue litmus to red. And, when we add it in the basic solution, it turns into the blue. Litmus is available in the form of strips of paper, known as litmus paper.   

Distilled water is neutral by nature. We can verify this by litmus paper. Distilled water will not show any change in colour when we add it to the litmus solution. It shows that distilled water is neither acidic or nor basic. 

NEUTRALISATION: When an acid is mixed with a base, they neutralise the effect of each other. The products formed on mixing an acid and a base are salt and water. The reaction between acid and base which produces salt and water with the evolution or release of heat is known as neutralization reaction.

Acid + Base → Salt + Water + Heat (released)

In a neutralisation reaction, salt are formed which is neutral by nature. The salt formed during a neutralisation reaction depends on which acid and which base are reacted with each other. For example: Sodium hydroxide is a base and hydrochloric acid is an acid. So, when sodium hydroxide is treated with hydrochloric acid, then a neutralisation reaction takes place to form sodium chloride (salt) and water.

Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water
         (NaOH)                  (HCl)                           (NaCl)        (H₂O)

The salt formed in this neutralisation reaction of sodium hydroxide and hydrochloric acid is sodium chloride (NaCl). It is known as common salt.

(i) Answer: False
Nitric acid is not right to turn the red litmus paper into blue. The explanation is that the colour of the paper does not change when an acid comes into contact with a red litmus paper. Only if an acid comes into contact with blue litmus paper will the colour of the litmus paper change into red. Nitric acid is an acid so it does not turns the litmus paper into blue.

(ii) Answer: False
This statement is false because sodium hydroxide is not an acid, it is base which turns red litmus to blue.

(iii) Answer: True
When acid and base reacts with each other, they form an neutral substance, this process is called neutralisation. When the solution of sodium hydroxide (a base) and hydrochloric acid (an acid) is mixed in test tube in proper ratio, both neutralize each other.

(iv) Answer: True
This statement is true because different solution show different colours in acidic or basic medium. Indicators are chemical compounds that change Colour according to pH of the solution they are put in. They are used to determine pH of the body (acidity/basicity).

(v) Answer: False
This statement is false because tooth decay is caused by acids which is formed in our mouth when the food particles left over all night. They constantly damage tooth enamel and thus cause tooth decay.

Drinks are eatable material so Dorji can check the medium of the drinks by tasting them, acid drink will be of souring taste, basic will be bitter in taste and neutral will not contain any taste. So, just by tasting and drinking them, we can serve the drinks on the demand of the customers. 

Note*: He can check the drinks medium of the drinks by using the litmus paper by putting one drop of each drinks on it. He can conclude which is acidic, basic and neutral.  

(a) An antacid tablet is taken during acidity because that time excess amount of acid is produced (HCL) in the stomach. Antacid is an bases that react with excess of acids and neutralize the effect of excessive acid and brings relief. Antacids help to treat heartburn (indigestion). They work by neutralizing the stomach acid that cause heartburn. We can buy many antacids without a prescription.

(b) Calamine solution is applied on the skin when an ant bites because, it injects formic acid into the skin. calamine solution is basic in nature. So, when we applied it on the skin, it neutralize the effect of formic acid. 

(c) Factory waste is neutralized before disposing it into the water bodies because it contains acids. The acidic nature of waste can harm the aquatic animals, so we add some basic chemicals to the waste to neutralize the effects of acid.

Turmeric indicator is of yellow colour, we can identify them by putting one drop of each solution to the turmeric indicator. If the liquid drop changes into red colour, then it must be a basic solution that is sodium hydroxide. If the drop will not change the colour of the turmeric indicator, then it must be an acid, that is hydrochloric acid. 

Sugar solution changes the colour of turmeric indicator to red because the mixture of basic and neutral solution is basic in nature.

Turmeric indicator + Sodium hydroxide → Colour change (yellow to red)  

Turmeric indicator + Hydrochloric acid (HCL) → No colour change

Turmeric indicator + Sugar solution → Colour change (yellow to red)