SHORT ANSWER -Class 10 – Science -Chemistry -chapter 2- Acids, Bases and Salt

SHORT ANSWER -Class 10 – Science -Chemistry -chapter 2- Acids, Bases and Salt

Q1. a) A milkman adds a very small amount of baking soda to fresh milk. Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

b) Mention pH range within which our bodyworks?

c) Explain how antacids give relief from acidity.

d) Mention the nature of toothpastes. How do they prevent tooth decay?

Ans: a) It is done to prevent the formation of lactic acid which spoils the milk

b) pH range 7.0 – 7.8

c) Antacids neutralizes excess acid in our body and gives relief.

d) Basic. Neutralize the acid formed in the mouth

Q2. a) Crystals of a substance changed their colour on heating in a closed test tube but regained it after some time when they were allowed to cool down. Name the substance and write its formula. Explain the phenomenon.

b) How is sodium carbonate prepared? Give two uses of the compound

Ans: a) Copper sulphate, CuSO4.5H2O. It is blue. It becomes white on heating due to loss of water molecules.

CuSO4.5H2O (heat)-> CuSO4+5 H2O

It regains its colour by absorbing water from atmosphere CuSO4+ 5 H2O -> CuSO4.5H2O

b) Prepared by passing CO2 through ammoniacal brine Used for production of washing powder & manufacture of glass

Q3. Ques. Explain why :

(i) Common salt becomes sticky during the rainy season

(ii) Blue vitriol changes to white upon heating

(iii) If a bottle full of concentrated sulphuric acid is left open in the atmosphere by accident, the acid starts flowing out of the bottle of its own.

Ans. (i) Common salt contains the impurity of magnesium chloride (MgCl2) which is of deliquescent nature. When exposed to the atmosphere, it becomes moist. Therefore, common salt becomes sticky during the rainy season.

(ii) Blue vitriol (CUSO 4 . 5H2O) upon heating changes to anhydrous copper sulphate (CUSO4) which is white in colour.

(iii) Concentrated sulphuric acid is highly hygroscopic. It absorbs moisture from air and gets diluted. Since the volume increases, the acid starts flowing out of the bottle.

Q4. Why does table salt become sticky during the rainy season?

Ans

Table salt becomes sticky on exposure during the rainy season because table salt generally contains a small percentage of magnesium chloride, as an impurity. Since, these impurities absorb moisture from the air due to their deliquescent nature; therefore it gets wet in the rainy season and becomes sticky.

5. On diluting an acid, it is advised to add acid to water and not water to acid. Explain why it is so advised?

Ans

Exothermic reaction: It is a reaction in which energy is emitted in the form of light or heat. Energy is released to the environment as the reaction proceeds.

The reaction of acid and water is an exothermic reaction since a lot of heat is liberated during the reaction. This heat changes some water to steam explosively which can splash the acid on our face or clothes and cause acid burns. Therefore,

While diluting an acid, it is preferred that the acid is added to water rather than the water being added to the acid.

Adding water to a concentrated acid releases a large amount of heat, which can cause an explosion and acid burns on the skin, clothing, and other body parts.

Hence, adding acid to water is safe, but adding water to acid is not.

Q6. (i) Give the constituents of baking powder

(ii) Why cake or bread swells on adding baking powder? Write chemical equation.

Ans

(i) Baking powder contain sodium hydrogen carbonate and tartaric acid.

(ii)It is due to carbon dioxide

2NaHCO3 (s) —> Na2CO3 (s) + CO2 (g)+ H2 O(l)

Q7. a) Write the name given to bases that are highly soluble in water. Give an example.

(b) How is tooth decay related to pH? How can it be prevented?

(c) Why does bee sting cause pain and irritation? Rubbing of baking soda on the sting area gives relief. How?

Ans.(a) Alkali, e.g. NaOH (Sodium hydroxide).

(b) Lower the pH, more will be tooth decay. Acid reacts with Ca3(PO4)2 and cause tooth decay.

It can be prevented by brushing teeth after every meal.

(c) It is due to formic acid. Sodium hydrogen carbonate (Baking soda) neutralises formic acid giving relief.

Q8. A white powder is added while baking breads and cakes to make them soft and fluffy. Write the name of the powder. Name its main ingredients. Explain the function of each ingredient. Write the chemical reaction taking place when the powder is heated during baking.

Ans. Baking powder.

It consist of sodium hyrogencarbonate and tartaric acid.

Sodium hydrogencarbonate gives CO2 which makes cake soft and fluffy. Tartaric acid neutralizes the bitterness due to sodium carbonate produced.

2NaHCO3 (s) —-> Na2 CO3 (s) + CO2 (g) + H2 O(l)

Q9. “Sodium hydrogencarbonate is a basic salt”. Justify the statement. How is it converted into washing soda? Explain.

Ans.Sodium hydrogencarbonate is a salt of sodium hydroxide (strong base) and carbonic acid (weak acid).

It is basic salt. It is converted into washing soda by heating followed by crystallization.

2NaHCO3 (aq) —-> Na2 CO3 –>  CO2 + H2 O

Q10. What are strong and weak acids? In the following list of acids, separate strong acids from weak acids. Hydrochloric acid, citric acid, acetic acid, nitric acid, formic acid, sulphuric acid.

Ans : Strong acids are those acids which are completely ionised in aqueous solution. Weak acids are those which do not ionise completely in aqueous solution. Strong acid: HCl, HNO3,H2SO4 Weak acid: Citric acid, acetic acid, formic acid.

 Q11. What happens when zinc sulphate solution is added to barium chloride solution? Identify the precipitate formed and type of reaction.

Ans: White precipitate of barium sulphate forms. It is a double displacement reaction.

When zinc sulfate solution (ZnSO4) is added to barium chloride solution (BaCl2), a precipitation reaction occurs. The balanced chemical equation for the reaction is:

ZnSO4 + BaCl2→ZnCl2 + BaSO4

In this reaction, zinc sulfate reacts with barium chloride to produce zinc chloride (ZnCl2) and barium sulfate (BaSO4 ). The barium sulfate formed is insoluble in water and appears as a white precipitate. So, the precipitate formed is BaSO4, and the type of reaction is a double displacement reaction with the formation of a precipitate.

Q12.  (a) Identify the compound of calcium which is yellowish white powder and is used for disinfecting drinking water. Write its chemical name and formula. How is it manufactured? Write the chemical equation for the reaction involved. Also list two other uses of the compound.

(b) Write the balanced chemical equation qf chlor-alkali process.

Ans. (a) The compound is bleaching powder (CaOCl2). Its chemical name is calcium oxychloride. It is manufactured by reaction of solid slaked lime with dry chlorine gas.

Ca(OH)2 + Cl2 –> CaOCl2 + H2 O

(i) It is used as disinfectant

(ii) It is used for preparation of chloroform.

(b)  2NaCl +2H2O –> 2NaOH +H2 + Cl2

Q13. Give two important uses of washing soda and baking soda.

Ans. Uses of washing soda:

(i)  As cleansing agent.

(ii) Removing permanent hardness of water.

(iii) Used in glass, soap and paper industries. Uses of baking soda:

 (a) For making baking powder.

(b)As ingredient of antacid.

Q14. Why do HCl, HNO3 etc. show acidic characters in aqueous solution while solutions of compounds like alcohol and glucose do not show acidic character?

Ans. Compounds like HCl and HNO3 release hydrogen ions in solution, therefore they show acidic character. While compounds like alcohol and glucose do not release hydrogen ions. Therefore, they do not show acidic properties.