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

Long Answer -Class 10 – Science -Chemistry -chapter 2- Acids, Bases and Salt

Q1. Write the chemical name and formula of common salt. List two main sources of common salt in nature. Write any three uses of common salt. How is it connected to our freedom struggle?

Ans. Sodium chloride (NaCl) also called common salt or table salt is the most essential part of our diet. Chemically it is formed by the reaction between solutions of sodium hydroxide and hydrochloric acid. Sea water is the major source of sodium chloride where it is present in the dissolved form alongwith other soluble salts such as chlorides and sulphates of calcium and magnesium. It is separated by some suitable method. Deposits of the salt are found in different parts of the world and are known as rock salt. The formation of rock salt is due to the slow evaporation of sea water which takes ages. When pure, it is a white crystalline solid. However, it is often brown due to the presence of impurities.

Sodium Chloride — Essential for Life

Sodium chloride is quite essential for life. Biologically, it has a number of functions to perform such as in muscle contraction, in conduction of nerve impulses in the nervous system and is also converted into hydrochloric acid in the stomach which helps in the digestion of food. When we sweat, there is loss of sodium chloride and some. Other salts along with water. This leads to muscle cramps. The loss has to be compensated suitably by giving certain salt preparations to the patients. These are called electrolytes. Electral powder is a very popular electrolyte.

Chemicals from Common Salt

Sodium chloride is also a very useful raw material for different chemicals. A few out of these are : hydrochloric acid (HCl), washing soda (Na 2CO 3-10H 2O), baking soda (NaHCO 3) etc. Upon electrolysis, a strong solution of the salt (brine), sodium hydroxide, chlorine and hydrogen are obtained.

Apart from these, it is used in the leather industry for the leather tanning. In severe colds, rock salt is spread on icy roads to melt ice. It is also a fertilizer for sugar beet.

6CaOCl2 (Bleaching powder) → Ca(ClO3)2 (Calcium chlorate) + 5CaCl2 (Calcium chloride)

Q2. (i) What are strong acids and weak acids ? Give an example for each.

(ii) A dry pellet of a common base ‘B’ when kept in the open absorbs moisture and turns sticky. The compound is also formed by the Chloralkali process. Identify B. What type of reaction occurs when B is treated with dilute hydrochloric acid ? Write the chemical equation.

Ans. (i) The strength of an acid is expressed in terms of its H+ ions releasing tendency in aqueous solution or in terms of its degree of dissociation

(α). Strong acids have a large value of α (close to one) while weak acids have comparatively less value. For example, hydrochloric acid (HCl) is a strong acid while acetic acid (CH 3COOH) is a weak acid.

(ii) The available information suggests that the base ‘B’ present in the pellet is sodium hydroxide (NaOH). It is of deliquescent nature. It absorbs moisture from the atmosphere and becomes sticky. The base reacts with dilute hydrochloric acid to form salt and water. This reaction is known as the neutralisation reaction.

NaOH (aq) + HCl(aq) ———–> NaCl (aq) + H 2O(l)

Q3. Metal compound A reacts with dilute Hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is Calcium chloride.

Ans: Understanding the chemical reaction:

Calcium carbonate reacts with dil hydrochloric acid to form calcium chloride and carbon dioxide is released as a by-product.

The gas evolved as carbon dioxide and is responsible for extinguishing the burning candle.

The balanced chemical equation to demonstrate the above-mentioned explanation can be given as:

CaCO₃(S) +2HCL(aq) à CaCl₂(s) +CO₂(g) + H₂ O(l)

Therefore, here the metal compound A is calcium carbonate and the effervescent is carbon dioxide. 

Q4. State reason for the following statements:

(i) Tap water conducts electricity whereas distilled water does not.

(ii) Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does.

(iii) During summer season, a milk man usually adds a very small amount of baking soda to fresh milk.

(iv) For a dilution of acid, acid is added into water and not water into acid.

(v) Ammonia is a base but does not contain hydroxyl group.

Ans.

(i) Tap water contains ions which conduct electricity, distilled water does not contain ions.

(ii)Dry HCl does not form ions but HCl gives H+ and Cl–.

(iii) Baking soda does not allow milk to change to lactic acid which makes milk sour.

(iv) Adding water to acid is highly exothermic. Therefore water is added to acid very slowly with cooling.

(v) Ammonia dissolves in water and forms H– Therefore, it is basic in nature.

Q5. (a) Mention the pH range within which our body works. Explain how antacids

give relief from acidity. Write the name of one such antacid.

(b) Fresh milk has a pH of 6. How does the pH will change as it turns to curd? Explain your answer.

(c) A milkman adds a very small amount of baking soda to fresh milk. Why does this milk take a longer time to set as curd?

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

Ans:

(a) Our stomach has pH equal to 2. Antacids neutralizes excess of acid in our body and gives relief from hyperacidity. Sodium hydrogencarbonate is one of such antacid.

(b) pH will decrease as it turns to curd because curd is acidic due to the presence of lactic acid.

(c) It takes longer time to set as curd as bacteria do not work well in presence of sodium hydrogencarbonate, i.e. fermentation will take place slowly.

(d) Toothpastes are basic in nature. They neutralize the acid formed in mouth which causes tooth decay.