Class 10 NCERT Acids Bases and Salts




Question 5.: Why is an aqueous solution of ammonium chloride acidic in nature?

 Answer: A basic component is the one that produces hydroxide (OH ) ions when dissolved in water. In an aqueous solution of ammonium chloride, ammonium ions (NH4+ ) first associate with H2O and form ammonia and hydroxide ions.

NH4Cl + H2O = NH4 + + HCl ( equation 1) NH4 + + H2O = NH3 + OH (equation 3) Since ammonium ions produce hydroxide ions, NH4+ are considered to be the basic components.

Question 6.: Dry ammonia has no action on litmus paper but a solution of ammonia in water turns red litmus paper blue. Why is it so?

Answer: In the dry state, Ammonia contains no hydroxyl ions. On dissolving in water, it forms NH4OH which dissociates to give NH4 + and OH ions. Thus the solution becomes basic and turns red litmus paper blue. NH3 (g) + H2O (l) —————> NH4OH (aq)

 NH4OH (aq) ————–> NH4+ (aq) + OH (aq)

Question 7.: What happens when (a) Solid sodium hydrogen carbonate is heated, (b) Chlorine gas is passed through dry slaked lime, (c) Gypsum is heated to a temperature of 373 K?

Answer: (a) When Solid sodium hydrogen carbonate is heated from sodium chloride Na2CO3.10H2O (washing soda). 2NaHCO3 ——–Heat———-> Na2CO3 +H2O+CO2 (b) when Chlorine gas is passed through dry slaked lime, Bleaching powder is produced. Bleaching powder is represented as CaOCl2, though the actual composition is quite complex. Ca(OH)2 + Cl2 → CaOCl2 + H2O (c) if Gypsum is heated to a temperature of 373 K. It forms plaster of Paris, CaSO4 ½ H2O CaSO4 2H2O — -373 K ——> CaSO4 ½ H2O + 3/2 H2O

Question 8. : Give the Names of Raw Materials that are required for the manufacture of washing soda by Solvay process. Write the reactions involved in the process.

Answer: Common salt : NaCl.Sodium chloride. Ammonia : NH3 .Ammonia Lime Stone : CaCO3.Lime stone (Calcium carbonate) H2O + NH3 + NaCl + CO2 ———–> NaHCO3 + NH4Cl

 2NaHCO3 ———- Heat——> Na2CO3+ CO2+H2O

CaCO3 ———– Heat > CO2+CaO

CaO + H2O ———> Ca (OH)2

Ca(OH)2+ 2NH4Cl ————> CaCl2 +2NH3+2H2O

Question 9.: What is efflorescence? Name one compound which shows efflorescence? support your answer with the reaction?

Answer: The processes of losing water of crystallisation, when the substance is exposed to air for a long time to form anhydrous or monohydric substance is called efflorescence. Na2CO3 10 H2O, Sodium carbonate decahydrate is one example.

Na2CO3 , 10 H2O ————–>Na2CO3 H2O + 9H2O

Question 10: why is calcium sulphate hemihydrate called ‘Plaster of Paris’?

Answer: Plaster of Paris gets its name from large gypsum deposits in Montmartre in Paris. We know that plaster of Paris, which is chemically calcium sulphate hemihydrate is obtained by heating gypsum. So the origin of the name ‘Plaster of Paris’ is the fact that the source material gypsum is found in large deposits at Montmartre in Paris

Question 11: Plaster of Paris should be stored in a moisture-proof container. Explain why?

Answer: Plaster of Paris should be stored in the moisture-proof container because Plaster of Paris is a powdered form of gypsum with less water of crystallisation. But when we keep in open it will react with water to for hard solid mass Gypsum. CaSO4. 1/2H2O + 3/ H2O ——- > CaSO4. 2H2O

 Question 12: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH3COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why?

Answer: -In the test tube, A fizzing occurs more vigorously. Because HCl is a stronger acid than acetic acid. Hence, HCl liberates hydrogen gas more vigorously, which causes fizzing more vigorously

Question 13: What is aqua-regia?

Answer: A mixture of concentrated nitric and hydrochloric acids. It is a highly corrosive liquid able to attack gold and other resistant substances.

Question 14: Why should curd and sour substances not be kept in brass and copper vessels?

Answer: Curd and sour substances contain acids. Acids react with metals to give salt and hydrogen gas. So, if such substances are kept in a copper container, the acid will react and the container will be corroded.

 Question 15: What is efflorescence?

Answer: Efflorescence is the loss of water (usually as evaporation to air) in an aqueous solution of salts, leaving behind crusts of solid salt crystals, and called efflorescent salts

Question 16: A farmer has found that the pH of the soil in his fields is 4.2. Name any two chemical materials which he can mix with soil to adjust its pH.

Answer: Quicklime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)

Question 17: Give the names and formulae of two (i) strong monobasic acids (ii) weak dibasic acids.

 Answer:  (i) Hydrochloric acid(HCl),nitric acid(HNO3) (ii) Carbonic acid (H2CO3),oxalic acid(C2H2O4)

Question 18: Why alkalies like sodium hydroxide and potassium hydroxide should not be left exposed to air?

Answer:  Because they are hygroscopic in nature and absorb moisture from the atmosphere in which they ultimately dissolve.

Question 19: State whether an aqueous solution of washing soda is acidic or alkaline.

Answer: An aqueous solution of washing soda is alkaline (turns red litmus blue).On dissolving in water it forms a strong base (NaOH) and weak acid(H2CO3) Na2CO3 + H2O → 2NaOH + H2CO3

Question 20: An old man complained of acute pain in the stomach. The doctor gave him a small antacid tablet and he got immediate relief. What actually happened?

Answer: The old person was suffering from acute acidity. An antacid tablet contains sodium hydrogen carbonate (NaHCO3). It reacts with acid (HCl) formed because of acidity and neutralise its effect.

Question 21. (a) Give Arrhenius definition of an acid and a base    (CBSE 2006)

(b) Choose a strong acid and a strong base from the following


Answer: (a): Acid: are characterised by acids, which dissociate in aqueous solution to form hydrogen ions (H+) and Acids are defined as a compound or element that releases hydrogen (H+) ions into the solution.

Bases: Which form hydroxide (OH) ions when dissolved in water.

(b) The strong acid is HCl and the Strong base is KOH.

Question 22. What happens chemically when quicklime is added?  (CBSE 2008)

Answer: Calcium hydroxide is formed when quicklime reacts with water and a large amount of heat is also released with a hissing sound as it is an exothermic reaction.

CaO  +  H2O ——–> Ca(OH)2 + Heat.

Question 23 What is baking powder? How does it make the cake soft and spongy? (CBSE 2008)

Answer: Baking powder is a mixture of sodium hydrogen carbonate and a mild edible(eatable) acid such as tartaric acid.

When baking powder is heated it releases carbon dioxide gas due to which the cake rises and become soft and spongy.

Question 24. Name the gas evolved when dilute HCl reacts with sodium hydrogen carbonate. How is it recognised? (CBSE 2008)

Answer: Carbon dioxide gas is evolved.

When it is allowed to pass through freshly prepared lime water, the lime water turns milky.

Ca(OH)2  +  CO2 ———–> CaCO3

(lime water)                             (milky)

Question 25. How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal? (CBSE 2008)

Answer: Hydrogen gas is evolved when an active metal reacts with hydrochloric acid. Evolved gas burns with a pop sound when a burning splinter is brought close to it.

Question 26. How does the flow of acid rainwater into a river make the survival of aquatic life in the river difficult? (A.I CBSE 2008)

Answer: Due to the flow of acid rainwater into a river, the pH value of the river water decreases, which makes the aquatic life difficult to survive?

Question 26. Two solutions A and B have pH values of 5 and 8 respectively. Which solution will be basic in nature? (CBSE 2008 C)

Answer: B will be of basic nature as pH= 8.

Question 27. A compound X of sodium is commonly used in the kitchen for making crispy pakoras. It is also used for curing acidity in the stomach. Identify X. What is its chemical formula? State the reaction which takes place when it is heated during cooking. (CBSE 2008 C)

Answer:  ‘X’ is baking soda, sodium hydrogen carbonate ( NaHCO3).

When it is heated during cooking, it decomposes into sodium carbonate, carbon dioxide and water.

2NaHCO3(s)———-> Na2CO3  +  CO2  +  H2O

Question 28. How is the pH of a solution of an acid-influenced when it is diluted? (CBSE 2008 F)

Answer: H+ ions concentration in moles per litre decreases when acid dissolved in water. And hence, pH increases because the solution becomes less acidic on dilution.

Question 29.  Arrange the following in increasing order of their pH values:

NaOH solution, blood, lemon juice. (CBSE 2008 F)

Answer: Lemon juice  <  Blood  <  NaOH solution

Question 30. What would be the colour of litmus in a solution of sodium carbonate? (CBSE 2009)

Answer: Red litmus turns blue in a solution of sodium carbonate.

Question 31. Why does tooth decay start when the pH of the mouth is lower than 5.5? (CBSE 2009)

Answer: Tooth decay starts when the pH of the mouth is lower than 5.5. It does not dissolve in water but is corroded when the pH in the mouth is below 5.5. Bacteria present in the mouth produce acids by the degradation of sugar and food particles remaining in the mouth after eating.

Question 32. Crystals of copper sulphate are heated in a test tube for some time

(a) What is the colour of copper sulphate crystals

(i) before heating and (ii) after heating?

(b) What is the source of liquid droplets seen on the inner side of the tube? (A.I CBSE 2009)

Answer: (a) (i) Colour of copper sulphate crystals before heating is blue.

               ( ii) After heating blue colour disappears and turns white.

(b) It is the water of crystallisation.

Question 33. Why does 1M HCl solution have a higher concentration of H+ ions than 1M CH3COOH (A.I CBSE 2009)

Answer: 1M HCl has a higher concentration of H+ ions as it releases H+ ions readily.

Question 34. What is an alkali? (CBSE 2009 F)

Answer: An alkali is defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0.

Question 35. (i) Name the products obtained when sodium hydrogen carbonate is heated.

(ii) Write the chemical equation for the reaction involved in the above. (A.I CBSE 2009)

Answer: (i) Sodium carbonate, carbon dioxide and water are formed when sodium hydrogen carbonate is heated.

(ii) Sodium carbonate, carbon dioxide and water are obtained.

2NaHCO3(s) ——–> Na2CO3(S) + CO2 (g) + H2O (l)

Question 36. Which of these has a higher concentration H+Ions – 1M HCl or 1M CH3COOH (A.I CBSE 2009)

Answer: 1M HCl has a higher concentration of H+Ions.

Question 37. Write the name and chemical formula of the calcium compound used for disinfecting drinking water. How is this compound manufactured? (A.I CBSE 2009)

Answer: Bleaching powder – CaOCl2 (Calcium oxychloride )

Preparation – CaOCl2 is formed when chlorine gas is passed through the slaked line.

Ca(OH)2 + Cl2——– -> CaOCl2 +H2O

Question 38. What is the colour of litmus in a solution of ammonium hydroxide? (CBSE 2009 F)

Answer: Red litmus turns blue in a solution of ammonium hydroxide.

Question 39. Which gas is generally liberated when a dilute solution of hydrochloric acid reacts with an active metal?

Answer: Hydrogen gas is liberated a dilute solution of hydrochloric acid reacts with an active metal.

Zn(S) +2HCl (dil) -> ZnCl2 (aq) + H2 (g)

Question 40: Fresh milk has a pH of 6. When it changes into curd (yoghurt) will it pH value increase or decrease? Why? (CBSE 2009)

Answer: The pH value of curd decreases when fresh milk turns into curd. It is due to the formation of lactic acid when milk changes into curd.