CH-2 chemistry ACIDS,BASES AND SALTS

1

How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?

1

ANS:     Bring a burning matchstick near the gas. It burns with ‘pop’ sound showing that it is hydrogen.

  

2

Which bases are called alkalies? Give an example of alkalies.

1

ANS:     Soluble bases are called alkalies, e.g. sodium hydroxide (NaOH).

  

3

Write a balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid indicating the physical state of the reactants and the products.

1

ANS:     Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + CO₂(g) + H₂O(l)

  

4

Write a balanced chemical equation for a neutralisation reaction, mentioning the physical state of the reactants and the products.

1

ANS:     NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

  

5

Name the gas usually liberated when a dilute acid reacts with a metal. What happens when a burning candle is brought near this gas?

1

ANS:     H₂ gas is liberated. It burns with pop sound when burning candle is brought near the gas.

  

6

What is the colour of litmus in a solution of ammonium hydroxide?

1

ANS:     Red litmus will turn blue in ammonium hydroxide.

  

7

Which gas is evolved when sodium hydrogencarbonate reacts with dilute hydrochloric acid?

1

ANS:     Carbon dioxide gas is evolved.

  

8

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

1

ANS:     Ammonia dissolves in water, forms ammonium hydroxide which is base and turns red litmus blue. NH₃ + H₂O → NH₄⁺ + OH^– Dry ammonia gas does not change into OH^– ions.

  

9

Name an acid-base indicator prepared at home.

1

ANS:     Beetroot extract.

  

10

What will happen to blue litmus when it is added to soda water?

1

ANS:     It changes to red.

  

11

What will happen to red litmus solution when it is added to bitter gourd extract? Is it acidic or basic?

1

ANS:     It changes to blue. It is basic in nature.

  

12

Name two synthetic indicators which are used to test acids and bases.

1

ANS:     (i) Phenolphthalein (ii) Methyl orange.

  

13

What is aqua regia?

1

ANS:     It is a mixture of conc. HCl and conc. HNO₃ in the ratio of 3 : 1.

  

14

Name the metals which are soluble in aqua regia.

1

ANS:     Au and Pt can dissolve in aqua regia.

  

15

Which planet contains yellowish clouds of sulphuric acid ?

1

ANS:     Venus.

  

16

Why does HCl(g) not conduct electricity when dissolved in toluene?

1

ANS:     It is because HCl cannot ionise in organic solvent like toluene.

  

17

What are antacids? Give one example.

1

ANS:     Those compounds which neutralise excess of acid in stomach, e.g. NaHCO₃.

  

18

Is toothpaste acidic or alkaline?

1

ANS:     It is alkaline.

  

19

What is cure for bee sting? Why?

1

ANS:     Baking soda, which is a mild base. It gives relief by neutralising the effect of formic acid.

  

20

A bud of petunia became reddish purple after first shower of rain. What does it indicate ?

1

ANS:     The rain was acid-rain.

  

21

Name an indicator which indicates the various levels of hydrogen ion concentration.

1

ANS:     Universal indicator.

  

22

What effect does the concentration of H⁺(aq) have on the acidic nature of the solution?

1

ANS:     Acidic nature increases with increase in conc. of H₃O⁺ ion.

  

23

The pH of a sample of vegetable soup was found to be 6.5. How is this soup likely to taste?

1

ANS:     The taste will be slightly sour as it is weakly acidic.

  

24

What effect does an increase in concentration of H⁺ (aq.) in a solution have on the pH of solution?

1

ANS:     Higher the concentration, lower will be pH of the solution.

  

25

What would be the colour of litmus in a solution of sodium carbonate?

1

ANS:     The red litmus will change to blue in sodium carbonate solution.

  

26

Which acid and base are used in formation of following salts (a) CuSO₄ (b) NaNO₃?

1

ANS:     (a) Cu(OH)₂ and H₂SO₄ (b) NaOH and HNO₃.

  

27

Why do acids react with salt?

1

ANS:     Salts of weak acid react with strong acid. Strong acid can replace weak acid from its salt.

  

28

Is pH of washing soda less than 7 or more than 7?

1

ANS:     pH of washing soda is more than 7.

  

29	Name the natural source each of the following acids (a) Citric acid (b) Oxalic acid (c) Lactic acid (d) Tartaric acid	2
	ANS:     (a) Lemon and orange (b) Tomatoes and Guava (c) Sour milk (curd) (d) Tamarind
30	15 mL of water and 10 mL of sulphuric acid are to be mixed in a beaker (a) State the method that should be followed with reason. (b) What is this process called?	2
	ANS:     (a) The acid is to be added slowly in water to prevent the mixture to be splashed. The reaction is highly exothermic, therefore, constant cooling should be done. (b) The process is called dilution.
31	Explain how an antacid works.	2
	ANS:     Hyperacidity is caused by excess of hydrochloric acid in stomach. Antacid is basic in nature. It neutralises excess of acid and gases relief from pain caused by hyperacidity.
32	What is observed when sulphur dioxide is passed through (a) water (b) lime water? Also write chemical equations for the reactions that take place.	2
	ANS:     (a) Sulphurous acid is formed. SO2 + H2O → H2SO3 (Sulphurous acid) (b) SO2 +   Ca(OH)2   →   CaSO3   + H2O                   Lime water     Calcium sulphite Lime water turned milky due to formation of calcium sulphite.
33	Given below are the pH values of four different liquids : 7.0, 14.0, 4.0, 2.0 Which of these could be that of (a) lemon juice, (b) distilled water, (c) 1 M sodium hydroxide solution, (d) tomato juice?	2
	ANS:     (a) Lemon juice pH = 2.0 (b) Distilled water pH = 7.0 (c) 1 M NaOH pH = 14.0 (d) Tomato juice pH = 4.0
34	A gas produced in the laboratory is highly soluble in water. Its colourless solution turns pink when a few drops of phenolphthalein is added to it. What is the nature of this gas?	2
	ANS:     The nature of gas is basic because it turns phenolphthalein pink. The following reaction takes place in aqueous solution, e.g. NH3 + H2O → NH4+ + OH–
35	A student detected the pH four unknown solutions A, B, C and D as follows: 11, 5, 7 and 2. Predict the nature of the solution.	2
	ANS:     A is basic, ‘B’ is acidic ‘C’ is neutral and ‘D’ is strongly acidic.
36	Classify the following salts into acidic, basic and neutral : Potassium sulphate, ammonium chloride, sodium carbonate, sodium chloride.	2
	ANS:     Neutral : Potassium sulphate, Sodium chloride Acidic : Ammonium chloride Basic : Sodium carbonate
37	The soil in a field is highly acidic. List any two materials which can be added to this soil to reduce its acidity. Give the reason for your choice.	2
	ANS:     (i) Lime, (CaO) can be added to neutralize acidity. (ii) Chalk, (CaCO3) can also be added to neutralize acidity. It is because CaO and CaCO3 are basic in nature which neutralize acid present in soil.
38	What happens when chlorine is passed over slaked lime at 313K ? Write chemical equation of the reaction involved and state two uses of the product obtained.	2
	ANS:     Bleaching powder is formed. Uses: (i) It is used as bleaching agent in paper and textile industries. (ii) It is used as disinfectant in purification of drinking water.
39	Tooth enamel is one of the hardest substances in our body. How does it undergo damage due to eating chocolates and sweets ? How do toothpastes prevent this damage ?	2
	ANS:     Tooth enamel is made up of calcium phosphate. Chocolates and sweets produce acid in our mouth which react with enamel (calcium phosphate) and leads to cavities. We should brush our teeth after every meals as well as after eating sweets, etc.

40	(a) Define olfactory indicators. Name two substances which can be used as olfactory indicators. (b) Choose strong acids from the following: CH3COOH, H2SO4, H2CO3, HNO3	3
	ANS:     (a) Those substances whose smell (odour) changes in acidic or basic solution are called olfactory indicators, e.g. onion and vanilla. (b) H2SO4 and HNO3 are strong acids.
41	Explain the action of dilute hydrochloric acid on the following with chemical equations: (a) Magnesium ribbon (b) Sodium hydroxide (c) Crushed egg shells	3
	ANS:     (a) Hydrogen gas will be formed, e.g. Mg (s) + 2HCl (dil) → MgCl2 (aq) + H2 (s) (b) Sodium chloride and water will be formed, e.g. NaOH + HCl → NaCl + H2O (c) Crushed egg shells are made up of CaCO3 which reacts with dil HCl to give brisk effervescence due to CO2, e.g. CaCO3 (s) + 2HCl → CaCl2 + H2O + CO2
42	(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?	3
	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 our teeth after every meal. (c) It is due to formic acid. Sodium hydrogencarbonate (Baking soda) neutralises formic acid giving relief.
43	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.	3
	ANS:     Baking powder. It consists of sodium hydrogencarbonate and tartaric acid. Sodium hydrogencarbonate gives CO2 which makes cake soft and fluffy. Tartaric acid neutralises the bitterness due to sodium carbonate produced. 2NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(l)
44	On passing excess carbon dioxide gas through lime water, it first turns milky and then becomes colourless. Explain why? Write all the chemical equations of the reactions involved.	3
	ANS:     Ca(OH)2 + CO2 → CaCO3(s) + H2O CaCO3(s) + CO2(g) + H2O(l) → Ca(HCO3)2 Lime water turns milky due to the formation of calcium carbonate. It becomes colourless when excess of CO2 is passed due to formation of Ca(HCO3)2 which is soluble in water.
45	“Sodium hydrogencarbonate is a basic salt”. Justify the statement. How is it converted into washing soda ? Explain.	3
	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 →Na2CO3 + CO2 + H2O
46	(a) What is universal indicator ? (b) Write the chemical equation involved in the preparation of sodium hydroxide. Name the process.	3
	ANS:     (a) Universal indicator is mixture of synthetic indicators which is used to find pH of solutions. (b) 2NaCl(aq) + 2H2O(l) →2NaOH(aq) + H2(g) + Cl2(g) It is called chlor-alkali process.
47	(a) Name the compound which is obtained from baking soda and is used to remove permanent hardness of water. (b) Write its chemical formula. (c) What happens when it is recrystallised from its aqueous solution?	3
	ANS:     (a) Sodium carbonate is obtained from baking soda and is used to remove hardness of water. (b) Na2CO3. (c) It changes to washing soda, Na2CO3·10H2O.
48	What is tooth enamel chemically? State the condition when it starts corroding. What happens when food particles left in the mouth after eating degrade? Why do doctors suggest use of tooth powder/toothpaste to prevent tooth decay?	3
	ANS:     It is made up of Calcium phosphate. It starts corroding by acid formed in mouth. These food particles left in mouth form acids which cause tooth decay. Tooth paste and tooth powder are basic and neutralise acid formed in mouth which prevents tooth decay.
49	Answer the following questions: (i) State the colour of phenolphthalein in soap solution. (ii) Name the by-product of chlor-alkali process which is used for the manufacture of bleaching powder. (iii) Name one indicator which specifies the various levels of H+ ion concentration.	3
	ANS:     (i) Pink (ii) Chlorine (iii) Universal indicator
50	What is baking soda chemically called ? Give reaction involved in its preparation. Write one of its uses.	3
	ANS:     Sodium hydrogencarbonate. NH3 + CO2 + H2O + NaCl → NaHCO3 + NH4Cl or Na2CO3 + CO2 + H2O → 2NaHCO3 It is used as an antacid.
51	Name the products formed in each case when (a) hydrochloric acid reacts with caustic soda. (b) granulated zinc reacts with caustic soda. (c) carbon dioxide is passed into lime water.	3
	ANS:     (a) Sodium chloride and water are formed.  (b) Sodium zincate and H2(g) are formed. (c) Calcium carbonate and water are formed.

52	State reasons for the following statements: (i) Stain of curry on a white cloth becomes reddish brown when soap is scrubbed on it and turns yellow again when the cloth in washed with plenty of water. (ii) Curd should not be kept in copper or brass vessels. What is done to protect it?	5
	ANS:     (i) Turmeric reacts with sodium hydroxide present in soap to form red coloured compound. It turns yellow again because sodium hydroxide becomes very dilute on adding lot of water and reaction stops. (ii) Curd contains lactic acid which reacts with copper or brass vessels and taste changes. Curd should be kept in glass, steel or ceramic container which does not react with lactic acid present in it.
53	State reason for the following statements: (a) Tap water conducts electricity whereas distilled water does not. (b) Dry hydrogen chloride gas does not turn blue litmus red whereas dilute hydrochloric acid does. (c) During summer season, a milk man usually adds a very small amount of baking soda to fresh milk. (d) For a dilution of acid, acid is added into water and not water into acid. (e) Ammonia is a base but does not contain hydroxyl group.	5
	ANS:     (a) Tap water contains ions which conduct electricity, distilled water does not contain ions. (b) Dry HCl does not form ions but HCl gives H+ and Cl–. (c) Baking soda does not allow milk to change to lactic acid which makes milk sour. (d) Adding water to acid is highly exothermic. Therefore, water is added to acid very slowly with cooling. (e) Ammonia dissolves in water forms OH–. Therefore, it is basic. NH3 + H2O → NH4+ + OH–
54	Equal length of magnesium ribbon are taken in two test tubes ‘A’ and ‘B’. H2SO4 is added to test tube ‘A’ and H2CO3 in the test tube ‘B’ in equal amounts: (a) Identify the test tube shaving vigorous reaction. (b) Give reason to support your answer. (c) Name the gas liberated in both the test tubes. How will you prove its liberation? (d) Write chemical equations for both reactions. (e) Out of the two acids taken above (i) which one will have lower pH value. (ii) lower H+ concentration respectively.	5
	ANS:     (a) ‘A’ will show vigorous reaction. (b) It is because H2SO4 is strong acid. (c) Hydrogen gas will be formed. Bring a burning splinter near the gas. It will burn with ‘pop’ sound. It shows gas liberated is hydrogen. (d) Mg + H2SO4 → MgSO4 + H2 Mg + H2CO3 → MgCO3 + H2 (e) ‘A’ (H2SO4) will have lower pH. ‘B’ (H2CO3) will have lower concentration of H+.
55	(a) Explain why is hydrochloric acid a strong acid and acetic acid, a weak acid. How can it be verified? (b) Explain why aqueous solution of an acid conducts electricity. (c) You have four solutions A, B, C and D. The pH of solution A is 6, B is 9, C is 12 and D is 7, (i) Identify the most acidic and most basic solutions. (ii) Arrange the above four solutions in the increasing order of H+ ion concentration. (iii) State the change in colour of pH paper on dipping in solution C and D.	5
	ANS:     (a) HCl is completely ionised in aqueous solution, whereas acetic acid is partially ionised in aqueous solution. HCl gives dark red colour with pH paper whereas CH3COOH gives orange colour. (b) It is because acid ionises in aqueous solution. (c) (i) ‘A’ is most acidic and ‘C’ is most basic. (ii) C (10–12) < B (10–9) < D (10–7) < A (10–6) (iii) pH paper will become blue in ‘C’ and green in ‘D’.
56	(a) Dry pellets of a base ‘X’ when kept in open absorbs moisture and turns sticky. The compound is also formed by chlor-alkali process. Write chemical name and formula of X. Describe chlor-alkali process with balanced chemical equation. Name the type of reaction that occurs when X is treated with dilute hydrochloric acid. Write the chemical equation. (b) While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid ?	5
	ANS:     X is Sodium hydroxide, NaOH When sodium chloride solution (brine solution) is electrolysed, sodium hydroxide solution is formed. H2 and Cl2 gases are liberated. This is chlor-alkali process. 2NaCl + 2H2O →2NaOH + H2 + Cl2 NaOH + HCl → NaCl + H2O    ‘X’ The above reaction is neutralisation reaction. (b) It is because process is highly exothermic. If water is added to acid, bottle of acid will break.
57	A student dropped few pieces of marble in dilute hydrochloric acid, contained in a test-tube. The evolved gas was then passed through lime water. What change would be observed in lime water? What will happen if excess of gas is passed through lime water? With the help of balanced chemical equations for all the changes explain the observations.	5
	ANS:     CaCO3 + 2HCl(dil.) → CaCl2 + H2O + CO2 Marble Calcium chloride Lime water will turn milky. Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l) If excess of CO2 gas is passed through lime water, milkiness will disappear due to the formation of Ca(HCO3)(aq) which is soluble in water. CaCO3(s) + H2O(l) + CO2(g) → Ca(HCO3)2(aq)
58	(a) The pH of soil A is 7.5 while that of soil B is 4.5. Which of the two soils A or B should be treated with powdered chalk to adjust its pH and why? (b) Name the chemical which is injected into the skin of a person: (i) During an ant’s sting (ii) During the nettle leaf sting. How can the effect of these stings be neutralised? (c) Explain how the pH change in the river water can endanger the lives of aquatic animals like fish?	5
	ANS:     (a) Soil ‘B’ is acidic. It should be treated with powdered chalk which is basic so as to adjust its pH. (b) (i) Formic acid, (HCOOH). (ii) Formic acid, (HCOOH). The effect of these stings neutralized by rubbing the skin with mild base like baking soda (NaHCO3). (c) If pH of river water changes, amount of oxygen dissolved in water may decrease. Acidic and basic water is harmful for skin of aquatic animals like fish.
59	(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 tooth pastes. How do they prevent tooth decay?	5
	ANS:     (a) Our stomach has pH equal to 2. Antacids neutralises 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) Tooth pastes are basic in nature. They neutralise the acid formed in mouth which causes tooth decay.
60	(a) State the chemical properties on which the following uses of baking soda are based: (i) as an antacid (ii) as soda-acid fire extinguisher (iii) to make bread and cake soft and spongy. (b) How washing soda is obtained from baking soda? Write balanced chemical equation.	5
	ANS:     (a) (i) It is weakly basic in nature and neutralises hyperacidity. (ii) It liberates CO2 with H2SO4, which extinguishes fire. (iii) It liberates CO2 on heating which makes bread and cake soft and sponge. (b) 2NaHCO3 →Na2CO3 + CO2 + H2O Baking soda on heating gives sodium carbonate which on crystallisation from aqueous solution gives washing soda, e.g. Na2CO3 + 10H2O → Na2CO3 . 10H2O

61	Kanchan takes milk which is undoubtly a ‘superstar’ when it comes to healthy and strong bones. It is best source of calcium required for proper growth of bones. Yoghurt can be preferred choice for those who do not like to drink milk. Cheese is also a great source of calcium. Low fat or fat free milk is better choice because it has calcium with little or no fat and is easy for body to absorb calcium. Magnesium, phosphorus and potassium are also present in milk. (a) Name the types of vitamins that are present in milk. (b) Which vitamin is essential for absorption of calcium present in milk? (c) What values are shown by Kanchan taking milk or milk products every day? (d) Why should obese (overweight) children take low fat or skimmed milk? (e) Some people offer milk to Lord Shiva and lot of milk gets wasted. Do you feel it is right practices? What values are possessed by people who do not follow this practices?	4
	ANS:     (a) A, D and B12 (b) Vitamin D (c) Kanchan is conscious about her health and has strong bones. (d) It is because they do not gain weight as skimmed milk contains 1.5% of fats. (e) Milk should not be wasted as it is precious. Those people who do not involve in these practices, are well aware of importance of milk and don’t waste it and are not superstitious.
62	Rohit takes cold drink every day. Cold drinks contain sugar and carbon dioxide which is dissolved in water along with preservative like sodium benzoate. Carbonic acid can cause tooth decay. It may also cause hyper acidity and weaken our bones. On the other hand, his friend Mohit take nimbu pani (lemonade) and fruit juice instead of cold drinks. (a) What is the formula of acid present in cold drink? (b) How do cold drinks cause tooth decay? (c) What is meant by hyperacidity? (d) What values are possessed by Mohit taking nimbu pani and fruit juices instead of cold drinks? (e) What are the diseases caused by weak bones?	4
	ANS:     (a) H2CO3 (Carbonic acid) (b) Enamel is made up of calcium phosphate which reacts with acid and causes tooth decay. (c) When we take acidic drinks, the pH of our stomach decrease making it more acidic. This condition is called hyperacidity. (d) Mohit is aware of harmful effects of cold drinks on his body and takes care of his health. (e) Arthritis and weak bones which may lead to fracture.
63	Mrs. Sharma adds iodised common salt to her meals. Iodised common salt is better than ordinary common salt. Mrs. Verma and her family take pickle, sauce, processed food and junk food which contains lot of sodium chloride. Her children are suffering from hypertension at a very young age. Doctors advised her that she should take less common salt. (a) Why do doctors advise us to take less common salt? (b) Why should we avoid junk food, pickle and processed food? (c) Why should we prefer iodised salt? (d) Why do doctors advise saline gargles in sore throat? (e) What values are possessed by Mrs. Sharma taking iodised common salt and in lesser amount?	4
	ANS:     (a) It is because excess of salt leads to high blood pressure. (b) It is because junk food is rich in common salt. (c) Iodised salt contains iodine which is essential for hormone, thyroxine which is a growth hormone. Iodine deficiency may lead to a disease called Goitre. (d) It acts as anti-viral agent and gives relief. (e) Mrs. Sharma is concerned about health of herself and her family.
64	Ram’s father is a farmer he uses lot of chemical fertilizers to increase fertility of soil. He uses lot of pesticides, weedicides and insecticides for healthy crops but these chemicals are non-biodegradable and cause water pollution. These chemicals are also mixed up with wheat, rice, barley etc., and consumed by us. But Shyam’s father use eco-friendly manure and bio-degradable pesticides. (a) What should be done before using fertilizer? (b) Which soil is generally best for crops? (c) Why should manure be preferred over chemical fertilizers? (d) Why should we avoid inorganic pesticides and use environmental friendly substances as pesticides? (e) What values are possessed by Shyam’s father who use eco-friendly substances to increase production of crops?	4
	ANS:     (a) Pit of soil should be tested so that it can be decided, which fertilizer and how much amount needed for a particular crop. (b) Neutral soil is best for crops. (c) It is because manures do not cause land pollution. (d) It is because they will not cause water pollution which is harmful for us. (e) Shyam’s father is responsible person and saves himself and others from contaminated water and land pollution.