2. ACIDS, BASES AND SALTS
Questions :
1. The group o f compounds that will dissociate partially in aqueous solution is
(a) Hydrochloric acid, Nitric acid
(b) Carbonic acid, phosphoric acid
(c) Hydrochloric acid, phosphoric acid
(d) Copper sulphate solution, sugar solution [SSLC April, 2018]
Ans : (b) C arbonic add, phosphoric add.
2. How is the concentration o f hydronium ions (H3O +) affected, when a solution o f an acid is diluted? [NCERT Exemplar]
Ans : When a solution o f an acid is diluted i.e., water is added to it, the concentration o f H 3O ions per unit volume decreases.
3. What is meant b y the term p H of a solution ? The p H of rain water collected from two cities A and B was found to b e 6 and 5 respectively. The water of which city is more acidic?
Ans : p H of a solution is a measure o f the H + j concentration in a solution. Lesser the pH, more acidic is the solution. Thus, rain water of city B is more acidic.
4. Explain why, an aqueous solution o f sodium sulphate is neutral while an aqueous o f sodium carbonate is basic in nature.
A ns : Sodium s ulpha te is a neutra l salt be c a us e it is ma de f r om s tr ong a c id a nd s tr ong base. Sodium c a r bona te is a basic salt be c a us e it is ma de f r om s tr ong ba s e a nd w e a k acid.
5. What is the change in p H values o f milk when it changes into curd? Explain.
Ans : pH decreases. It is due to the formation o f lactic acid in curd. Since, acids generally have a low p H value, so, when milk changes to curd its p H will decrease.
6. Dry HC I gas does not change the colour o f dry blue litmus paper. Give reasons.
Ans : In dry state, HC l gas does not give any H + ions. It does not behave as an acid. That is why the colour of litmus paper does not change.
7. Write chemical equations that shows aluminium oxide reacts with acid as well as base.
Ans: (i) Al2O3 + 6HCl → 2 AlCl3 + 3 H2O (ii) Al2O3 + 2NaOH → 2N aAlO 2 + H2O
8. Though the compounds such as glucose and alcohol have, hydrogen atoms in their molecules yet they are not categorized as acids. Why?
Ans : Glucose and alcohol d o not produce H+ ions, when dissolved in water. The acids contain hydrogen. When acid is dissolved in water, it produces H+ ions.
9. (a) What is the action o f litmus on: [SSLC July, 2018]
(i) Dry ammonia gas (ii) Solution of ammonia gas in water (b) State the observations you would make o n adding sodium hydroxide to aqueous solution of: (i) F errous sulphate (ii) Aluminium chloride. Give balanced chemical equations.
Ans: (a) (i) N o change in colour.
(ii) Red litmus turns blue.
(b) (i) 2NaOH(aq) + FeSO4(aq) → Sodium Ferrous Fe(OH)2(s) + Na2 SO4 Ferrous Sodium Hydroxide s u lp h ate hydroxide s ulphate
(ii) 3NaOH (aq) + AlCl3(aq) → Sodium Aluminium Al(OH)3(s) A luminium Hydroxide chloride Hydroxide
10. N ame the acid present in the following: (i) Tomato, (ii) Vinegar, (iii) Tamarind
Ans: (i) Tomato : Malic acid and citric acid (ii) Vinegar: Acetic acid (iii) Tamarind : Tartaric acid and citric acid.
11. To the three solutions listed below, a few drops o f phenolphthalein and blue litmus were added separately. S pecify the colour change in each case, if any:
(i) S odium carbonate
(ii) Hydrochloric acid
(iii) Sodium chloride
Ans:
12. A student takes three beakers A, B and C filled with aqueous solution of glucose, alcohol and hydrochloric acid respectively as shown in the following figure:
(i) State your observation in terms of glowing of bulb when the switch is on.
(ii) Justify your observations b y giving reason in each case.
(iii) Mention the change noticed with appropriate reason if the content o f beaker B is replaced b y sodium hydroxide solution.
Ans: (i) Bulb A and B do not glow but bulb C glows.
(ii) Glucose and alcohol solutions d o not conduct electricity as they d o not have ions. Dil. HC I contains ions so the flow of ions is responsible for the flow of current.
(iii) After replacement, bulb glows in B as N a O H solution contains ions (N a + and O H − ions)
13. A metal is treated with dilute sulphuric acid. The gas evolved is collected b y the method shown in the figure:
(i) Name the gas.
(ii) Is the gas soluble o r insoluble in water?
(iii) Is the gas lighter o r heavier than air?
(iv) How will you test the gas?
(v) If the metal used a bove is zinc then write the chemica l equation for the evolution of gas.
(vi) Write one industrial use of the gas evolved.
Ans: (i) Hydrogen gas.
ii) Is soluble in water.
(iii) Gas is lighter than air.
(iv) Test for H 2 Gas: Bring a burning matchstick near the gas jar. It b urns with a pop sound.
(v) Zn(s) + H2SO4(dil) → ZnSO4(aq) + H2 (g)↑ ½ X 6
(vi) Liquid hydrogen is used as a fuel in rockets.
14. While eating food, you happen to spill some curry o n your white shirt. You immediately scrub it with soap. What happens to its yellow colour o n scrubbing with soap? Why? What happens to this stain when the shirt is washed with plenty o f water?
Ans : The colour changes from yellow to reddish brown. S oap is basic in nature and the colour of turmeric changes from yellow to reddish brown in basic medium. The stain turns yellow again.
15. In the following schematic diagram for the preparation o f hydrogen gas as shown in figure, what would happen if the following changes are made? [SSLC April, 2018]
(a) In place o f zinc granules, same amount o f zinc dust is taken in the test tube.
(b) Instead o f dilute sulphuric acid, dilute hydrochloric acid is taken.
(c) In place o f zinc, copper turnings are taken.
(d) Sodium hydr oxide is ta ke n in pla c e of dilute sulphur ic a c id a nd the test tube is heated.
Ans: (a) Rate of reaction will b e faster as surface area increases. -
(b) N o net effect will b e there as both are the strong acids.
(c) N o reaction will take place as copper is less reactive than hydrogen.
(d) Zinc can also react with N aO H to give hydrogen gas. 2NaOH + Zn → ∆ Na2ZnO2 + H2
16.(i) Define universal indicator. F or what purpose it is used?
(ii) Two solutions A and B have p H values of 3.0 and 9.5 respectively. Which of these will turn litmus solution from blue to red and which will turn phenolphthalein from colourless to pink?
(iii) Water is a neutral substance. What colour will you get when you add a few drops of universal indicator to a test tube containing distilled wate r?
Ans: (i) Universal indicator is a mixture of many different indication (or dyes) which give different colours at different p H values o f the entire p H scale. The colour produced b y universal indicator is used to find the p H value o f acid o r base b y matching the colour with the colours o n p H colour chart.
(ii) Solution A is acidic and will turn litmus solution from blue to red. Solution B is basic and will turn phenolphthalein from colourless to pink.
(iii) Green colour will be obtained.
17. (i) A local magician was showing magic in a village street. He took egg shell and poured a solution over it. As a result, effervescences were formed. When he took a burning matchstick over it, it went off. (a) Identify the solution poured and the substance present in egg shell.
(b) What is the reason behind effervescences?
(c) Write its balanced chemical equation.
(d) Give the common name of the substance p re sent in the egg shell.
(ii) Draw a labelled diagram to show that acid solution in water conducts electricity.
Ans : (i) (a) Solution is an acid. S ubstance in egg shell is calcium carbonate.
(b) Effervescence is due to the evolution of C O 2 gas.
(c) 2HNO3 + CaCO3 → 2Ca(NO 3)2 + H2O + C O 2
(d) Lime stone.(ii)
18. The p H values of four solutions F , Q , R and S are 7.8, 1.0, 13.0 and 1.4 respectively. The solution having highest hydrogen ion concentration among them is: [ S S LC July 2018]
(i) P (ii) Q (iii) R (iv) S
Ans: (ii)
Q 19. In a bakery, baking powder was not added while preparing cake. The cake obtained was hard and small in size. What is the reason for this? [SSLC July 2018]
Ans : The reason for the hardness is absence o f banking powder. Baking powder when added releases C arbon dioxide gas, which gets trapped in the wet dough and bubbles out slowly making the cake to rise and hence, soft and spongy.
20. N ame the sodium compound which is used for softening hard water. [N C ERT Exemplar]
Ans : S odium carbonate.
21. What is the common name of the compound C aO C12? [NCERT Exemplar]
Ans: Bleaching powder.
22. Classify the following salts into acidic, basic and neutral — Potassium sulphate, Ammonium chloride, Sodium ca rbo nate, Sodium chloride.
Ans : Neutral Salts: P otassium sulphate, sodium chloride Basic Salts: S odium carbonate Acidic Salts: Ammonium chloride.
23. Explain why, an aqueous solution o f sodium sulphate is neutral while an aqueous solution o f sodium carbonate is basic in nature.
Ans : S odium sulphate when dissolves in water forms strong add and strong base so its aqueous solution is neutral. S odium carbonate, o n the other hand, when dissolves in water forms an alkaline solution which turns red litmus blue. Thus, its aqueous solution is basic in nature .
24. The colour o f copper sulphate solution changes when an iron nail is dipped in it. State the reason .giving chemical equation for the reaction involved .
Ans: C uS O 4 + Fe → F eS O 4 + C u (Blue) (Green) Iron is more reactive than copper. Hence it displaces copper from copper sulphate and due to formation o f F eS O 4, the colour changes from blue to green.
25. (i) How chloride of lime chemically differs from calcium chloride?
(ii) What happens when chloride o f lime reacts with sulphuric acid? Write chemical equation involved. (iii) Mention two uses of chloride of lime.
Ans: (i) Chloride of lime is calcium hypochiorite C aO C12 which is alkaline while calcium chloride is C aC12 which is neutral.
(ii) 2CaOC12(s) + H2SO4(aq) → C aS O 4(aq) + C aC12(aq) + 2HClO(aq)
(iii) (a) Bleaching powder is used for bleaching purposes in textile and paper industry and in laundry. (b) It is also used as a disinfectant for water.
Textbook Exercises
1. A solution turns red litmus blue, its p H is likely to b e
(a) 1 (b) 4 (c) 5 (d) 10
Ans: (d) 10
2. A solution reacts with crushed egg shells to give a gas that turns lime -water milky. The solution contains (a) NaCl (b) HCl (c) LiCl (d) KCl Ans: (b) HCl
3. 1 0 ml of a solution of N aO H is found to b e completely neutralised b y 8 ml of a given solution of HC 1. If we take 2 0 ml of the same solution of N aO H, the amount of HC l solution (the same solutions as before) required to neutralise it, will b e
(a) 4 ml (b) 8ml (c) 12ml (d) 16ml
Ans: (d) 16ml
4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Ans: (c) Antacid
5. Write word equations and then balance equations for the reaction taking place when
(a) D ilute sulphuric acid reacts with zinc granules.
(b) D ilute hydrochloric acid reacts with magnesium ribbon.
(c) D ilute sulphuric acid reacts with aluminium powder
(d) Dilute hydrochloric acid reacts with iron fillings.
Ans: (a) Zinc + Sulphuric acid → Zinc sulphate + Hydrogen Zn(s) + H2SO 4 (aq) → ZnSO4(aq) + H2(g)
(b) Magnesium + Hydrochloric acid → Magnesium Chloride + Hydrogen Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
(c) Aluminium + Sulphuric acid → Aluminium sulphate + Hydrogen 2Al(s) + 3H2SO4(aq) → Al2(SO4)3(aq) + 3H2(g)
(d) Iron + hydrochloric acid → Ferrous chloride + Hydrogen Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
6. C ompounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an activity to prove it.
Ans : When electric current is passed through alcohol and glucose solution, we will observe that the current does not pass through these solutions due to lack o f ions.
7. Why does distilled water not conduct electricity, whereas rain water does?
Ans : In rainwater, various acids are present which give ions b y dissociating and therefore can conduct electricity whereas distilled water is salt free so it does not conduct electricity.
8. Why d o acids not show acidic behaviour in the absence o f water?
Ans : In the absence of water, acids cannot b e dissociated to give H ions. Therefore they can’t show acidic behaviour.
9. F ine solutions A,B,C , D and E when tested with universal indicator showed p H as 4, 1, 11, 7 and 9 respectively. Which solution is
(a) neutral
(b) strongly alkaline
(c) strongly acidic
(d) weakly acidic
(d) weakly alkaline? Arrange the p H in increasing order of W concentration.
Ans: (a) D (b) C (c) B (d) A (e) E C (10 −11M) < E (10 − 9M) < D (10 − 7M) < A (10 − 4M) < B (10 −1M)
10. Equal lengths o f magnesium ribbons are taken in test tube A and B. HC I is added to test tube
A, while C H3C O O H 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?
Ans : The fizzing will be faster in test tube A because H C 1 is stronger acid than acetic acid. Mg + 2HCl → F a s t MgCl2 + H2 Mg + 2CH3COOH → (CH3COO)2 Mg + H2
11. F resh milk has a p H o f 6. How d o you think the p H will change as it turns into curd? Explain your answer.
Ans : When milk is turned into curd then is p H Value will decrease due to the production of lactic acid in curd which is acidic in nature.
12. A milkman adds a very small amount o f baking soda to fresh milk
(a) Why does he shift the p H of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a bug time to set as curd?
Ans : (a) O n addition of small amount of baking soda, the milk will b e slightly alkaline to stop the curdling.
(b) This milk takes a long time to set as curd because the lactic acid produced reacts with the baking soda.
13. Plaster the Paris should b e stored in a moisture proof container. Explain why?
Ans : By absorbing moisture, P O P can change into hard mass (gypsum), so it should b e stored in moisture proof container. CaSO4 . (POP) 1 2 H2O + 1 2 H2O → (W ater) CaSO4. 2H2O (Gyps um)
14. What is a neutralisation reaction? Give two examples.
Ans : Neutralisation reactions are those chemical reactions in which salt and water are formed b y the reaction o f acids and bases. Ex. (i) KOH + HCl → KCl + H2O (Potas s ium (Hydro (Potas ium (W ater) Hydroxide) Chioric) Chloride) (ii) Ca(OH)2 + H2SO4 → C aS O 4 + 2 H2O (Calcium (Sulphuric (Calcium (water) hydroxide) acid) s ulphate)
15. Give two important uses o f washing soda and baking soda.
Ans : Two important uses of washing soda are:
(i) It is used in the preparation o f washing powder
(ii) It is used in softening o f water. Two important uses of baking soda are:
(i) It is used as antacid to remove the acidity o f stomach.
(ii) It is used in the designing of soda acid fire extinguisher.
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