CBSE Class 10 Science HOTs Acids Bases And Salts Set 02

Question. Adding which of the following to a colourless solution would give an indication that the solution could possibly be hydrochloric acid?
(a) Copper metal strips
(b) Silver metal strips
(c) Calcium carbonate
(d) Sodium chloride
Answer: (c) Calcium carbonate

Question. Which fruit is basic in nature?
(a) Apples
(b) Strawberries
(c) Oranges
(d) Banana
Answer: (d) Banana

Question. An oxide of an element ‘P’ is added to acid where it forms salt and water. The table shows the possible values of pH and type of element before the reaction.

Which option is correct?
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Answer: (a) 

Question. Which of the following will be present in dilute aqueous solution of sulphuric acid?
(a) \( H_3O^+ + SO_4^{2-} \)
(b) \( H_3O^+ + OH^- \)
(c) \( OH^- + SO_4^{2-} \)
(d) \( H_3O^+ + SO_2 \)
Answer: (a) \( H_3O^+ + SO_4^{2-} \)

Question. A student learnts that plants grow when the pH of the soil is slightly acidic. Which range of pH is most suited for plant growth?
(a) 1 – 3
(b) 5.5 – 7
(c) 7 – 9
(d) 11 – 14
Answer: (b) 5.5 – 7

Question. With the reference to four gases \( CO_2 \), \( CO \), \( Cl_2 \) and \( O_2 \), which one of the option in the table is correct?

Answer: (b) ​​​​

Question. Anita added a drop each of diluted acetic acid and diluted hydrochloric acid on pH paper and compared the colors. Which of the following is the correct conclusion?
(a) pH of acetic acid is more than that of hydrochloric acid.
(b) pH of acetic acid is less than that of hydrochloric acid.
(c) Acetic acid dissociates completely in aqueous solution.
(d) Acetic acid is a strong acid.
Answer: (a) pH of acetic acid is more than that of hydrochloric acid.

Question. Which of the given options correctly represents the parent acid and base of calcium carbonate?

Answer: (b) 

Question. How will you protect yourself from the heat generated while diluting a concentrated acid?
(a) By adding acid to water with constant stirring.
(b) By adding water to acid with constant stirring.
(c) By adding water to acid followed by base.
(d) By adding base to acid with constant stirring.
Answer: (a) By adding acid to water with constant stirring.

Question. Which of the following oxide(s) is/are soluble in water to form alkalies?
(i) \( Na_2O \)
(ii) \( SO_2 \)
(iii) \( K_2O \)
(iv) \( NO_2 \)
(a) (i) and (iii)
(b) (i) only
(c) (ii) and (iv)
(d) (iii) only
Answer: (a) (i) and (iii)

Question. Which of the options in the given table are correct?

(a) (i) and (ii)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (iii) and (iv)
Answer: (c) (ii) and (iii)

Question. Select from the following the statement which is true for bases.
(a) Bases are bitter and turn blue litmus red.
(b) Bases have a pH less than 7.
(c) Bases are sour and change red litmus to blue.
(d) Bases turn pink when a drop of phenolphthalein is added to them.
Answer: (d) Bases turn pink when a drop of phenolphthalein is added to them.

Question. Study the following table and choose the correct option:

Answer: (d) 

Question. Consider the pH value of the following acidic samples:

The decreasing order of their \( H^+ \) ion concentration is
(a) 3 > 4 > 1 > 2
(b) 2 > 1 > 3 > 4
(c) 2 > 1 > 4 > 3
(d) 3 > 4 > 2 > 1
Answer: (c) 2 > 1 > 4 > 3

Question. An aqueous solution with pH = 1 is
(a) strongly acidic
(b) strongly basic
(c) neutral
(d) weakly acidic
Answer: (a) strongly acidic

Question. Curd cannot be stored in
(i) Brass vessel
(ii) Copper vessel
(iii) Steel
(iv) Bronze
(a) (i), (ii), (iii)
(b) (ii), (iii), (iv)
(c) (i), (ii), (iv)
(d) (i), (iii), (iv)
Answer: (c) (i), (ii), (iv)

Question. When a small amount of acid is added to water, the phenomena which occur are:
(A) Dilution
(B) Neutralisation
(C) Formation of \( H_3O^+ \) ions
(D) Salt formation
The correct statements are:
(a) (A) and (C)
(b) (B) and (D)
(c) (A) and (B)
(d) (C) and (D)
Answer: (a) (A) and (C)

Question. \( NaHCO_3 \) is formed by reaction of
(a) \( NaOH + H_2CO_3 \)
(b) \( NaCl + H_2CO_3 \)
(c) \( Na_2CO_3 + HCl \)
(d) \( NaOH + Na_2CO_3 \)
Answer: (c) \( Na_2CO_3 + HCl \)

Question. pH of \( H_2O \) is
(a) 7
(b) 8
(c) 9
(d) 10
Answer: (a) 7

Question. \( Ag_2S \) reacts with \( H_2SO_4 \) to form
(a) \( AgSO_4 \)
(b) \( Ag_2SO_4 + H_2S \)
(c) \( Ag_2O + H_2S \)
(d) \( AgOH + H_2S \)
Answer: (b) \( Ag_2SO_4 + H_2S \)

Question. A solution turns the colour of turmeric to reddish brown. If the same solution is poured on universal indicator the colour would change to
(a) violet
(b) blue
(c) red
(d) green
Answer: (a) violet

Question. Acid present in tomato is
(a) methanoic acid
(b) acetic acid
(c) lactic acid
(d) oxalic acid
Answer: (d) oxalic acid

Very Short Answer Type Questions

Question. To prepare a salad dressing, Parag adds a solution of sodium chloride in distilled water to vinegar. State what change will occur in the following:
(a) the pH of the vinegar
(b) the acidity of the vinegar

Answer: (a) The pH will increase because vinegar (dil. solution of acetic acid) is acidic in nature whereas \( NaCl(aq) \) is neutral (pH = 7).
(b) The acidity will decrease because \( NaCl(aq) \) is being added i.e., water is being increased. Acidity decreases with dilution.

Question. Dipti has three flasks containing dilute hydrochloric acid, dilute sulphuric acid and dilute sodium hydroxide respectively. The flasks are not labeled and she does not have any pH indicator.
(a) Which of the solutions will she be able to identify just by making mixtures of pairs of the substances?
(b) What observation will help her to make this identification?

Answer: (a) The dilute sodium hydroxide
(b) The flasks containing mixtures of sodium hydroxide with hydrochloric acid and with sulphuric acid will be warm to touch.
\( NaOH + HCl \longrightarrow NaCl + H_2O + \text{heat} \)
\( 2NaOH + H_2SO_4 \longrightarrow Na_2SO_4 + 2H_2O + \text{heat} \)
Neutralisation reactions are exothermic.

Question. (a) What is the colour of litmus in a solution of ammonium hydroxide?
(b) A bud of Petunia became reddish purple after first shower of rain. What does it indicate?

Answer: (a) Red litmus will turn blue in ammonium hydroxide.
(b) The rain was acid-rain.

Question. (a) How will you test for the gas which is liberated when hydrochloric acid reacts with an active metal?
(b) Which gas is evolved when sodium hydrogencarbonate reacts with dilute hydrochloric acid?

Answer: (a) Bring a burning matchstick near the gas. It burns with ‘pop’ sound showing that it is hydrogen.
(b) Carbon dioxide gas is evolved.

Question. (a) A few drops of sulphuric acid is added to water before electrolysis, why?
(b) What effect does the concentration of \( H^+(aq) \) have on the acidic nature of the solution?

Answer: (a) It makes water better conductor.
(b) Acidic nature increases with increase in conc. of \( H_3O^+ \) ion.

Question. (a) Name the metals which are soluble in aqua regia.
(b) What will you do if conc. \( H_2SO_4 \) falls on your hand?

Answer: (a) Au and Pt can dissolve in aqua regia.
(b) Put hand in running water till irritation stops.

Question. Out of the two — hydrochloric acid and acetic acid, which one is considered a strong acid and why? Write the name/molecular formula of one more strong acid.
Answer: Hydrochloric acid is a strong acid because it has more number of hydronium \( (H_3O^+) \) ions whereas acetic acid contains less number of hydronium \( (H_3O^+) \) ions. So it is a weak acid. Sulphuric acid \( (H_2SO_4) \) is also a strong acid.

Question. What are the amphoteric oxides? Give an example. 
Answer: Amphoteric oxides have both acidic and basic properties. They form salts when they react with acids. They also react with alkalis to form complex salts. Examples: \( Al_2O_3 \).

Question. Define olfactory indicators. Name two substances which can be used as olfactory indicators.
Answer: Those substances whose smell (odour) changes in acidic or basic solution are called olfactory indicators, e.g. onion and vanilla.

Question. Classify the following into strong acids and weak acids. \( CH_3COOH, H_2SO_4, H_2CO_3, HNO_3 \)
Answer: \( H_2SO_4 \) and \( HNO_3 \) are strong acids. \( CH_3COOH, H_2CO_3 \) and \( H_2SO_3 \) are weak acids.

Question. Explain the action of dilute hydrochloric acid on the following with chemical equations: 
(a) Magnesium ribbon
(b) Crushed egg shells
Answer: (a) Hydrogen gas will be formed, e.g.
\( Mg (s) + 2HCl (dil) \rightarrow MgCl_2 (aq) + H_2 (s) \)
(b) Crushed egg shells are made up of \( CaCO_3 \) which reacts with dil \( HCl \) to give brisk effervescence due to \( CO_2 \), e.g.
\( CaCO_3 (s) + 2HCl \rightarrow CaCl_2 + H_2O + CO_2 \)

Question. Write two example of neutralization reaction.
Answer: Examples of neutralization reaction are as follows:
(i) \( HCl (aq) + NH_4OH (aq) \rightarrow NH_4Cl (aq) + H_2O \)
(ii) \( NaOH (aq) + HCl (aq) \rightarrow NaCl (aq) + H_2O \)

Question. A clear solution of slaked lime is made by dissolving \( Ca(OH)_2 \) in an excess of water. This solution is left exposed to air. The solution slowly goes milky as a faint white precipitate forms. Explain why a faint white precipitate forms, support your response with the help of a chemical equation.
Answer: Calcium hydroxide reacts with carbon dioxide present in the atmosphere to form calcium carbonate which results in milkiness/white ppt / Formation of calcium carbonate 
\( Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \) (1 mark)

Question. Keerti added dilute hydrochloric acid to four metals and recorded her observations as shown in the table given below:
Metal | Gas Evolved
Copper | Yes
Iron | Yes
Magnesium | No
Zinc | Yes
Select the correct observation(s) and give chemical equation(s) of the reaction involved. 
Answer: \( Fe + HCl \rightarrow FeCl_2/ FeCl_3 + H_2 \) (1 mark)
\( Zn + HCl \rightarrow ZnCl_2 + H_2 \) (1 mark)

Short Answer Type Questions 

Question. The pH of three solutions is given in the table. Answer the questions that follow.
Solution | pH
P | 1
Q | 7
R | 14
(a) Which of these solutions could possibly react with zinc metal to produce hydrogen gas?
(b) Which of these solutions could be formed by the reaction of a metal oxide with water?
(c) Which of these solutions could be the raw material for the industrial manufacture of chlorine? 
Answer: (a) – Solution P because it is acid (strong), will displace \( H_2 \) by reacting with zinc.
– Solution R because it is strong base, zinc reacts with strong base to form \( H_2(g) \).
(b) Solution R because it is strong base, formed by reaction of metal oxide with water to form metal hydroxide (base).
(c) Solution Q is of \( NaCl \) (pH = 7) which on electrolysis will produce chlorine gas on industrial scale as a by-product in manufacture of \( NaOH \).

Question. Sunlta carried out the following reactions in the laboratory:
(i) Complete neutralisation of one mole of sodium carbonate with hydrochloric acid
(ii) Complete neutralisation of one mole of sodium bicarbonate with hydrochloric acid
She found that the amount of carbon dioxide formed in both the reactions was the same.
(a) Is her finding correct? Justify your answer.
(b) How does the amount of salt formed in case (i) compare with the amount of salt formed in case (ii)? 
Answer: (a) Yes, her finding is correct. 1 mole of \( CO_2 \) is produced in both the cases.
\( Na_2CO_3(s) + 2HCl(dil.) \rightarrow 2NaCl(aq) + H_2O + CO_2(g) \) …(i)
\( NaHCO_3(s) + HCl(dil.) \rightarrow NaCl(aq) + H_2O(l) + CO_2(g) \) …(ii)
(b) The amount of salt (2 mole \( NaCl \)) formed in case (i) is twice the amount of salt (1 mole \( NaCl \)) formed in case (ii).

Rajesh was given a substance and asked to identify it. He conducted three tests on the substance and recorded the results below.
P. It releases carbon dioxide, water and a sodium salt on heating with water.
Q. It turns universal indicator greenish-blue.
R. It can be prepared from ammonia as a raw material. 

Question. What substance was Rajesh given?
Answer: Sodium bicarbonate \( (NaHCO_3) \).

Question. Give ONE use of the substance based on the properties mentioned in P and Q.
Answer: – Used as antacids
– Used as baking soda
– Used as a first aid in acidic insect bites
– Used in soda acid-fire extinguisher (any one)

Question. Rajesh later read that recrystallisation of the sodium salt formed in P gives another basic salt that is used in manufacture of borax. Identify the sodium salt formed in P.
Answer: Sodium carbonate \( (Na_2CO_3) \) is used for manufacture of borax \( (Na_2B_4O_7 \cdot 10H_2O) \)

Question. While constructing a house, a builder selects marble flooring and marble top for kitchen where vinegar and juices of lemon, tamarind etc. are more often used for cooking. Will you agree to this selection and why? 
Answer: No, I won’t agree with the selection because vinegar, juices of lemon and tamarind contains organic acids like acetic acid, citric acid and tartaric acid. Marble is calcium carbonate. If accidentally juices of citrus fruits will fall on the marble floor, it will react with the calcium carbonate and will leave mark on the floor which will look really bed.

Question. 1 g of solid sodium chloride is taken in a clean and dry test tube and 2 mL of conc. sulphuric acid is added to it. If the gas evolved is tested first with dry and then with wet blue litmus paper, in which case will the litmus paper change colour? Give reason for your answer with chemical equation for the reaction. 
Answer: The colour change will be observed in wet blue litmus paper because moisture/water is needed for the formation of hydronium ions due to which colour change of litmus paper take place.
\( 2NaCl + H_2SO_4 \rightarrow Na_2SO_4 + 2HCl \)

Question. 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the contents are warmed, a gas evolves which is bubbled through a soap solution before testing. Write the equation of the chemical reaction involved and the test to detect the gas. Name the gas which will be evolved when the same metal reacts with dilute solution of a strong acid. 
Answer: (a) Zinc reacts with NaOH to form sodium zincate and hydrogen gas
\( Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2 \)
(Sodium zincate and Hydrogen gas)
(b) When a burning splinter is brought near the gas, the splinter will gives out a ‘pop’ sound which shows the gas evolved is hydrogen.
(c) Hydrogen gas will be evolved when Zn metal reacts with dilute solution of strong acid.
\( Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2 \uparrow \)

Long Answer Type Questions

Question. A remarkable property of acids is that they can ‘dissolve’ metals. When metals are added to an acid, they disintegrate and disappear into the acid.
(a) State one other common observation when metals ‘dissolve’ in acids. Explain the reason for this observation.
(b) If the acid with the ‘dissolved’ metal is evaporated, can we get the metal back? Why or why not?
(c) In this question, the word ‘dissolve’ is used within quotes. This is because it is not actually an example of dissolving. What is the MAIN difference between a metal ‘dissolving’ in an acid and sugar dissolving in water? 
Answer: (a) Observation: Bubbling is seen.
Reason: Because hydrogen is produced, e.g.
\( Zn(s) + H_2SO_4(dil.) \rightarrow ZnSO_4(aq) + H_2(g) + heat \)
Alternative solution:
Observation: The vessel becomes warm.
Reason: Because it is an exothermic reaction.
(b) – No, because metal has reacted with acid to form salt solution.
– The metal is present as a part of a salt solution.
(c) Metal dissolving in acid is a chemical change while sugar dissolving in water is a physical change. In chemical change, metal cannot be recovered whereas in sugar solution, sugar can be obtained by crystallisation.

Question. 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. 
Answer: (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, acid is added to water very slowly with cooling.
(e) Ammonia dissolves in water forms \( OH^- \). Therefore, it is basic.
\( NH_3 + H_2O \rightarrow NH_4^+ + OH^- \)

Question. (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?
Answer: (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 \( (NaHCO_3) \).
(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.