CBSE Class 10 Science HOTs Acids Bases And Salts Set 04

Understanding the Chemical Properties of Acids and Bases

Question. When sodium bicarbonate reacts with dilute hydrochloric acid, the gas evolved is
(a) Hydrogen; it gives pop sound with burning matchstick.
(b) Hydrogen; it turns lime water milky.
(c) Carbon dioxide; it turns lime water milky.
(d) Carbon dioxide; it blows off a burning match stick with a pop sound.
Answer: (c) Carbon dioxide; it turns lime water milky.

Question. Acid present in tomato is :
(a) Methanoic acid
(b) Acetic acid
(c) Lactic acid
(d) Oxalic acid
Answer: (d) Oxalic acid

Question. Select a pair of olfactory indicators from the following:
(a) Clove oil and vanilla essence
(b) Onion and turmeric
(c) Clove oil and litmus paper
(d) Vanilla and methyl orange
Answer: (a) Clove oil and vanilla essence

Question. Which of the options in the given table are correct?
Option | Natural Source | Acid Present
(i) Orange | Oxalic acid
(ii) Sour milk | Lactic acid
(iii) Ant sting | Methanoic acid
(iv) Tamarind | Acetic acid
(a) (i) and (ii)
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (iii) and (iv)
Answer: (c) (ii) and (iii)

Question. Three test tubes A, B and C contain distilled water, an acidic solution and a basic solution respectively. When red litmus solution is used for testing these solutions, the observed colour changes respectively will be
(a) A – no change; B – becomes dark red ; C – becomes blue
(b) A – becomes light red; B – becomes blue; C – becomes red
(c) A – becomes red; B – no change; C – becomes blue
(d) A – becomes light red; B – becomes dark red; C – becomes blue
Answer: (d) A – becomes light red; B – becomes dark red; C – becomes blue

Question. A visually challenged student, has to perform a lab test to detect the presence of acid in a given solution. The acid-base indicator preferred by him will be :
(a) Blue litmus
(b) Clove oil
(c) Red cabbage extract
(d) Hibiscus extract
Answer: (b) Clove oil

Question. Which one of the following natural sources contains Oxalic acid ?
(a) Tomato
(b) Tamarind
(c) Ant sting
(d) Nettle sting
Answer: (a) Tomato

VSA 

Question. State the purpose for which litmus is used in laboratories.
Answer: Litmus is used as an acid-base indicator to determine whether a given solution is acidic or basic in nature. It turns red in acidic medium and blue in basic medium.

SA I 

Question. A student took a small amount of copper oxide in a conical flask and added dilute hydrochloric acid to it with constant stirring. He observed a change in colour of the solution.
(i) Write the name of the compound formed and its colour.
(ii) Write a balanced chemical equation for the reaction involved.
Answer: (i) The compound formed is copper(II) chloride and its colour is blue-green.
(ii) \( CuO_{(s)} + 2HCl_{(aq)} \rightarrow CuCl_{2(aq)} + H_2O_{(l)} \)

Question. Blue litmus solution is added to two test tubes A and B containing dilute HCl and NaOH solution respectively. In which test tube a colour change will be observed? State the colour change and give its reason.
Answer: A colour change will be observed in test tube A containing dilute HCl. The blue litmus solution will turn red because acids turn blue litmus red. No colour change will be observed in test tube B as bases do not change the colour of blue litmus.

Question. With the help of an example, explain what happens when a base reacts with a non-metallic oxide. What do you infer about the nature of non-metal oxide?
Answer: When a base reacts with a non-metallic oxide, it forms salt and water. For example, calcium hydroxide reacts with carbon dioxide to form calcium carbonate and water.
\( Ca(OH)_2 + CO_2 \rightarrow CaCO_3 + H_2O \)
Since this is a neutralisation reaction between a base and an oxide to produce salt and water, we infer that non-metallic oxides are acidic in nature.

Question. What is observed when carbon dioxide gas is passed through lime water
(i) for a short duration?
(ii) for a long duration? Also write the chemical equations for the reactions involved.
Answer: (i) For a short duration, lime water turns milky due to the formation of insoluble calcium carbonate.
\( Ca(OH)_{2(aq)} + CO_{2(g)} \rightarrow CaCO_{3(s)} + H_2O_{(l)} \)
(ii) For a long duration, the milkiness disappears due to the formation of soluble calcium hydrogen carbonate.
\( CaCO_{3(s)} + H_2O_{(l)} + CO_{2(g)} \rightarrow Ca(HCO_3)_{2(aq)} \)

SA II 

Question. (a) Write a balanced equation to show the reaction that occurs when a piece of aluminium is dipped in a dilute solution of (i) sulphuric acid and (ii) sodium hydroxide
(b) Write the colour of the solution formed when copper oxide is treated with hydrochloric acid. Give reason for this observation.
Answer: (a) (i) \( 2Al_{(s)} + 3H_2SO_{4(dil)} \rightarrow Al_2(SO_4)_{3(aq)} + 3H_{2(g)} \)
(ii) \( 2Al_{(s)} + 2NaOH_{(aq)} + 2H_2O_{(l)} \rightarrow 2NaAlO_{2(aq)} + 3H_{2(g)} \)
(b) The colour of the solution is blue-green. This is because copper oxide reacts with hydrochloric acid to form copper(II) chloride salt which is blue-green in colour.

Question. 2 mL of sodium hydroxide solution is added to a few pieces of granulated zinc metal taken in a test tube. When the content 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: The equation is: \( Zn_{(s)} + 2NaOH_{(aq)} \xrightarrow{\Delta} Na_2ZnO_{2(aq)} + H_{2(g)} \)
Test: When a burning matchstick is brought near the gas bubbles, it burns with a 'pop' sound, confirming it is hydrogen gas. When the same metal reacts with a dilute strong acid, hydrogen gas is evolved.

Question. OR
Write the names of the product formed when zinc reacts with NaOH. Also write the balanced chemical equation for the reaction involved. Write a test to confirm the presence of the gas evolved during this reaction.
Answer: The products formed are sodium zincate and hydrogen gas.
Equation: \( Zn_{(s)} + 2NaOH_{(aq)} \rightarrow Na_2ZnO_{2(aq)} + H_{2(g)} \)
Test: Bring a burning splinter near the evolved gas; it will burn with a pop sound, indicating the presence of hydrogen gas.

LA 

Question. A cloth strip dipped in onion juice is used for testing a liquid ‘X’. The liquid ‘X’ changes its odour. Which type of an indicator is onion juice? The liquid ‘X’ turns blue litmus red. List the observations the liquid ‘X’ will show on reacting with the following :
(a) Zinc granules
(b) Solid sodium carbonate
Write the chemical equations for the reactions involved.
Answer: Onion juice is an olfactory indicator. Since liquid 'X' turns blue litmus red, it is an acid.
(a) With Zinc granules: Evolution of hydrogen gas which burns with a pop sound.
\( Zn + 2HX \rightarrow ZnX_2 + H_2 \)
(b) With Solid sodium carbonate: Brisk effervescence due to the evolution of carbon dioxide gas which turns lime water milky.
\( Na_2CO_3 + 2HX \rightarrow 2NaX + H_2O + CO_2 \)

What Do All Acids and All Bases have in Common?

Question. Sodium hydroxide is termed as alkali while ferric hydroxide is not because :
(a) Sodium hydroxide is a strong base, while ferric hydroxide is a weak base.
(b) Sodium hydroxide is a base which is soluble in water while ferric hydroxide is also a base but it is not soluble in water.
(c) Sodium hydroxide is a strong base while ferric hydroxide is a strong acid.
(d) Sodium hydroxide and ferric hydroxide both are strong base but the solubility of sodium hydroxide in water is comparatively higher than that of ferric hydroxide.
Answer: (b) Sodium hydroxide is a base which is soluble in water while ferric hydroxide is also a base but it is not soluble in water.

Question. Hydronium ions are formed by the reaction between
(a) sodium hydroxide and water
(b) calcium chloride and water
(c) hydrogen chloride gas and water
(d) ethanol and water.
Answer: (c) hydrogen chloride gas and water

Question. Concentrated \( H_2SO_4 \) is diluted by adding drop by drop
(a) water to acid with constant stirring
(b) acid to water with constant stirring
(c) water to acid followed by a base
(d) base to acid followed by cold water.
Answer: (b) acid to water with constant stirring

Question. Consider the following compounds
I. HCl II. \( C_2H_5OH \)
III. \( C_6H_{12}O_6 \) IV. \( H_2SO_4 \)
Which of these compounds do not conduct electricity in solution?
(a) I and II
(b) II and III
(c) III and IV
(d) I and IV
Answer: (b) II and III

SA I 

Question. On diluting an acid, it is advised to add acid to water and not water to acid. Explain why it is so advised?
Answer: The process of dissolving an acid in water is highly exothermic. If water is added to concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. Therefore, acid is always added to water dropwise with constant stirring.

SA II 

Question. (i) Suggest a safe procedure of diluting a strong concentrated acid.
(ii) Name the salt formed when sulphuric acid is added to sodium hydroxide and write its pH.
(iii) Dry HCl gas does not change the colour of dry blue litmus paper. Why?
Answer: (i) Add concentrated acid slowly into water with constant stirring.
(ii) Sodium sulphate (\( Na_2SO_4 \)). Its pH is approximately 7 as it is a salt of a strong acid and a strong base.
(iii) Acidic behaviour of a substance is due to the presence of \( H^+ \) ions. HCl gas dissociates into ions only in the presence of water. In dry state, no \( H^+ \) ions are produced, so it does not show acidic properties.

Question. (i) Draw a labelled diagram to show the preparation of hydrogen chloride gas in laboratory.
(ii) Test the gas evolved first with dry and then with wet litmus paper. In which of the two cases, does the litmus paper show change in colour?
(iii) State the reason of exhibiting acidic character by dry HCl gas/HCl solution.
Answer: (ii) The colour change is observed with wet blue litmus paper which turns red.
(iii) HCl solution exhibits acidic character because it dissociates into hydronium ions (\( H_3O^+ \)) in aqueous solution. Dry HCl gas does not show acidic character as it lacks moisture required for ionisation.

Question. Complete and balance the following chemical equations :
(i) \( NaOH_{(aq)} + Zn_{(s)} \rightarrow \)
(ii) \( CaCO_{3(s)} + H_2O_{(l)} + CO_{2(g)} \rightarrow \)
(iii) \( HCl_{(aq)} + H_2O_{(l)} \rightarrow \)
Answer: (i) \( 2NaOH_{(aq)} + Zn_{(s)} \rightarrow Na_2ZnO_{2(aq)} + H_{2(g)} \)
(ii) \( CaCO_{3(s)} + H_2O_{(l)} + CO_{2(g)} \rightarrow Ca(HCO_3)_{2(aq)} \)
(iii) \( HCl_{(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)} + Cl^-_{(aq)} \)

Question. How the following substances will dissociate to produce ions in their solutions?
(i) Hydrochloric acid (ii) Nitric acid
(iii) Sulphuric acid (iv) Sodium hydroxide
(v) Potassium hydroxide (vi) Magnesium hydroxide
Answer: (i) \( HCl \rightarrow H^+ + Cl^- \)
(ii) \( HNO_3 \rightarrow H^+ + NO_3^- \)
(iii) \( H_2SO_4 \rightarrow 2H^+ + SO_4^{2-} \)
(iv) \( NaOH \rightarrow Na^+ + OH^- \)
(v) \( KOH \rightarrow K^+ + OH^- \)
(vi) \( Mg(OH)_2 \rightarrow Mg^{2+} + 2OH^- \)

LA 

Question. (a) Illustrate an activity to investigate whether all compounds containing hydrogen are acidic.
(b) What happens when hydrochloric acid and sodium hydroxide are dissolved in water? Explain by giving equation of each.
Answer: (a) Set up a circuit as shown in the experimental setup. Test solutions like glucose, alcohol, HCl and NaOH. The bulb glows for HCl and NaOH solutions because they ionise and conduct electricity, indicating acidic/basic nature. The bulb does not glow for glucose and alcohol, showing they are not acidic even though they contain hydrogen.
(b) HCl dissociates into hydronium and chloride ions:
\( HCl + H_2O \rightarrow H_3O^+ + Cl^- \)
NaOH dissociates into sodium and hydroxide ions:
\( NaOH \xrightarrow{H_2O} Na^+ + OH^- \)

How Strong are Acid or Base Solutions?

Question. Fresh milk has a pH of 6. To delay its curdling, a chemical substance is added to it, which is
(a) sodium carbonate
(b) baking powder
(c) sodium hydroxide (caustic soda)
(d) baking soda (sodium hydrogen carbonate).
Answer: (d) baking soda (sodium hydrogen carbonate).

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. Consider the pH value of the following acidic samples.
S.No. Sample pH Value
1. Lemon Juice 2.2
2. Gastric Juice 1.2
3. Vinegar 3.76
4. Dil. Acetic acid 3.0
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. A solution gives yellowish orange colour when a few drops of universal indicator are added to it. This solution is of
(a) lemon juice
(b) sodium chloride
(c) sodium hydroxide
(d) milk of magnesia.
Answer: (a) lemon juice

Question. An aqueous solution ‘A’ turns phenolphthalein solution pink. On addition of an aqueous solution ‘B’ to ‘A’ the pink colour disappears. The following statement is true for solution ‘A’ and ‘B’ :
(a) A is strongly basic and B is a weak base.
(b) A is strongly acidic and B is a weak acid.
(c) A has pH greater than 7 and B has pH less than 7.
(d) A has pH less than 7 and B has pH greater than 7.
Answer: (c) A has pH greater than 7 and B has pH less than 7.

Question. The acid produced in our stomach during digestion of food is
(a) Hydrochloric acid
(b) Oxalic acid
(c) Lactic acid
(d) Acetic acid
Answer: (a) Hydrochloric acid

SA I

Question. Two solutions M and N give red and blue colour respectively with a universal indicator.
(i) In which solution will the hydrogen ion concentration be more? Justify your answer.
(ii) If both M and N solutions are mixed and the resultant mixture is tested with a universal indicator, it turns green. What is the nature of the salt formed? Justify your answer.
Answer: (i) Solution M will have more hydrogen ion concentration as it shows red colour on pH scale which indicates it is highly acidic.
(ii) The salt formed is neutral because the mixture of an acid and a base results in a neutral solution (pH 7), which turns universal indicator green.

Question. Out of the two hydrochloric acid and acetic acid, which one is considered as strong acid and why ? Write the name/molecular formula of one more strong acid.
Answer: Hydrochloric acid is a strong acid because it completely ionises in aqueous solution. Nitric acid (\( HNO_3 \)) or Sulphuric acid (\( H_2SO_4 \)) are other examples of strong acids.

Question. Out of HCl and \( CH_3COOH \), which one is a weak acid and why? Explain with the help of an example.
Answer: \( CH_3COOH \) is a weak acid because it only partially ionises in water, producing a low concentration of \( H^+ \) ions. For example, a 0.1 M solution of acetic acid has a higher pH than a 0.1 M solution of HCl.

Question. Explain how an antacid works.
Answer: During indigestion, the stomach produces too much hydrochloric acid, causing pain and irritation. Antacids are mild bases (like magnesium hydroxide) that react with and neutralise the excess acid, providing relief.

SA II 

Question. (a) You are provided with concentrated sulphuric acid. Describe the process of preparing a dilute solution of sulphuric acid.
(b) What is the effect of dilution on (\( H_3O^+/OH^- \)) ratio ?
(c) If the \( H_3O^+ \) ion concentration is increased in a solution, will the pH increase or decrease ? What are the probable colours of pH paper if the pH range is 0·5 to 2·0 ?
Answer: (a) Add concentrated acid drop by drop into water while stirring constantly.
(b) On dilution, the concentration of ions per unit volume decreases.
(c) The pH will decrease as \( H_3O^+ \) ion concentration increases. The probable colours will be red or dark orange.

Question. (a) What does pH scale measure ?
(b) Write its range.
(c) State the significance of highest and lowest values of pH scale.
Answer: (a) It measures the hydrogen ion concentration in a solution.
(b) Its range is 0 to 14.
(c) Low pH (0-2) indicates high acidity. High pH (12-14) indicates high basicity.

Question. (a) Three acidic solutions A, B and C have pH = 0, 3 and 5 respectively.
(i) Which solution has highest concentration of \( H^+ \) ions?
(ii) Which solution has the lowest concentration of \( H^+ \) ions?
(b) How concentrated sulphuric acid can be diluted? Describe the process.
Answer: (a) (i) Solution A (pH 0). (ii) Solution C (pH 5).
(b) Concentrated acid should be added to water dropwise with constant stirring to dissipate the heat generated.

Question. A compound P forms the enamel of teeth. It is the hardest substance of the body. It doesn’t dissolve in water but gets corroded when the pH is lowered below 5.5.
(a) Identify the compound P.
(b) How does it undergo damage due to eating chocolate and sweets? What should we do to prevent tooth decay?
Answer: (a) Compound P is calcium phosphate.
(b) Bacteria in the mouth produce acid by degradation of sugar and food particles. When pH falls below 5.5, tooth enamel is corroded. Brushing teeth twice daily with toothpaste (which is basic) helps neutralise excess acid and prevents decay.