Get summaries, questions, answers, solutions, notes, extras, PDF and guides for Chapter 2 Acid, Bases and Salts: Class 10 Science textbook, which is part of the syllabus for students studying under SEBA (Assam Board), NBSE (Nagaland Board), TBSE (Tripura Board), CBSE (Central Board), MBOSE (Meghalaya Board), BSEM (Manipur Board), WBBSE (West Bengal Board), and all other boards following the NCERT books. These solutions, however, should only be treated as references and can be modified/changed.
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Summary
This chapter introduces acids, bases, and salts. Acids generally have a sour taste, like lemons. Bases often taste bitter and feel slippery, like soap. Since tasting unknown chemicals is unsafe, special substances called indicators are used. Indicators change their color to show if a substance is an acid or a base. For example, litmus paper turns red in an acid and blue in a base. Turmeric, a common yellow spice, turns reddish-brown when it comes into contact with a base. Some plants, like red cabbage, also have parts that can act as indicators.
Acids and bases show distinct chemical behaviors. When acids react with many metals, such as zinc, they produce hydrogen gas. This gas can make a “pop” sound if a flame is brought near it. Bases can also react with certain metals to produce hydrogen. When acids react with substances known as metal carbonates or metal hydrogencarbonates, like baking soda, they release carbon dioxide gas. This is the same gas that creates bubbles in fizzy drinks and can make limewater, a clear solution, turn milky.
A significant reaction occurs when acids and bases are mixed: they neutralize each other. This means they cancel out each other’s properties, forming a salt and water. For example, if a person has too much acid in their stomach, an antacid, which is a base, can help neutralize the excess acid. Compounds of metals with oxygen, called metallic oxides, usually behave like bases. Compounds of non-metals with oxygen, known as non-metallic oxides, generally behave like acids.
When dissolved in water, acids produce hydrogen ions (H⁺). These ions are responsible for the acidic properties of the solution. Bases, when dissolved in water, produce hydroxide ions (OH⁻), which cause the basic properties. Solutions containing these ions can conduct electricity. The strength of an acid or a base is determined by the number of H⁺ or OH⁻ ions it produces. A pH scale, ranging from 0 to 14, is used to measure this strength. A pH of 7 is neutral, like pure water. A pH value below 7 indicates an acidic solution, with lower numbers meaning stronger acidity. A pH value above 7 indicates a basic solution, with higher numbers meaning stronger basicity. Living organisms and the environment are sensitive to pH levels. For instance, acid rain can harm fish and other aquatic life. Tooth decay can begin if the pH in our mouth becomes too low.
Salts are compounds typically formed from the reaction between an acid and a base. Common table salt, or sodium chloride, is a familiar example. Salts can be neutral, acidic, or basic, depending on the strengths of the acid and base that created them. Some salts have water molecules incorporated into their crystal structure; this is called water of crystallization. For example, blue copper sulphate crystals contain this water. If these crystals are heated, the water is removed, and they turn white. Plaster of Paris is another useful salt that hardens when mixed with water and is used for making casts for broken bones. Common salt also serves as a raw material for producing many important chemicals, such as sodium hydroxide (used in soap manufacturing), baking soda (used in cooking and as an antacid), and washing soda (used for cleaning).
Textbook solutions
Intext Questions and Answers I
1. You have been provided with three test tubes. One of them contains distilled water and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube?
Answer: If the colour of red litmus does not change then it is acid. If the colour of red litmus changes to blue then it is base. If there is a slight change in the colour of red litmus (such as purple) then it is distilled water.
Intext Questions and Answers II
1. Why should curd and sour substances not be kept in brass and copper vessels?
Answer: Sour substances contain acids. The reaction of a metal with an acid can be summarised as Acid + Metal → Salt + Hydrogen gas. Thus, sour substances react with brass and copper vessels, which are metals, to form salt and hydrogen gas.
2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Answer: When an acid reacts with a metal, hydrogen gas is usually liberated. For example, when dilute sulphuric acid reacts with zinc granules, hydrogen gas is formed. To test for the presence of this gas, one can take a burning candle near a gas filled bubble, and it burns with a pop sound.
3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Answer: Metal carbonates and hydrogen carbonates react with acids to give a corresponding salt, carbon dioxide and water. The gas evolved, carbon dioxide, causes effervescence and extinguishes a burning candle. If one of the compounds formed is calcium chloride, the metal compound A is calcium carbonate. The balanced chemical equation for the reaction is CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + CO₂(g) + H₂O(l)
Intext Questions and Answers III
1. Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?
Answer: Acids like HCl and HNO₃ show acidic characters in aqueous solutions because they produce hydrogen ions, H+(aq), or hydronium ions (H₃O+), in solution, which are responsible for their acidic properties. The separation of H+ ions from acid molecules cannot occur in the absence of water. Compounds such as alcohol and glucose also contain hydrogen but do not produce hydrogen ions in aqueous solution, and therefore they do not show acidic character.
2. Why does an aqueous solution of an acid conduct electricity?
Answer: An aqueous solution of an acid conducts electricity because the electric current is carried through the solution by ions. Acids produce hydrogen ions (H+ or H₃O+) and corresponding anions when dissolved in water, and these ions are responsible for the conduction of electricity.
3. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer: Dry HCl gas does not change the colour of dry litmus paper because the acidic nature of a substance is due to the formation of H+(aq) ions in solution. The separation of H+ ions from HCl molecules cannot occur in the absence of water. Therefore, dry HCl gas does not produce the necessary ions to show acidic behaviour and change the litmus paper colour.
4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer: While diluting an acid, it is recommended that the acid should be added slowly to water with constant stirring, and not water to the acid. This is because the process of dissolving an acid in water is a highly exothermic one. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns, and the glass container may also break due to excessive local heating.
5. How is the concentration of hydronium ions (H3O+) affected when a solution of an acid is diluted?
Answer: When a solution of an acid is diluted, mixing the acid with water results in a decrease in the concentration of hydronium ions (H3O+ or H+(aq)) per unit volume.
6. How is the concentration of hydroxide ions (OH-) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer: When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide ions (OH-) in the solution increases, as bases generate hydroxide ions in water, and adding more base adds more of these ions
Intext Questions and Answers IV
1. You have two solutions, A and B. The pH of solution A is 6 and pH of solution B is 8. Which solution has more hydrogen ion concentration? Which of this is acidic and which one is basic?
Answer: On the pH scale, a higher hydronium ion concentration results in a lower pH value. Values less than 7 on the pH scale represent an acidic solution, while as the pH value increases from 7 to 14, it represents an increase in OH- ion concentration, indicating an increase in the strength of the alkali or base. Therefore, solution A with a pH of 6 has more hydrogen ion concentration and is acidic, while solution B with a pH of 8 is basic.
2. What effect does the concentration of H+ (aq) ions have on the nature of the solution
Answer: The concentration of H+(aq) ions has a significant effect on the nature of the solution. It is stated that acids produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties. The strength of acids depends on the number of H+ ions produced. Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids. A higher concentration of hydronium ions also leads to a lower pH value, which indicates a more acidic solution.
3. Do basic solutions also have H+ (aq) ions? If yes, then then why are these basic?
Answer: Yes, basic solutions also have H+(aq) ions. However, it is the formation of OH-(aq) ions in solution that is responsible for the basic nature of a substance. While both H+(aq) and OH-(aq) ions are present in aqueous solutions, in basic solutions, the concentration of OH-(aq) ions is higher than the concentration of H+(aq) ions. As the pH value increases from 7 to 14, representing a basic solution, there is an increase in OH- ion concentration and a decrease in H+ ion concentration.
4. Under what soil condition do you think a farmer would treat the soil of his fields with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)?
Answer: Plants require a specific pH range for their healthy growth. Quick lime (calcium oxide) is a metallic oxide, which is said to be basic in nature. Slaked lime (calcium hydroxide) is a base, and chalk (calcium carbonate) is a metal carbonate. Metal oxides and metal carbonates react with acids. Therefore, a farmer would likely treat the soil of his fields with quick lime, slaked lime, or chalk when the soil is acidic, in order to neutralise the excess acid and adjust the pH to a range suitable for plant growth.
Intext Questions and Answers V
1. What is the common name of the compound CaOC₁₂?
Answer: The common name of the compound CaOC₁₂ is bleaching powder.
2. Name the substance which on treatment with chlorine yields bleaching powder.
Answer: The substance which on treatment with chlorine yields bleaching powder is dry slaked lime..
3. Name the sodium compound which is used for softening hard water.
Answer: The sodium compound which is used for softening hard water is washing soda.
4. What will happen if a solution of sodium hydrocarbonate is heated? Give the equation of the reaction involved.
Answer: If a solution of sodium hydrogen carbonate is heated, it decomposes to form sodium carbonate, water, and carbon dioxide. The equation of the reaction involved is 2NaHCO₃ Heat → Na₂CO₃ + H₂O + CO₂.
5. Write an equation to show the reaction between Plaster of Paris and water.
Answer: The equation to show the reaction between Plaster of Paris and water is CaSO₄.½H₂O + 1½H₂O → CaSO₄.2H₂O..
Exercise Questions and Answers
1. A solution turns red litmus blue, its pH is likely to be
(a) 1 (b) 4 (c) 5 (d) 10
Answer: (d) 10
Explanation: A solution that turns red litmus blue is basic. Bases change the colour of red litmus to blue. The pH scale measures pH generally from 0 (very acidic) to 14 (very alkaline). Values less than 7 represent an acidic solution, while a pH of 7 is neutral. As the pH value increases from 7 to 14, it represents an increase in OH- ion concentration, meaning an increase in the strength of alkali. Therefore, a solution turning red litmus blue is likely to have a pH of (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
Answer: (b) HCl
Explanation: Crushed egg-shells are primarily calcium carbonate. Metal carbonates react with acids to give a corresponding salt, carbon dioxide, and water. The carbon dioxide gas produced turns lime water (calcium hydroxide solution) milky due to the formation of a white precipitate of calcium carbonate. Among the given options, HCl (hydrochloric acid) is an acid. Therefore, the solution contains (b) HCl.
3. 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be
(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL
Answer: (d) 16 mL
Explanation: The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. If 10 mL of NaOH solution is completely neutralised by 8 mL of HCl solution, this establishes a specific ratio for neutralisation between these two solutions. If we take 20 mL of the same NaOH solution (which is double the initial amount), then double the amount of the same HCl solution will be required to neutralise it. Therefore, the amount of HCl solution required will be 2 × 8 mL = 16 mL. So the answer is (d) 16 mL.
4. Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic
Answer: (c) Antacid
Explanation: During indigestion, the stomach produces too much acid, which causes pain and irritation. To get rid of this pain, people use bases called antacids. These antacids neutralise the excess acid. Therefore, the type of medicine used for treating indigestion is (c) Antacid.
5. Write word equations and then balanced equations for the reaction taking place when –
(a) dilute sulphuric acid reacts with zinc granules.
(b) dilute hydrochloric acid reacts with magnesium ribbon.
(c) dilute sulphuric acid reacts with aluminium powder.
(d) dilute hydrochloric acid reacts with iron filings.
Answer: The reaction of a metal with an acid can be summarised as: Acid + Metal → Salt + Hydrogen gas. In these reactions, the metal displaces hydrogen atoms from the acids as hydrogen gas and forms a compound called a salt.
(a) dilute sulphuric acid reacts with zinc granules.
Word equation: Dilute sulphuric acid + Zinc granules → Zinc sulphate + Hydrogen gas
Balanced equation: H₂SO₄(aq) + Zn(s) → ZnSO₄ + H₂
(b) dilute hydrochloric acid reacts with magnesium ribbon.
Word equation: Dilute hydrochloric acid + Magnesium ribbon → Magnesium chloride + Hydrogen gas
Balanced equation: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)
(c) dilute sulphuric acid reacts with aluminium powder.
Word equation: Dilute sulphuric acid + Aluminium powder → Aluminium sulphate + Hydrogen gas
Balanced equation: 3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)
(d) dilute hydrochloric acid reacts with iron filings.
Word equation: Dilute hydrochloric acid + Iron filings → Iron(II) chloride + Hydrogen gas
Balanced equation: 2HCl(aq) + Fe(s) → FeCl₂(aq) + H₂(g)
6. Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.
Answer: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. To prove this, one can perform an activity. Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, etc. Fix two nails on a cork, and place the cork in a 100 mL beaker. Connect the nails to the two terminals of a 6 volt battery through a bulb and a switch, as shown in Figure 2.3. Now pour some dilute HCl in the beaker and switch on the current. Repeat with dilute sulphuric acid. Then repeat the experiment separately with glucose and alcohol solutions. It is observed that the bulb starts glowing in the case of acids like HCl and sulphuric acid, indicating a flow of electric current through the solution carried by ions. However, glucose and alcohol solutions do not conduct electricity, meaning they do not produce ions in solution. Acids contain H⁺ ion as cation and anion, and the cation present in acids is H⁺, which suggests that acids produce hydrogen ions, H⁺(aq), in solution, which are responsible for their acidic properties and conductivity. Since glucose and alcohol solutions do not conduct electricity, they do not produce these hydrogen ions in solution and are therefore not categorised as acids, despite containing hydrogen.
7. Why does distilled water not conduct electricity, whereas rain water does?
Answer: The flow of electric current through a solution is carried by ions. Acidic and basic solutions in water conduct electricity because they produce hydrogen and hydroxide ions respectively. Distilled water is pure water and does not contain significant amounts of dissolved substances that would produce ions. Rain water, however, is not pure; it dissolves gases from the atmosphere and may pick up other impurities. These dissolved substances can dissociate in water to form ions. Therefore, rain water conducts electricity because it contains ions, while distilled water does not conduct electricity because it lacks significant ions
8. Why do acids not show acidic behaviour in the absence of water?
Answer: Hydrogen ions in acids are produced in the presence of water. The separation of H+ ion from acid molecules cannot occur in the absence of water. The cation present in acids is H+, and acids produce hydrogen ions, H+(aq), in solution, which are responsible for their acidic properties. Therefore, acids do not show acidic behaviour in the absence of water because the hydrogen ions responsible for acidic properties are produced only when the acid is in a water solution
9. Five solutions A,B,C,D and E when tested with universal indicator showed pH as 4,1,11,7 and 9, respectively. Which solution is
(a) neutral?
(b) strongly alkaline?
(c) strongly acidic?
(d) weakly acidic?
(e) weakly alkaline?
Arrange the pH in increasing order of hydrogen-ion concentration.
Answer: Based on the pH values, solution D with a pH of 7 is neutral. Solution C with a pH of 11 is strongly alkaline. Solution B with a pH of 1 is strongly acidic. Solution A with a pH of 4 is weakly acidic. Solution E with a pH of 9 is weakly alkaline. The pH values in increasing order of hydrogen-ion concentration are 11, 9, 7, 4, 1, corresponding to solutions C, E, D, A, B.
10. 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: The fizzing will occur more vigorously in test tube A where hydrochloric acid is added. This is because hydrochloric acid is a strong acid, which means it gives rise to more hydrogen ions in solution compared to acetic acid, which is a weak acid and gives less hydrogen ions, even when the amount and concentration of both acids are the same. The rate of reaction with the metal depends on the concentration of hydrogen ions, leading to more vigorous evolution of hydrogen gas with the stronger acid.
11. Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.
Answer: Fresh milk has a pH of 6. As milk turns into curd, lactic acid is formed due to the action of bacteria. Acids lower the pH of a solution. Therefore, the pH of the milk will decrease as it turns into curd.
12. A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
Answer: (a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline by adding baking soda because baking soda is a mild basic salt. Making the milk slightly alkaline helps to prevent it from turning sour quickly due to the formation of lactic acid by bacteria.
(b) This milk takes a long time to set as curd because the bacteria responsible for curdling produce lactic acid. Curdling occurs when the pH drops sufficiently due to the accumulation of lactic acid. By making the milk slightly alkaline, the milkman increases the initial pH, so the bacteria need to produce more lactic acid over a longer period to reach the pH required for curdling.
13. Plaster of Paris should be stored in a moisture-proof container. Explain why?
Answer: Plaster of Paris is calcium sulphate hemihydrate. It should be stored in a moisture-proof container because it reacts with water to form gypsum, which is a hard solid mass. Contact with moisture would cause it to set into a hard mass, making it unusable.
14. What is a neutralisation reaction? Give two examples.
Answer: The reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. For example, sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water. Calcium hydroxide reacts with carbon dioxide to produce calcium carbonate and water.
15. Give two important uses of washing soda and baking soda.
Answer: Two important uses of washing soda are that it is used in glass, soap, and paper industries, and it is used as a cleaning agent for domestic purposes. Two important uses of baking soda are that it is used for making baking powder, and it is used as an ingredient in antacids.
Extras
Additional MCQs (Knowledge Based)
1. If someone is suffering from acidity after overeating, which of the following would be a suitable remedy?
A. Lemon juice
B. Vinegar
C. Baking soda solution
D. Orange juice
Answer: C. Baking soda solution
37. Which of the following is a strong acid?
A. Acetic acid
B. Citric acid
C. Carbonic acid
D. Hydrochloric acid
Answer: D. Hydrochloric acid
Additional MCQs (Competency Based)
1. Assertion (A): When a piece of zinc metal is added to dilute sulphuric acid, a gas is evolved which burns with a pop sound.
Reason (R): Metals that are more reactive than hydrogen displace hydrogen from dilute acids, forming a salt and hydrogen gas.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
19. A solution turns red litmus paper blue. Its pH is likely to be:
(a) 1
(b) 4
(c) 5
(d) 10
Answer: (d) 10
Additional Questions and Answers
1. What colour change does blue litmus paper undergo in an acidic solution?
Answer: Acids change the colour of blue litmus to red.
31. Compare the reactions of acids with metals, metal carbonates and metallic oxides.
Answer: Acids react differently with metals, metal carbonates, and metallic oxides.
- Reaction of Acids with Metals:
- When an acid reacts with a metal, the metal displaces hydrogen atoms from the acid as hydrogen gas, and a compound called a salt is formed.
- The general equation is: Acid + Metal → Salt + Hydrogen gas
- For example, zinc granules react with dilute sulphuric acid to produce zinc sulphate and hydrogen gas.
- Reaction of Acids with Metal Carbonates and Metal Hydrogencarbonates:
- All metal carbonates and metal hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide gas, and water.
- The general equation is: Metal carbonate/Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water
- For example, sodium carbonate reacts with dilute hydrochloric acid to produce sodium chloride, carbon dioxide, and water.
- Reaction of Acids with Metallic Oxides:
- When an acid reacts with a metallic oxide, a salt and water are formed. This reaction is similar to the neutralisation reaction between an acid and a base, which indicates that metallic oxides are basic in nature.
- The general equation is: Metal oxide + Acid → Salt + Water
- For example, copper oxide (a metallic oxide) reacts with dilute hydrochloric acid to form copper(II) chloride (a salt) and water, and the solution becomes blue-green.
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