Class 10 Science - Chapter 2: Acids, Bases & Salts

Q1) 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?

Ans: Red litmus paper will turn blue in the basic solution. If it stays red, dilute a small amount of that solution with water and test again; if it remains red, it's the acidic solution. The remaining tube contains the distilled water.

Q1) Why should curd and sour substances not be kept in brass and copper vessels?

Ans: Acidic foods, like curd, should not be kept in brass or copper vessels because the acids react with these metals, producing potentially harmful metallic compounds that can contaminate the food.

Q2) 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?

Ans: When an acid reacts with any metal then salt and hydrogen gas are formed.

Metal + Acid ----> Salt + Hydrogen gas

Eg: When a burning candle is bought near hydrogen gas then it burns with a pop sound.

Q3) 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?

Ans: The metal compound ‘A’ is salt of calcium.

A metallic compound, designated as 'A', releases a gas when it comes into contact with dilute hydrochloric acid. This gas has the property of extinguishing a flame. Given that calcium chloride is a product of this reaction, and the gas that extinguishes flames is carbon dioxide, it can be concluded that 'A' is calcium carbonate.

Balanced equation :-

CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

(Calcium Carbonate)      (Calcium Chloride)

Q1) Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show acidic character?

Ans: Substances such as hydrochloric acid (HCl) and nitric acid (HNO₃) exhibit acidic properties when dissolved in water because they dissociate, or break apart, to form hydrogen ions (H⁺). These hydrogen ions are responsible for the acidic nature of the solutions. Conversely, compounds like alcohol and glucose, when dissolved in water, do not produce hydrogen ions. Therefore, their aqueous solutions do not display acidic characteristics.

Q2) Why does an aqueous solution of an acid conduct electricity?

Ans: When acids are dissolved in water, they break down into charged particles, known as ions. These ions, being mobile within the solution, allow for the flow of electrical current.

Q3) Why does dry HCl gas not change the colour of the dry litmus paper?

Ans: The color change of litmus paper occurs due to the presence of hydrogen ions in an aqueous environment. Dry hydrogen chloride (HCl) gas lacks the necessary water to facilitate the dissociation of HCl into these ions. Therefore, without the presence of hydrogen ions in solution, the dry litmus paper does not undergo a color change.

Q4) While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?

Ans: When diluting an acid, it's crucial to add the acid slowly to water, rather than the reverse. The process of acid dilution releases heat. Adding water to concentrated acid can cause a rapid and intense heat release, potentially leading to boiling and splashing, which poses a risk of chemical burns. Adding the acid to water allows for a more controlled heat dissipation, especially with continuous stirring.

Q5) How is the concentration of hydronium ions H₃O⁺ affected when a solution of an acid is diluted?

Ans: Diluting an acid reduces the number of hydronium ions (H₃O⁺) present within a given volume of the solution. Consequently, the concentration of H₃O⁺ decreases, which corresponds to a decrease in the acid's strength.

Q6) How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?

Ans: Adding more base to a sodium hydroxide solution increases the quantity of hydroxide ions (OH⁻) present, thus raising the concentration of OH⁻ in the solution.

Q1) 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?

Ans: Solution A, with a pH of 6, has a higher hydrogen ion concentration than solution B, which has a pH of 8. Solution A is acidic, and solution B is basic. A lower pH signifies a higher hydrogen ion concentration and an acidic solution, while a higher pH indicates a lower hydrogen ion concentration and a basic solution.

Q2) What effect does the concentration of H⁺ (aq) ions have on the nature of the solution?

Ans: A higher concentration of hydrogen ions (H⁺) makes a solution more acidic, while a lower concentration makes it more basic (or alkaline).

Q3) Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?

Ans: Yes, basic solutions do contain hydrogen ions (H⁺). However, the concentration of hydroxide ions (OH⁻) is significantly greater than the concentration of hydrogen ions in these solutions. This imbalance, with a surplus of OH⁻ ions, is what defines a solution as basic.

Q4) 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) ?

Ans: A farmer would use quicklime, slaked lime, or chalk to treat their fields when the soil is too acidic. These substances neutralize the excess acidity, making the soil more suitable for growing crops.

Q1) What is the common name of the compound CaOCl₂?

Ans: Bleaching powder.

Q2) Name the substance which on treatment with chlorine yields bleaching powder.

Ans: Slaked lime or calcium hydroxide [Ca(OH)₂] .

Q3) Name the sodium compound which is used for softening hard water.

Ans: Sodium carbonate is used for softening hard water.

Q4) What will happen if a solution of sodium hydro-carbonate is heated? Give the equation of the reaction involved.

Ans: Heating a solution of sodium bicarbonate (sodium hydrogen carbonate) causes it to decompose, producing sodium carbonate, water, and carbon dioxide gas. The chemical equation for this reaction is

2NaHCO₃(aq) + Heat → Na₂CO₃(aq) + H₂O(l) + CO₂(g)

Q5) Write an equation to show the reaction between Plaster of Paris and water.

Ans:

CaSO₄.½H₂O + 1½H₂O → CaSO₄.2H₂O

Q1) A solution turns red litmus blue, its pH is likely to be

1. 1
2. 4
3. 5
4. 10

Ans: (d) 10

Q2) A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains

1. NaCl
2. HCl
3. LiCl
4. KCl

Ans: (b) HCl.

Q3) 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

1. 4 mL
2. 8 mL
3. 12 mL
4. 16 mL

Ans: (d) 16 mL

Q4) Which one of the following types of medicines is used for treating indigestion?

1. Antibiotic
2. Analgesic
3. Antacid
4. Antiseptic

Ans: (c) Antacid is used for treating indigestion.

Q5) Write word equations and then balanced equations for the reaction taking place when -

1. dilute sulphuric acid reacts with zinc granules.
2. dilute hydrochloric acid reacts with magnesium ribbon.
3. dilute sulphuric acid reacts with aluminium powder.
4. dilute hydrochloric acid reacts with iron filings.

Ans:

(a) Zinc + Sulphuric acid ---> Zinc sulphate + Hydrogen

H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)

(b) Magnesium + Hydrochloric acid ----> Magnesium chloride + Hydrogen gas

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

(c) Aluminum + Sulphuric acid ----> Aluminum sulphate + Hydrogen gas

3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) Iron + Hydrochloric acid --> Iron chloride + Hydrogen

Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Q6) Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it.

Ans: Alcohol and glucose both contain hydrogen but are not categorized as acids, and this can be proved by an activity.

Material required :- Beaker, nails, battery, connecting wires, bulb, switch and alcohol.

Procedure : Do experiment as follows-

1. Take ethyl alcohol in the beaker.
2. When the switch is turned on you will observe that the bulb does not glow.
3. Now take glucose solution in place of alcohol but still bulb does not glow.

Q7) Why does distilled water not conduct electricity, whereas rain water does?

Ans: Pure water, obtained through distillation, lacks dissolved charged particles. Consequently, it does not facilitate the flow of electrical current. Conversely, naturally occurring rainwater gathers various dissolved substances, including those that form ions. These ions, being mobile charge carriers, enable the passage of electricity through the rainwater.

Q8) Why do acids not show acidic behaviour in the absence of water?

Ans: The characteristic acidic actions of a substance only become apparent when water is present. Without water, the process that generates the positively charged hydrogen components, which give rise to acidic traits, cannot occur. Therefore, in a dry environment, these substances do not display their typical acidic behavior.

Q9) 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

1. neutral?
2. strongly alkaline?
3. strongly acidic?
4. weakly acidic?
5. weakly alkaline?

Arrange the pH in increasing order of hydrogen-ion concentration

Ans:

• (a) Neutral : D (pH 7)
• (b) Strongly alkaline : C (pH 11)
• (c) Strongly acidic : B (pH 1)
• (d) Weakly acidic : A (pH 4)
• (e) Weakly alkaline : E (pH 9)

Increasing order of hydrogen-ion concentration (which means decreasing order of pH): C (pH 11), E (pH 9), D (pH 7), A (pH 4), B (pH 1).

Q10) 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?

Ans: Two test tubes, A and B, each contain an equal length of magnesium ribbon. Test tube A receives hydrochloric acid, while test tube B receives acetic acid, with both acids having the same volume and concentration. The reaction in test tube A will produce more noticeable bubbling, or fizzing. This is because hydrochloric acid is a strong acid, meaning it readily releases a greater quantity of hydrogen ions compared to acetic acid, which is a weak acid. The higher concentration of hydrogen ions in test tube A results in a faster and more vigorous reaction with the magnesium ribbon, leading to increased fizzing.

Q11) Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

Ans: When milk changes to curd, the pH will reduce because curd is acidic in nature and the acids present in it decreases the pH.

Q12) A milkman adds a very small amount of baking soda to fresh milk.

1. Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
2. Why does this milk take a long time to set as curd?

Ans:

(a) The milkman shifts the pH of the fresh milk from 6 to slightly alkaline because in alkaline condition, milk does not get curd easily.

(b) Since this milk is slightly basic than usual milk, so, acids produced to set the curd are neutralized by the base. Therefore, it takes a longer time for the curd to set.

Q13) Plaster of Paris should be stored in a moisture-proof container. Explain why?

Ans: Plaster of Paris reacts with moisture to form gypsum and gets into a hard mass. Therefore, it should be stored in moisture-proof container.

Q14) What is a neutralisation reaction? Give two examples.

Ans: A reaction in which the acid and base react to form a salt and water is called as neutralization reaction.

For Eg :-

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

Mg(OH)₂(aq) + 2HCl(aq) → MgCl₂(aq) + 2H₂O(l)

Q15) Give two important uses of washing soda and baking soda.

Ans:

1. Uses of washing soda :

• (i) Used as cleansing agent.
• (ii) Removing permanent hardness of water.

2. Uses of baking soda :

• (i) Used for making baking powder
• (ii) Used as ingredient of antacid.