Text Question & Answers
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?
Step-I: Dip the given red litmus paper in each of the three test tubes. The tube in which red litmus turns blue contains a basic solution. Now, we will use this blue litmus in next step.
Step-II: Dip this blue litmus paper now in the remaining two test tubes. The tube in which blue litmus again changes back into red contains an acidic solution.
Step-III: The tube in which neither red litmus nor blue litmus turns red contains distilled water.
Why should curd and sour substances not be kept in brass and copper vessels?
Curd and other sour foodstuffs contain acids, which can react with the metal of the vessel to form poisonous metal compounds, which can cause food poisoning and hence damage our health.
Therefore, it is advised not to store foodstuffs that contain acidic substances in brass or copper containers.
Also Check: Electric charges and field notes
Also Check: Class 10 Chemistry Acids bases and salts notes
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?
When acids react with metals, salts are produced with the liberation of hydrogen gas:
When gas is made to pass through a soap solution, soap bubbles are formed with H₂ gas in them.
Taking a burning candle near the bubbles, the gas burns with a pop sound proving that the gas inside is H₂, which is highly inflammable.
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.
The gas that extinguishes a burning candle is carbon dioxide, which is formed by the action of dilute hydrochloric acid on a metal carbonate and produces effervescence. Since one of the compounds formed is calcium chloride, it shows that the metal compound is calcium carbonate.
Thus, the metal compound A is calcium carbonate (CaCO₃). Calcium carbonate reacts with dilute hydrochloric acid to form calcium chloride, carbon dioxide, and water. This can be written as:
Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions, while solutions of compounds like alcohol and glucose do not show acidic character?
An acid is a substance that dissociates on dissolving in water to produce hydrogen ions ([H⁺(aq)]).
The substances like HCl, H₂SO₄, show acidic character because they dissociate in aqueous solutions to produce hydrogen ions ([H⁺(aq)]).
On the other hand, substances like alcohol (C₂H₅OH) and glucose (C₆H₁₂O₆) do not ionize to liberate H⁺ ions in their aqueous solutions, hence are not acidic in nature.
Why does an aqueous solution of acid conduct electricity?
An acid upon dissolving in water dissociates into its ions (cations and anions). When electrodes of different electrode potential are introduced in such an aqueous solution, the ions of acid get migrated towards oppositely charged electrodes, and it is due to this migration of ions of acidic solution which is responsible for the conduction of electricity.
Why does dry HCl gas not change the color of dry litmus paper?
HCl gas shows its acidic character only in an aqueous solution. This is because in an aqueous solution it completely ionizes to liberate H⁺ ions. Hence, changes the color of litmus paper.
However, when HCl gas is in dry form, it does not show acidic behavior because it does not ionize to liberate H⁺ ions. Since dry HCl gas does not contain any hydrogen ions in it and it does not show acidic behavior. That is why there is no change observed on litmus paper.
While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Dilution of concentrated acid with water is a highly exothermic process. Thus, diluting an acid should be done by adding concentrated acid to water gradually with stirring.
Because the heat is evolved gradually and is easily absorbed by the large amount of water.
If, however, water is added to a concentrated acid to dilute it, a large amount of heat is evolved at once. The heat generated may cause the mixture to splash the acid on our face or clothes and cause acid burns.
How is the concentration of hydronium ions ([H₃O⁺]) affected when a solution of an acid is diluted?
When the concentrated solution of an acid is diluted by mixing water, the concentration of H⁺ ions (or H₃O⁺ ions) in solution increases due to an increase in the extent of ionization.
However, the concentration of hydronium ([H₃O⁺]) ions per unit volume decreases.
How is the concentration of hydroxide ions ([OH⁻]) affected when excess base is dissolved in a solution of sodium hydroxide?
The concentration of OH⁻ ions increases per unit volume on dissolving excess base in the solution of sodium hydroxide.
NCERT Solutions for Class 10 Science Acids Bases and Salts
These acids bases and salts solutions are very short and crisp to help you understand and remember the concepts very well.
You have two solutions A and B. The pH of solution A is 6 and the pH of solution B is 8.
(i) Which solution has more hydrogen ion concentration?
(ii) Which of this is acidic and which one is basic?
(i) The pH of a solution is inversely proportional to its hydrogen ion concentration. This means that the solution having a lower pH will have more hydrogen ion concentration. In this case, solution A (having a lower pH of 6) will have more hydrogen ion concentration than solution B.
(ii) We know a solution with pH < 7 is acidic, and a solution with pH > 7 is basic. Therefore, solution A with pH = 6 is acidic, and solution B with pH = 8 is basic in nature.
What effect does the concentration of H⁺(aq) ions have on the nature of the solution?
Acids produce hydrogen ions in water. So, when an acid is added to an aqueous solution, the concentration of hydrogen ions in the aqueous solution increases. The solution thus formed will have more hydrogen ions, and therefore, its acidic character increases.
Do basic solutions also have H⁺(aq) ions? If yes, then why are these basic?
Basic solutions also have H⁺ ions in addition to OH⁻ ions. They are basic because, in these solutions, the OH⁻ ion concentration is more than the H⁺ ion concentration.
Under what soil condition do you think that a farmer would spread or treat the soil of his fields with quick lime (calcium oxide), slaked lime (calcium hydroxide), or chalk (calcium carbonate)?
For healthy growth of plants, the soil should neither be alkaline nor highly acidic. A quick lime or slaked lime or chalk, all being alkaline substances, would be added to the soil by the farmer when the soil is highly acidic, in order to decrease its acidic nature.
What is the common name of the compound CaOCl₂?
The common name of the compound CaOCl₂ is bleaching powder.
Name the substance that, on treatment with chlorine, yields bleaching powder.
Dry slaked lime (calcium hydroxide) Ca(OH)₂ is the substance that, on treatment with chlorine, yields bleaching powder (calcium oxychloride):
Name the sodium compound, which is used for softening hard water.
Sodium carbonate (washing soda) is used for softening hard water.
What will happen if a solution of sodium hydrogen carbonate is heated? Give the equation of the reaction involved.
Sodium hydrogen carbonate upon heating decomposes into sodium carbonate, carbon dioxide, and water. The equation for the reaction involved is as follows:
Sodium bicarbonate → Sodium carbonate + Carbon dioxide + Water.
What happens when water is added to plaster of Paris? Write an equation to show the reaction between Plaster of Paris and water.
Plaster of Paris has a very remarkable property of setting into a hard mass on wetting with water. So, when water is added to Plaster of Paris, it sets into a hard mass in about half an hour.
The setting of Plaster of Paris is due to the hydration of Plaster of Paris. The hydrated crystals of Plaster of Paris are called gypsum, which sets to form a hard, solid mass:
Plaster of Paris + Water → Gypsum.
A solution turns red litmus blue, its pH is likely to be
(a) 1
(b) 4
(c) 5
(d) 10
Ans: (d) 10
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
10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of a given solution of HCl. If we take 20 ml of the same solution of NaOH, the amount of HCl solution (the same solution as before) required to neutralize it will be
(a) 4 ml
(b) 8 ml
(c) 12 ml
(d) 16 ml
Ans: (d) 16 ml
Which one of the following types of medicines is used for treating indigestion?
(a) Antibiotic
(b) Analgesic
(c) Antacid
(d) Antiseptic
Ans: (c) Antacid
Write word equations and then balanced equations for the reactions taking place when:
(a) Dilute sulphuric acid reacts with zinc granule
(b) Dilute hydrochloric acid reacts with magnesium ribbon
(c) Dilute sulphuric acid reacts with aluminium powder
(d) Dilute sulphuric acid reacts with iron fillings
(a) The word equation is
Zinc + sulphuric acid → Zinc sulphate + Hydrogen
The balanced equation will be:
(b) The word equation is
Magnesium + Hydrochloric acid → Magnesium Chloride + Hydrogen
The balanced equation will be:
(c) The word equation is
Aluminium + Sulphuric acid → Aluminium Sulphate + Hydrogen
The balanced equation will be:
(d) The word equation is
Iron + Hydrochloric acid → Iron (II) chloride + Hydrogen
The balanced equation will be:
Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Why?
Ans: An acid is a substance which dissociates on dissolving in water to produce hydrogen ions (H+).
Alcohols and glucose, though containing hydrogen, do not ionize in the solution to produce (H+) ions and are hence not acidic in nature. This is proved by the fact that their solutions do not conduct electricity.
Why does distilled water not conduct electricity, whereas rainwater does?
Ans: A liquid conducts electric current if it contains ions. This is because the electric current is carried by the ions. Distilled water does not conduct electricity because it does not contain any ion or any ionic compound (like acids, bases, or salts) dissolved in it.
On the other hand, rainwater, while falling to the earth through the atmosphere, dissolves acidic gases such as carbon dioxide, sulphur dioxide, etc., from the air and forms acids.
These acids provide ions to rainwater due to their ionization, and it is because of the presence of these ions that rainwater conducts electricity.
For example: Rainwater, while falling to the earth through the atmosphere, dissolves carbon dioxide from the air and forms carbonic acid (H2CO3). Carbonic acid provides hydrogen ions, \(H^+ \), and carbonate ions, \(CO_3^{2-} \), to rainwater as shown:
So, due to the presence of carbonic acid (which provides ions to rainwater), rainwater conducts electricity.
Why do acids not show acidic behaviour in the absence of water?
Ans: The acidic behaviour of an acid is due to the presence of hydrogen ions (H+). An acid produces (H+) ions on dissociation in the presence of water. In the absence of water, (H+) ions are not produced. Hence, acids do not show acidic behaviour in the absence of water.
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:
(i) Neutral?
(ii) Strongly alkaline?
(iii) Strongly acidic?
(iv) Weakly acidic?
(v) Weakly alkaline?
The pH of a solution is inversely proportional to its hydrogen ion concentration. This means that the solution having a lower pH will have more hydrogen ion concentration.
- Solutions with \( \text{pH} < 7 \) are acidic, with \( \text{pH} > 7 \) are basic, and with \( \text{pH} = 7 \) are neutral.
- Therefore, the solutions with \( \text{pH} = 4, 1 \) are acidic, and solutions with \( \text{pH} = 9, 11 \) are basic.
- The solution with \( \text{pH} = 7 \) is neutral.
This means:
- Solution with \( \text{pH} = 1 \) is Strongly acidic.
- Solution with \( \text{pH} = 4 \) is Weakly acidic.
- Solution with \( \text{pH} = 7 \) is Neutral.
- Solution with \( \text{pH} = 9 \) is Weakly alkaline.
- Solution with \( \text{pH} = 11 \) is Strongly alkaline.
| Solution | pH | Nature |
|---|---|---|
| D | 7 | Neutral |
| C | 11 | Strongly Alkaline |
| B | 1 | Strongly Acidic |
| A | 4 | Weakly Acidic |
| E | 9 | Weakly Alkaline |
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. In which test-tube will fizzing occur more vigorously and why?
Acetic acid (CH3COOH) is a weak acid, whereas hydrochloric acid (HCl) is a strong acid.
Fizzing occurs in the test tube due to the evolution of hydrogen gas by the action of acid on magnesium ribbon.
Since hydrochloric acid is a strong acid, a large amount of hydrogen gas is liberated in test-tube A. So fizzing occurs more vigorously in test-tube A than in test-tube B.
Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain.
When milk turns into curd, lactic acid is formed, which increases the acidic character of the milk. We know that the more acidic a substance is, the lower its pH.
Thus, when milk changes into curd, its pH will decrease. Therefore, the pH of the milk will change to below 6 when milk turns into curd.
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?
Fresh milk is acidic and becomes more acidic easily and turns sour in taste. In the presence of baking soda, milk becomes alkaline and does not turn sour easily because the alkali does not allow the milk to become more acidic easily.
Why does this milk take a long time to set as curd?
When the milk sets to curd, lactic acid is formed, and its pH decreases, i.e., it becomes more acidic. The presence of alkali does not allow it to become more acidic easily. Hence, it will take a long time to set as curd.
Plaster of Paris should be stored in a moisture-proof container. Why?
Plaster of Paris is CaSO4 · ½ H2O. It absorbs moisture to become gypsum with the formula CaSO4 · 2H2O and sets to a hard mass.
If Plaster of Paris is not stored in a moisture-proof container, it will absorb moisture and become gypsum, which sets to a hard mass. This will make the plaster of Paris useless after some time. Hence, it should be stored in a moisture-proof container.
What is a neutralization reaction? Give two examples.
A neutralization reaction is a reaction in which an acid reacts with a base to form salt and water.
Examples:
(i) Sodium hydroxide (base) reacts with hydrochloric acid (acid) and forms Sodium Chloride (Salt) and water:
(ii) Magnesium hydroxide (base) reacts with carbonic acid (acid) and forms Magnesium Carbonate (Salt) and water: