RowQ
The Vault
RowQ
The Vault
CBSE Class 10 Chemistry · 11 questions · 26 marks
From the sourness of lemon juice to the soapy feel of shampoo, acids and bases shape everyday chemistry. This chapter explores how these two families of substances behave, react, and neutralise each other, and how salts are born from that neutralisation.
A solution turns red litmus blue and has a pH of 11. This solution is most likely to be:
Answer
Sodium hydroxide solution — it is a strong base (pH greater than 7) that turns red litmus blue, unlike the acidic lemon juice and HCl or the neutral distilled water.
Which gas is produced when sodium bicarbonate reacts with hydrochloric acid, and how is it tested?
Answer
Carbon dioxide gas, tested by passing through lime water which turns milky — NaHCO3 + HCl -> NaCl + H2O + CO2, and CO2 turns freshly prepared lime water milky due to formation of CaCO3.
What is formed when Plaster of Paris is mixed with water?
Answer
Gypsum, CaSO4.2H2O, which sets into a hard mass — Plaster of Paris (CaSO4.1/2H2O) absorbs water and hardens back into gypsum, which is why it is used for casts and moulds.
Which of the following correctly describes the chlor-alkali process?
Answer
Electrolysis of concentrated sodium chloride solution to give NaOH, Cl2, and H2 — at the cathode H2 is released, at the anode Cl2 is released, and NaOH is formed in solution, which is why it is called the chlor-alkali process.
Assertion (A): Dry HCl gas does not turn dry blue litmus paper red. Reason (R): Acids show their acidic character (release H+ ions) only in the presence of water.
Answer
Both A and R are true and R is the correct explanation of A. HCl gas ionises to release H+ ions only when dissolved in water, so in the absence of water it cannot show acidic behaviour and dry litmus paper stays unchanged.
Why should curd and sour substances not be kept in brass or copper vessels? Explain briefly.
Answer
Curd and other sour foods contain acids (like lactic acid), which can react with the metal of brass or copper vessels to form toxic metal salts. These salts can dissolve into the food and cause food poisoning if consumed, so acidic foods are best stored in glass, ceramic, or stainless steel containers instead.
A milkman adds a very small amount of baking soda to fresh milk before storing it. State why he does this and what effect it has on the pH of the milk.
Answer
Baking soda (sodium bicarbonate, NaHCO3) is a mild base, so adding a small amount shifts the milk's pH slightly towards the basic side (above its natural pH of about 6). This delays the souring of milk because it takes longer for the milk to become acidic enough (due to bacterial action producing lactic acid) for it to curdle or spoil.
What happens when zinc granules are added to a solution of sodium hydroxide? Write the balanced chemical equation.
Answer
Zinc, being an amphoteric metal, reacts with the strong base sodium hydroxide to form sodium zincate and hydrogen gas is liberated with effervescence. The balanced equation is: Zn + 2NaOH -> Na2ZnO2 + H2. This shows that some metals like zinc and aluminium can react with both acids and bases, which is why they are called amphoteric metals.
Define the pH scale. Explain, with examples, the importance of pH in (a) our digestive system, (b) plant and animal life, and (c) soil for agriculture.
Answer
The pH scale is a numerical scale from 0 to 14 that indicates how acidic or basic a solution is, based on the concentration of H+ ions; pH less than 7 is acidic, pH equal to 7 is neutral, and pH more than 7 is basic. (a) Our stomach secretes hydrochloric acid to maintain an acidic pH (around 1.5-3.5) that helps digest food and kills harmful bacteria; excess acid causes indigestion and pain, which is relieved by taking a mild base (antacid) like milk of magnesia to neutralise the excess acid. (b) Plants and animals function within a narrow pH range; for example, our body works normally only between pH 7.0 and 7.8, and if the pH of rainwater falls below 5.6 (acid rain), it can damage aquatic life in rivers and lakes as fish and other organisms cannot survive the changed pH. (c) Soil has an optimum pH range for healthy crop growth; if soil is too acidic, farmers add slaked lime (calcium hydroxide) to neutralise it, and if it is too alkaline, organic matter is added, since most crops need a nearly neutral soil pH to absorb nutrients efficiently.
What is water of crystallisation? Describe the preparation and one important use of Plaster of Paris, and explain why it must be stored in a moisture-proof container.
Answer
Water of crystallisation is the fixed number of water molecules chemically combined with one formula unit of a salt in its crystalline form, for example gypsum, CaSO4.2H2O, contains two water molecules per formula unit. Plaster of Paris is prepared by heating gypsum to about 373 K, which removes three-quarters of its water of crystallisation: CaSO4.2H2O -> CaSO4.1/2H2O + 3/2 H2O. It is a white powder that, when mixed with water, slowly absorbs water and hardens into a solid mass of gypsum again, which is why it is used to support fractured bones (plaster casts) and to make decorative moulds. It must be stored in an airtight, moisture-proof container because exposure to moist air would cause it to slowly absorb atmospheric water and set into a useless hard lump before it can be used.
In a school laboratory, a student tests four unlabelled solutions A, B, C, and D with universal indicator paper. Solution A turns the paper red, B turns it green, C turns it dark blue, and D turns it orange. Based on this, answer: (a) Arrange the solutions in increasing order of pH. (b) Which solution is most likely to be a strong acid like dilute HCl? (c) Which solution is neutral? (d) Which solution could be a weak base like sodium bicarbonate solution?
Answer
(a) In increasing order of pH: A less than D less than B less than C. (b) Solution A, since red indicates the lowest pH range (strongly acidic), matching a strong acid like dilute HCl. (c) Solution B, since green on universal indicator corresponds to pH around 7, which is neutral. (d) Solution D, since orange indicates a mildly acidic to near-neutral range that is less acidic than red, though a weak base like sodium bicarbonate would actually show a colour on the mildly basic side (light blue/green); given the options, D (orange, mildly acidic) is closest to a very weak base's near-neutral behaviour is less likely than C, so C (dark blue, clearly basic) best represents a base such as sodium bicarbonate solution.
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