Questions
15
Q1
What are acid-base indicators? Give examples of natural and synthetic indicators, and explain how an olfactory indicator works with the help of an activity.
Acid-base indicators are dyes or mixtures of dyes that show different colours in acidic and basic media, helping identify the nature of a solution. Natural indicators include litmus (a purple dye from lichen) and turmeric (which turns reddish-brown in a base). Synthetic indicators include methyl orange and phenolphthalein. Olfactory indicators are substances whose odour changes in acidic or basic media. For example, NCERT tests cloth strips soaked with onion extract using dilute HCl and dilute NaOH, then compares the odour after rinsing. Vanilla essence and clove oil are similarly tested by adding them separately to dilute HCl and dilute NaOH and observing whether their smell changes.
Q2
Describe the reaction of zinc granules with dilute sulphuric acid. How can you test that the gas evolved is hydrogen? Write the balanced chemical equation.
Zinc granules react with dilute sulphuric acid to produce zinc sulphate and hydrogen gas: . Bubbles appear on the surface of zinc granules and the metal gradually dissolves. To test for hydrogen, pass the gas through a soap solution to form soap bubbles filled with the gas; bring a burning candle near a gas-filled bubble — the gas burns with a characteristic pop sound, confirming it is hydrogen.
Q3
How do metal carbonates and metal hydrogencarbonates react with acids? Describe with the examples of sodium carbonate and sodium hydrogencarbonate reacting with dilute HCl. How is the evolved gas identified?
Metal carbonates and hydrogencarbonates react with acids to produce the corresponding salt, carbon dioxide gas, and water. Sodium carbonate with HCl: . Sodium hydrogencarbonate with HCl: . The evolved CO₂ is identified by passing it through lime water ( solution), which turns milky due to the formation of insoluble calcium carbonate: .
Q4
What is a neutralisation reaction? Explain with the help of a simple activity involving sodium hydroxide and hydrochloric acid. Give the balanced chemical equation.
A neutralisation reaction is the reaction between an acid and a base to produce a salt and water. In Activity 2.6, 2 mL of dilute NaOH is taken in a test tube and two drops of phenolphthalein indicator are added — the solution turns pink (phenolphthalein is pink in a basic medium). When dilute HCl is added drop by drop, the pink colour disappears, indicating that the base has been completely neutralised by the acid. Equation: . The ionic basis is: .
Q5
What do all acids and all bases have in common? Explain the role of water in making a substance show acidic or basic behaviour, giving examples of HCl and NaOH.
All acids produce hydrogen ions () in aqueous solutions, which are responsible for their acidic character. All bases produce hydroxide ions () in aqueous solutions, which are responsible for their basic character. Water plays a crucial role: dry HCl gas does not change the colour of dry blue litmus paper, but moist litmus turns red — because ions separate from HCl only in the presence of water. In water, HCl dissociates as: . The ion does not exist alone; it combines with water to form the hydronium ion (). Similarly, NaOH dissolves in water to produce ions: .
Q6
Why should an acid always be added to water and not water to the acid while diluting? What happens to the concentration of ions when an acid is diluted?
Mixing an acid or a base with water is highly exothermic — it releases a large amount of heat. If water is added to a concentrated acid, the heat released can cause the water to boil instantly, splashing the concentrated acid out of the container and causing severe burns. The container itself may also break due to localised excessive heating. When acid is added slowly to water with stirring, the large volume of water absorbs and dissipates the heat safely. When an acid is diluted, the concentration of ions per unit volume decreases because the same number of ions is now spread over a larger volume of solution.
Q7
What is the pH scale? How does it help classify solutions? Distinguish between strong acids and weak acids, giving one example of each. Does a basic solution contain ions?
The pH scale is a scale from 0 to 14 that measures the hydrogen ion concentration in a solution, where 'p' stands for 'potenz' (German for power). A pH of 7 is neutral; values below 7 are acidic; values above 7 are basic. A strong acid produces a higher concentration of ions (e.g., HCl) while a weak acid produces a lower concentration (e.g., ) — even at the same molar concentration. Yes, basic solutions do contain ions (from the self-ionisation of water), but the ion concentration far exceeds the ion concentration, making the solution basic. pH measures the balance: higher means lower pH.
Q8
How is pH relevant to our digestive system and dental health? Explain the role of antacids and the chemical basis of tooth decay.
The stomach produces hydrochloric acid (HCl) for digestion. During indigestion, excess acid is produced, causing pain and irritation. Antacids — mild bases like milk of magnesia () — are taken to neutralise this excess acid. Tooth decay starts when the pH inside the mouth falls below 5.5. Tooth enamel, made of calcium hydroxyapatite (a crystalline form of calcium phosphate), is the hardest substance in the body but corrodes when the mouth pH drops below 5.5. Bacteria in the mouth break down sugars and food particles, producing acids that lower the pH. Toothpastes, which are generally basic, help neutralise the excess acid and prevent decay.
Q9
Explain how pH affects plant growth and aquatic life. What is acid rain, and what happens when soil becomes too acidic? Mention how a farmer can treat acidic soil.
Plants require a specific pH range for healthy growth and can only survive within a narrow range. Soil pH can be tested (Activity 2.12) by mixing a soil sample with water, filtering, and testing the filtrate with universal indicator paper. Acid rain — rainwater with a pH below 5.6 — flows into rivers, lowering the pH of river water and making survival difficult for aquatic life. If soil is too acidic, a farmer can treat it by adding quicklime (), slaked lime (), or chalk () — all basic substances that neutralise the excess acid in the soil.
Q10
What is meant by the 'family of salts'? How does the pH of a salt depend on the strengths of the acid and base from which it is formed? Give an example of each — an acidic salt, a basic salt, and a neutral salt.
Salts that share the same positive radical (cation, from the base) or the same negative radical (anion, from the acid) belong to the same family. For example, NaCl and belong to the sodium salts family (same cation), while NaCl and KCl belong to the chloride salts family (same anion). The pH of a salt depends on the relative strengths of its parent acid and base: a salt from a strong acid and strong base has pH = 7 (neutral, e.g., NaCl); from a strong acid and weak base has pH < 7 (acidic, e.g., ); from a weak acid and strong base has pH > 7 (basic, e.g., or ).
Q11
Describe the chlor-alkali process. Why is it so named? List the three main products obtained and give one important use of each.
The chlor-alkali process is the electrolysis of brine (aqueous sodium chloride solution) to produce sodium hydroxide, chlorine gas, and hydrogen gas: . It is named 'chlor-alkali' because the products are chlorine ('chlor') and sodium hydroxide ('alkali'). At the anode (+), chlorine gas is released. At the cathode (-), hydrogen gas is released, and NaOH solution is formed near the cathode. Uses: NaOH — making soaps and detergents; Cl₂ — water treatment and PVC production; H₂ — fuel and manufacturing ammonia for fertilisers.
Q12
How is bleaching powder prepared? Write the chemical equation and give its uses.
Bleaching powder is produced by the action of chlorine gas on dry slaked lime (): . Although the actual composition is complex, it is represented as and commonly called calcium oxychloride. Uses: (i) bleaching cotton and linen in the textile industry, wood pulp in paper factories, and washed clothes in laundries; (ii) as an oxidising agent in chemical industries; (iii) making drinking water free from germs.
Q13
How is baking soda prepared? Write its chemical equation for heating during cooking. Explain the difference between baking soda and baking powder, and how baking soda is useful as an antacid.
Baking soda (sodium hydrogencarbonate, ) is produced from sodium chloride: . On heating during cooking, it decomposes: . The released CO₂ makes bread and cakes rise. Baking powder is a mixture of baking soda and a mild edible acid (like tartaric acid) — when heated or mixed with water, the acid reacts with to produce CO₂ more efficiently: . As an antacid, baking soda neutralises excess stomach acid because it is a mild, non-corrosive basic salt.
Q14
How is washing soda obtained from sodium chloride? What is water of crystallisation? Give the formulae of washing soda and gypsum with their respective water of crystallisation.
Washing soda () is obtained from sodium chloride through a sequence: first, baking soda () is produced from NaCl; baking soda on heating gives sodium carbonate (); recrystallisation of sodium carbonate from water yields washing soda: . Water of crystallisation is the fixed number of water molecules chemically attached to one formula unit of a salt. Washing soda has 10 water molecules () and gypsum has 2 water molecules ().
Q15
How is Plaster of Paris prepared from gypsum? Write the chemical equation for its setting when mixed with water. Why should Plaster of Paris be stored in a moisture-proof container?
Plaster of Paris is prepared by heating gypsum () at 373 K: . Gypsum loses water molecules to form calcium sulphate hemihydrate (Plaster of Paris). When mixed with water, it rehydrates and sets into a hard solid mass of gypsum: . Plaster of Paris must be stored in a moisture-proof container because it readily absorbs moisture from the air and sets into a hard lump, becoming unusable.