Which of the following methods is suitable for preventing an iron frying pan from rusting?
(a) Applying grease
(b) Applying paint
(c) Applying a coating of zinc
(d) All of the above
Answer: (c) Applying a coating of zinc
Explanation: While grease and paint prevent rusting, they burn off when the pan is heated for cooking. Zinc coating (Galvanization) is more durable and protects iron even if the layer is scratched. (Though for cooking utensils, grease is avoided, zinc is the technical answer for iron protection in this context).
Q3
An element reacts with oxygen to give a compound with a high melting point. This compound is also soluble in water. The element is likely to be:
(a) Calcium (b) Carbon (c) Silicon (d) Iron
Answer: (a) Calcium
Reason: Calcium reacts with oxygen to form Calcium Oxide ($\text{CaO}$), which is an ionic compound having a high melting point. $\text{CaO}$ dissolves in water to form Calcium Hydroxide (Lime water).
Food cans are coated with tin and not with zinc because:
(a) Zinc is costlier than tin.
(b) Zinc has a higher melting point than tin.
(c) Zinc is more reactive than tin.
(d) Zinc is less reactive than tin.
Answer: (c) Zinc is more reactive than tin.
Explanation: If food cans are coated with Zinc, it might react with the organic acids present in the food to form poisonous substances, leading to food poisoning. Tin is less reactive and safe for food storage.
Q5
You are given a hammer, a battery, a bulb, wires and a switch.
(a) How could you use them to distinguish between metals and non-metals?
(b) Assess the usefulness of these tests.
(a) Methods:
1. Hammer Test (Malleability): Beat the sample with a hammer. If it flattens into a thin sheet, it is a Metal. If it breaks/shatters (brittle), it is a Non-metal.
2. Circuit Test (Conductivity): Set up a circuit with the battery and bulb. Insert the sample. If the bulb glows, the sample conducts electricity and is a Metal.
(b) Usefulness: These physical tests are generally reliable for solids but have exceptions (e.g., Graphite is a non-metal but conducts electricity; Sodium is a metal but is brittle/soft). Chemical tests are more definitive.
Q6
What are amphoteric oxides? Give two examples of amphoteric oxides.
Definition: Metal oxides that react with both acids and bases to produce salt and water are called amphoteric oxides. They show properties of both acidic and basic oxides.
Name two metals which will displace hydrogen from dilute acids, and two metals which will not.
Metals that displace Hydrogen:
Magnesium (Mg) and Zinc (Zn). (Because they are more reactive than Hydrogen).
Metals that DO NOT displace Hydrogen:
Copper (Cu) and Silver (Ag). (Because they are less reactive than Hydrogen in the reactivity series).
Q8
In the electrolytic refining of a metal M, what would you take as the anode, the cathode and the electrolyte?
Anode: Thick block of Impure Metal M.
Cathode: Thin strip of Pure Metal M.
Electrolyte: A salt solution of the metal M (e.g., $\text{CuSO}_4$ for Copper).
On passing current, pure metal from the anode dissolves into the electrolyte and deposits on the cathode. Impurities settle as anode mud.
[Image of electrolytic refining of copper]
Q9
Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it. (a) What will be the action of gas on dry and moist litmus paper? (b) Write a balanced chemical equation.
(a) Action on Litmus:
The gas formed is Sulphur Dioxide ($\text{SO}_2$).
– Dry Litmus: No change (needs moisture to form acid).
– Moist Blue Litmus: Turns Red. $\text{SO}_2$ reacts with moisture to form Sulphurous Acid ($\text{H}_2\text{SO}_3$), which is acidic.
Give reasons:
(a) Platinum, gold and silver are used to make jewellery.
(b) Sodium, potassium and lithium are stored under oil.
(c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.
(d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction.
(a) They are highly lustrous (shiny) and very unreactive (do not corrode), so they retain their shine for a long time.
(b) They are highly reactive alkali metals. They react vigorously with oxygen and moisture in the air and can catch fire. Oil prevents contact with air.
(c) Aluminium forms a thin, stable layer of Aluminium Oxide ($\text{Al}_2\text{O}_3$) on its surface. This layer prevents further corrosion and makes it durable for heating.
(d) It is easier to extract metal from its oxide (by reduction with Carbon) than from its carbonate or sulphide. Hence, ores are first roasted/calcined to oxides.
Q13
You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.
Copper vessels tarnish due to the formation of a green layer of Basic Copper Carbonate. Lemon/Tamarind contains mild acids (Citric/Tartaric acid). The acid reacts with the basic carbonate layer (Neutralization) and dissolves it, restoring the shine of the copper vessel.
Q14
Differentiate between metal and non-metal on the basis of their chemical properties.
Metals
Non-Metals
Form Basic Oxides.
Form Acidic/Neutral Oxides.
Lose electrons to form Cations (+).
Gain electrons to form Anions (-).
React with dilute acids to give H₂ gas.
Do not react with dilute acids.
Q15
A man went door to door posing as a goldsmith… He dipped a lady’s bangles in a solution, they sparkled but became light. What was the solution?
The solution was Aqua Regia (Royal Water).
It is a freshly prepared mixture of Concentrated Hydrochloric Acid and Concentrated Nitric Acid in the ratio 3:1. Aqua Regia is highly corrosive and is one of the few reagents able to dissolve Gold. The outer layer of gold dissolved, reducing the bangle’s weight.
Q16
Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).
1. Conductivity: Copper is a better conductor of heat than steel, which is efficient for heating water.
2. Reactivity with Steam: Iron (Steel) reacts with steam to form Iron Oxide ($\text{Fe}_3\text{O}_4$) and corrodes. Copper does not react with water or steam at any temperature.