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CBSE Class 10 Chemistry · 11 questions · 26 marks
Why does gold stay shiny for centuries while iron rusts within weeks? This chapter compares the physical and chemical behaviour of metals and non-metals, ranks metals by their reactivity, and traces how ores are turned into the metals we use every day.
Which property is generally shown by non-metals but NOT by metals?
Answer
They are poor conductors of heat and electricity (with a few exceptions) — most non-metals are insulators or poor conductors, unlike metals, which are typically good conductors, malleable, and ductile.
Which of these metals will NOT react with dilute hydrochloric acid to produce hydrogen gas?
Answer
Copper — since copper lies below hydrogen in the reactivity series, it cannot displace hydrogen from dilute acids, unlike magnesium, zinc, and iron, which lie above hydrogen.
In the formation of an ionic compound like magnesium chloride, MgCl2, what happens to the magnesium atom?
Answer
It loses two electrons to form Mg2+ — magnesium (2, 8, 2 electronic configuration) loses its two valence electrons to attain a stable noble gas configuration, and these electrons are gained by two chlorine atoms to form 2 Cl- ions, giving the ionic compound MgCl2.
Which method is most suitable for extracting a highly reactive metal such as sodium from its molten compound?
Answer
Electrolytic reduction — highly reactive metals like sodium, potassium, calcium, and aluminium are extracted by electrolysis of their molten compounds because they cannot be reduced by carbon.
Assertion (A): Aluminium is used to make cooking utensils even though it is a reactive metal. Reason (R): Aluminium forms a thin, tough layer of aluminium oxide on its surface that protects it from further reaction with air and moisture.
Answer
Both A and R are true and R is the correct explanation of A. The protective Al2O3 layer formed on aluminium's surface prevents further corrosion, which is why it remains safe and durable for everyday use in utensils despite being fairly reactive.
What is meant by the terms 'roasting' and 'calcination' in the extraction of metals? Give one example of each.
Answer
Roasting is the process of heating a sulphide ore strongly in the presence of excess air to convert it into the metal oxide, for example, 2ZnS + 3O2 -> 2ZnO + 2SO2. Calcination is the process of heating a carbonate ore strongly in limited or absence of air to convert it into the metal oxide, for example, ZnCO3 -> ZnO + CO2. Both processes are used to prepare metal oxides, which are then reduced to obtain the free metal.
Explain why sodium and potassium are stored in kerosene oil.
Answer
Sodium and potassium are extremely reactive metals that react vigorously with the oxygen and moisture present in air, often catching fire due to the heat released. Storing them under kerosene oil prevents their direct contact with air and moisture, protecting them from unwanted reactions and keeping them safe to handle.
Describe the process of thermite reaction and state one practical use of this reaction.
Answer
The thermite reaction is a highly exothermic displacement reaction in which aluminium, being more reactive, displaces iron from iron(III) oxide: Fe2O3 + 2Al -> 2Fe + Al2O3 + heat. So much heat is released that the iron produced is in the molten state. This reaction is used practically to join railway tracks or cracked machine parts, as the molten iron fills the gap and solidifies into a strong joint.
Explain, with balanced equations, how metal oxides can be classified as basic, amphoteric, or (rarely) acidic. How is the amphoteric nature of aluminium oxide demonstrated experimentally?
Answer
Most metal oxides are basic in nature because they react with acids to form salt and water, for example, CuO + 2HCl -> CuCl2 + H2O, behaving as a base. Some metal oxides are amphoteric, meaning they react with both acids and bases to form salts and water, showing dual character; aluminium oxide is a classic example. Its amphoteric nature is demonstrated because it reacts with hydrochloric acid, Al2O3 + 6HCl -> 2AlCl3 + 3H2O (acting as a base), and it also reacts with sodium hydroxide solution, Al2O3 + 2NaOH -> 2NaAlO2 + H2O (acting as an acid). A very small number of metal oxides can behave in an acidic manner under special conditions, but this is uncommon compared to the vast majority of basic metal oxides; non-metal oxides, by contrast, are typically acidic, e.g. CO2 dissolved in water forms carbonic acid.
What is corrosion? Explain the process of rusting of iron with its chemical formula, and describe three methods used to prevent corrosion.
Answer
Corrosion is the slow and gradual destruction of a metal's surface due to its reaction with substances present in the environment, such as moisture, air, or acids, converting the shiny metal into an undesirable compound. Rusting is the corrosion of iron, in which iron reacts with oxygen and moisture in air to form a reddish-brown flaky coating of hydrated iron(III) oxide, represented as Fe2O3.xH2O; both oxygen and water must be present together for rusting to occur. Corrosion can be prevented in several ways: (1) Painting or applying grease/oil on the metal surface to create a barrier that keeps out air and moisture. (2) Galvanising, which is coating iron or steel with a thin protective layer of zinc, since zinc corrodes preferentially and also forms a protective layer, thus protecting the iron underneath. (3) Alloying, such as mixing iron with chromium and nickel to make stainless steel, which greatly increases resistance to corrosion compared to pure iron.
A shopkeeper stores iron nails in an open box near a window where they are frequently exposed to humid air. Over a few weeks, the nails develop a reddish-brown flaky coating and become weaker. Meanwhile, aluminium window frames in the same shop remain shiny and undamaged despite similar exposure. Based on this, answer: (a) Name the phenomenon affecting the iron nails and write its chemical formula. (b) Why does aluminium not show similar visible damage despite being a reactive metal? (c) Suggest one method the shopkeeper could use to protect the iron nails from this damage.
Answer
(a) The phenomenon is rusting, a form of corrosion, forming hydrated iron(III) oxide, Fe2O3.xH2O. (b) Aluminium does not show visible damage because it reacts quickly with oxygen in air to form a thin, tough, and tightly adhering layer of aluminium oxide (Al2O3) on its surface, which protects the metal underneath from further reaction, unlike the flaky, non-protective rust layer formed on iron. (c) The shopkeeper could protect the iron nails by applying a coat of oil or grease, or by galvanising them (coating with zinc), to prevent direct contact between the iron and the air/moisture that causes rusting.
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