Questions Related to NCERT

Amphoteric oxides are oxides that react with both acids and bases to form salt and water. This means they show both acidic and basic behavior.

Examples of amphoteric oxides:

1. Aluminum oxide (Al₂O₃)
2. Zinc oxide (ZnO)

(a) How could you use a hammer, a battery, a bulb, wires and a switch to distinguish between samples of metals and non-metals?

• Hammer Test (Malleability Test):
  • Hit the sample gently with the hammer.
  • If the sample flattens or bends without breaking, it is likely a metal (metals are malleable).
  • If the sample breaks or shatters, it is likely a non-metal (non-metals are brittle).
• Electrical Conductivity Test:
  • Make a simple circuit using the battery, bulb, wires, and switch.
  • Place the sample between two wire ends in the circuit.
  • Turn on the switch and observe the bulb.
  • If the bulb lights up, the sample conducts electricity and is likely a metal.
  • If the bulb does not light up, the sample does not conduct electricity and is likely a non-metal.

(b) Assess the usefulness of these tests in distinguishing between metals and non-metals:

• These tests are useful because they are based on basic properties:
  • Metals are usually malleable and conduct electricity.
  • Non-metals are usually brittle and poor conductors of electricity.
• The tests are simple and can be done easily in a classroom or lab.
• However, there are some limitations:
  • Some non-metals like graphite can conduct electricity.
  • Some metals like mercury are liquid and cannot be hammered.
  • Metalloids may not clearly behave like metals or non-metals.
  • Surface coatings or rust on metals can affect the results.

The correct answer is: (c) zinc is more reactive than tin.

Because, food cans are coated with tin and not zinc primarily because zinc is more reactive than tin. If zinc were used, it could react with the acids present in food, potentially leading to contamination or spoilage. Tin, being less reactive, is safer and more stable in contact with food, making it the preferred coating material.

(a) Calcium : - Calcium reacts with oxygen to form calcium oxide (CaO). Calcium oxide has a high melting point. It is soluble in water, forming calcium hydroxide. This matches both conditions.

(b) Carbon :- Carbon forms carbon dioxide or carbon monoxide when it reacts with oxygen. These are gases, not solids with high melting points. Does not match.

(c) Silicon :- Silicon forms silicon dioxide (SiO₂), which has a high melting point. But it is not soluble in water. Does not match.

(d) Iron :-  Iron forms iron oxides when it reacts with oxygen. They have high melting points but are not soluble in water. Does not match.

Correct answer:
(a) Calcium

Rusting of iron happens when iron comes in contact with oxygen and moisture (water) from the air. To prevent rusting, we need to stop air and water from reaching the iron surface.

Let’s look at each method:

(a) Applying grease –Grease forms a layer that prevents air and moisture from touching the iron.
(b) Applying paint –Paint also acts as a barrier and protects the iron from rusting.
(c) Applying a coating of zinc –This method is called galvanization. Zinc protects the iron by forming a coating and also by sacrificing itself (zinc reacts before iron does).

All these methods help in preventing rusting.

So correct answer: - (d) All of the above

To determine if a displacement reaction will occur, we use the reactivity series of metals. In a displacement reaction, a more reactive metal displaces a less reactive metal from its salt solution.

Reactivity series :--  Potassium (K) > Sodium (Na) > Calcium (Ca) > Magnesium (Mg) > Aluminium (Al) > Zinc (Zn) > Iron (Fe) > Lead (Pb) > Hydrogen (H) > Copper (Cu) > Mercury (Hg) > Silver (Ag) > Gold (Au)

Now, let’s check each pair:

1. NaCl solution and copper metal :- Copper is less reactive than sodium. So, copper cannot displace sodium from NaCl. No reaction.
2. MgCl₂ solution and aluminium metal :-- Aluminium is more reactive than magnesium. So, aluminium can displace magnesium from MgCl₂. So Displacement reaction occurs.
3. FeSO₄ solution and silver metal :-- Silver is less reactive than iron. So, silver cannot displace iron from FeSO₄. No reaction.
4. AgNO₃ solution and copper metal :-- Copper is more reactive than silver. So, copper can displace silver from AgNO₃. Displacement reaction occurs.

Alloys are mixtures of two or more metals, or a metal and a non-metal. They are made to improve the properties of metals, like making them stronger, harder, or resistant to rust.

Examples:

• Brass = Copper + Zinc
• Bronze = Copper + Tin
• Steel = Iron + Carbon
• Stainless Steel = Iron + Chromium + Nickel
• Solder = Lead + Tin

Why are alloys made?

• To make metals stronger
• To prevent rusting (corrosion)
• To improve hardness or appearance

Metals that do not corrode easily are called less reactive or noble metals. These metals do not react easily with air, water, or acids.

Examples:

• Gold (Au)
• Platinum (Pt)
• Silver (Ag) (corrodes very slowly)
• Aluminium (Al) (forms a protective oxide layer that prevents further corrosion)

Why they don’t corrode easily:

Gold and platinum are very unreactive, so they stay shiny and do not rust or tarnish. Aluminium forms a thin layer of aluminium oxide on its surface, which protects it from further corrosion.

The chemical process used for obtaining a metal from its oxide is called Reduction. In this process, oxygen is removed from the metal oxide to get the pure metal.

Example:

When iron oxide (Fe₂O₃) is heated with carbon (C), iron metal is obtained:

Fe₂O₃ + 3C → 2Fe + 3CO

Here, carbon takes away the oxygen from iron oxide, and iron is left behind.

So, reduction is the process used to extract metals from their oxides.

Some metals are very unreactive, which means they do not easily react with air, water, or other substances. Because of this, they are found in nature in their free or native state, meaning in pure form, not combined with other elements.  Examples:

1. Gold (Au) :- 
   1. Gold is a highly unreactive metal.
   2. It does not react with air or moisture, so it is found in the earth as shiny yellow metal lumps or nuggets.
   3. That's why we often find pure gold in riverbeds or underground rocks.
2. Platinum (Pt) :- 
   1. Platinum is also very unreactive and does not corrode or tarnish.
   2. It is found in its natural state, often mixed with other precious metals, but not combined chemically.
   3. It is rare and valuable, used in jewellery and scientific instruments.

Mineral: - A naturally occurring substance found in the earth that has a definite chemical composition. Example: Iron, gold, salt.

Ore:-  A type of mineral from which we can extract a metal easily and profitably. Example: Bauxite is an ore of aluminium.

Gangue:- The unwanted materials (like sand, mud, etc.) that are mixed with the ore. These are removed during the process of extracting the metal.

Ionic compounds have high melting points because the forces of attraction between the oppositely charged ions are very strong.

• Ionic compounds are made of positive and negative ions.
• These ions are held together by strong forces of attraction.
• A lot of heat is needed to break these bonds.
• That’s why ionic compounds have high melting points.

Examples of ionic compounds with high melting points:

• Sodium chloride (NaCl)
• Magnesium oxide (MgO)

These compounds are formed by ionic bonding, where metals transfer electrons to non-metals.

Formation of Sodium Oxide (Na₂O)

Sodium (Na) has 1 electron in its outer shell
Oxygen (O) has 6 electrons in its outer shell and needs 2 electrons to complete its octet.
So:
2 sodium atoms each give 1 electron (total 2 electrons).
1 oxygen atom accepts those 2 electrons.
Electron transfer:
Na → Na⁺ + e⁻
O + 2e⁻ → O²⁻

Ionic compound formed:
2Na⁺ + O²⁻ → Na₂O

Formation of Magnesium Oxide (MgO)

Magnesium (Mg) has 2 electrons in its outer shell.
Oxygen (O) needs 2 electrons to complete its octet.
So:
1 magnesium atom gives 2 electrons.
1 oxygen atom accepts 2 electrons.
Electron transfer:
Mg → Mg²⁺ + 2e⁻
O + 2e⁻ → O²⁻

Ionic compound formed:
Mg²⁺ + O²⁻ → MgO

Electron-dot structures show the valence electrons (outermost electrons) of an element as dots around the chemical symbol.

1. Sodium (Na)
Atomic number: 11
Electronic configuration: 2, 8, 1 (1 valence electron)
Electron-dot structure:
Na·

2. Oxygen (O)
Atomic number: 8
Electronic configuration: 2, 6 (6 valence electrons)
Electron-dot structure:
··
·O·
··
(6 dots are placed around the O symbol to represent 6 valence electrons)

3. Magnesium (Mg)
Atomic number: 12
Electronic configuration: 2, 8, 2 (2 valence electrons)
Electron-dot structure:
·Mg·

When zinc is added to a solution of iron(II) sulphate (FeSO₄), the following observations and reaction occur:

Observation:

• The green color of the iron(II) sulphate solution fades.
• A grey deposit of iron forms on the zinc metal.

This happens because zinc is more reactive than iron, so it displaces iron from its salt solution.

Chemical Reaction:
Zinc + Iron(II) sulphate → Zinc sulphate + Iron

Chemical equation:
Zn + FeSO₄ → ZnSO₄ + Fe

When dilute hydrochloric acid (HCl) is added to a reactive metal, hydrogen gas (H₂) is produced.

Chemical Reaction of Iron with Dilute Sulphuric Acid (H₂SO₄):- 

Iron reacts with dilute sulphuric acid to form iron(II) sulphate (FeSO₄) and hydrogen gas (H₂).

Chemical equation:
Fe + H₂SO₄ → FeSO₄ + H₂↑

1.  Reaction of iron with steam:
Iron reacts with steam to form iron(III) oxide and hydrogen gas.
Chemical equation:
3Fe + 4H₂O → Fe₃O₄ + 4H₂

2.  Reaction of calcium with water:
Calcium reacts slowly with cold water to form calcium hydroxide and hydrogen gas.
Chemical equation:
Ca + 2H₂O → Ca(OH)₂ + H₂

3.  Reaction of potassium with water:
Potassium reacts very quickly and violently with cold water. It forms potassium hydroxide and hydrogen gas. The reaction releases a lot of heat.
Chemical equation:
2K + 2H₂O → 2KOH + H₂

Malleable

• A material is malleable if it can be hammered or rolled into thin sheets without breaking.
• This property shows the ability of a metal to withstand compression.
• Example: Gold and aluminium are highly malleable metals.

Ductile

• A material is ductile if it can be drawn into thin wires without breaking.
• This property shows the ability of a metal to stretch without breaking under tension.
• Example: Copper and silver are highly ductile metals.

Here are examples of metals that fit each of the given criteria:

(i) Metal that is a liquid at room temperature: Mercury (Hg)
– Mercury is the only metal that is liquid at room temperature (about 25°C).

(ii) Metal that can be easily cut with a knife: Sodium (Na)
– Sodium is a soft metal and can be easily cut with a knife. (Potassium is another similar example.)

(iii) Metal that is the best conductor of heat: Silver (Ag)
– Silver is the best conductor of heat among all metals.

(iv) Metal that is a poor conductor of heat: Lead (Pb)
– Lead is a metal but is a poor conductor of heat compared to most other metals.

Function of an Earth Wire:

The earth wire provides a safe path for electric current to flow into the ground if there is any fault in the appliance, such as when the live wire touches the metallic casing. This prevents electric shocks by ensuring that the current does not pass through the user.

Why is it necessary to earth the metallic casing of electric appliances?

• The metallic casing can become live if there is a fault, causing electric shock if touched.
• By connecting the casing to the earth wire, any leakage current flows safely to the ground instead of through the person.
• This protects users from electric shocks and helps in quickly blowing the fuse or tripping the circuit breaker by causing a large current flow.