GSEB Solutions for Class 10 Science and Technology – Metals

GSEB Solutions for Class 10 Science and Technology – Metals (English Medium)

GSEB SolutionsMathsScience
Question 1:

Select the proper choice from the given muitiple choices :

Question 1.1:

What is the chemical formula of alumina ?

Solution :

A. Al₂O₂
Alumina is obtained from Bauxite, the ore of aluminium by Bayer’s process.

Question 1.2:

Which of the following reactions is called roasting ?

Solution :

B. 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g)
The method of conversion of sulphide-containing ore to metal oxides is called roasting. For converting sulphide-containing ores to metal oxides, the ore is heated for a long time in the presence of excess of air.

Question 1.3:

Which of the following is an alloy ?

Solution :

C. 22 carat gold
22 carat gold is an alloy of 22 parts of pure gold and 2 parts of copper or silver.

Question 1.4:

During which reaction dihydrogen gas is not produced under normal conditions ?

Solution :

C. Metal + dilute nitric acid
Nitric acid is a strong oxidising agent. Thus, under normal conditions, dihydrogen gas is not produced by the reaction of metals with dilute nitric acid.

Question 1.5:

In which of the following, displacement reaction is possible ?

Solution :

D. Solution of AgNO₃ + coin of copper
Copper is a more active metal than Ag; thus, Ag can be displaced from the solution of AgNO₃ by coin of copper.

Question 1.6:

Which of the following reactions is not possible ?

Solution :

D. Cu(s) + FeSO₄(aq) → CuSO₄(aq) + Fe(s)
Copper is a less active metal than Fe. Hence, it cannot displace Fe from FeSO₄ solution.

Question 1.7:

By which reaction metal is obtained from metal     oxide ?

Solution :

B. Reduction
Chemical reduction or electrochemical reduction methods help in reducing metal oxides to obtain a pure metal.

Question 1.8:

Which of the following statements is incorrect ?

Solution :

B. The melting points and boiling points of metals are low.
The melting points and boiling points of metals are generally high.

Question 1.9:

Which alloy is used to solder the electric wires ?

Solution :

C. Lead + tin
The melting point of an alloy of lead and tin is less. Thus, it is used to solder electric wires.

Question 1.10:

Which metal is available in highest proportion on the earth ?

Solution :

C. Aluminium
Aluminium makes up almost 8% of the Earth’s crust by weight.

Question 1.11:

Which metal is used in thermometer ?

Solution :

B. Mercury
Mercury metal is liquid at room temperature and sensitive to changes in temperature. Thus, it is used in a thermometer.

Question 1.12:

Which of the following substances is hygroscopic ?

Solution :

C. Anhydrous calcium chloride
Anhydrous calcium chloride is highly hygroscopic and needs to be kept in a tightly sealed container. It is commonly used as a desiccant.

Question 2:

Answer the following questions in brief :

Question 2.1:

In which three sections elements are classified ?

Solution :

Elements are classified as metals, non-metals and semi-metals (metalloids).

Question 2.2:

Which metals are available in free state in nature ?

Solution :

Gold, silver and platinum are available in the free state in nature.

Question 2.3:

In which form ores are available in nature ?

Solution :

Ores are available in combined forms in nature.

Question 2.4:

Write names and formulas of two ores of iron.

Solution :

Two ores of iron are

• Haematite – Fe₂O₃
• Magnetite – Fe₃O₄

Question 2.5:

Write names and formulas of two ores of copper.

Solution :

Two ores of copper are

• Cuprite – Cu₂O
• Copper pyrite -CuFeS₂

Question 2.6:

Write names of the three methods for concentration of ores.

Solution :

Three methods of concentration of ores are
Centrifugation on the basis of difference in densities
Froth floatation
Magnetic separation

Question 2.7:

Which two methods are used for reduction of metal oxides ?

Solution :

Methods used for reduction of metal oxides are

• Chemical reduction
• Electrochemical reduction

Question 2.8:

Write names of three methods used for refining of metals.

Solution :

Three methods used for refining of metals are

• Electrolysis
• Liquefaction
• Zone refining

Question 2.9:

What has been taken as anode and cathode for refining of metals in electrolysis method ?

Solution :

In the refining of metals by electrolysis method, a rod of impure metal is taken as anode and a rod of pure metal is taken as cathode.

Question 2.10:

Which chief impurities are present in bauxite ?

Solution :

Chief impurities present in bauxite are

• Iron oxide (Fe₂O₃)
• Sand (SiO₂)

Question 2.11:

Which two substances are added along with alumina to obtain aluminium from alumina by electrochemical reduction ?

Solution :

Two substances added along with alumina to obtain aluminium from alumina by electrochemical reduction are

• Cryolite (Na₃AlF₆)
• Feldspar (CaF₂)

Question 2.12:

Which substance is called slag ?

Solution :

Calcium silicate (CaSiO₃) formed as a result of the reaction of calcium oxide and silica in the blast furnace is called slag.

Question 2.13:

Mention use of slag.

Solution :

Slag formed during the extraction of iron is used for the construction of roads.

Question 2.14:

Mention use of slag.

Solution :

The general principle taken into consideration in the determination of the activity series of metals is ‘A less active metal can be displaced from the solution of its salt by a more active metal’.

Question 2.15:

Mention names of two softer metals.

Solution :

Two soft metals are

• Sodium
• Potassium

Question 2.16:

Write names of two metals which are bad conductors of electricity.

Solution :

Two metals which are bad conductors of electricity are

• Lead
• Mercury

Question 2.17:

Write names of two metal oxides which form alkali by dissolving in water

Solution :

Two metal oxides which form an alkali by dissolving in water are

• Sodium oxide (Na₂O)
• Potassium oxide (K₂O)

Question 2.18:

Which two metals bum with explosion in reaction with water ?

Solution :

Two metals which burn with an explosion in the reaction with water are

• Sodium
• Potassium

Question 2.19:

Mention two examples of metals forming metal hydride with hydrogen.

Solution :

Two metals which form metal hydride with hydrogen are

• Potassium
• Calcium

Question 2.20:

Which two metals are generally not corroded ?

Solution :

Metals which are generally not corroded include

• Gold
• Silver

Question 2.21:

Stainless steel is a homogeneous mixture of which three metals ?

Solution :

Stainless steel is a homogeneous mixture of

• Iron
• Nickel
• Chromium

Question 2.22:

Mention examples of two alloys.

Solution :

Two alloys are

• Steel
• Bronze

Question 2.23:

Write chemical formula of rust.

Solution :

The chemical formula of rust is Fe₂O₃.xH₂O.

Question 2.24:

Explain the following terms :

Solution :

1. Minerals
   The inorganic elements or compounds which are available naturally from the Earth’s crust are called minerals.
2. Ore
   If the proportion of a certain metal is more in a mineral and if its extraction is advantageous, then the mineral is called an ore.
3. Metallurgy
   The process of separating a metal from its ore and purifying or refining it is called metallurgy.
4. Centrifugation
   Centrifugation is the process of separation of substances on the basis of differences in their densities. In metallurgy, this process helps to separate impurities and particles of metal which have different densities.
5. Roasting
   Roasting is the method which converts sulphide-containing ore into metal oxide for ease of metal extraction.
6. Calcination
   Calcination is the method of converting carbonate-containing ore into metal oxide for ease of metal extraction.
7. Anodic mud
   Anodic mud is the insoluble impurities which get collected at the bottom of the anode during electrolysis while refining metals.
8. Metallic corrosion
   The erosion reaction of any metal with water or moisture when it comes in its contact is called metallic corrosion.
9. Alloy
   An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal.
10. Galvanizing
    Galvanizing is the method of applying a layer of zinc metal on the surface of iron to prevent corrosion.

Question 2.25:

Mention the formulas, names  and physical states of the products in the following reactions :

Solution :

1. 2PbS(s) + 3O₂(g) → 2PbO(s) + 2SO₂(g)
2PbO – Lead oxide
SO₂ – Sulphur dioxide gas

2. MgCO₃(s) MgO(s) + CO₂(g)
MgO – Magnesium oxide
CO₂ – Carbon dioxide gas

3. Fe₂O₃(s) + 3CO(g) 2Fe(s) + 3CO₂(g)
Fe(s) – Iron metal
CO₂ – Carbon dioxide gas

4. Fe₂O₃(s) + 2Al(s) 2Fe(l) + Al₂O₃(s)
Fe – Iron metal
Al₂O₃(s) – Alumina

5. 2Al(OH)₃(s)  Al₂O₃(s) + 3H₂O(g)
Al₂O₃ – Alumina
3H₂O – Water

6. CaO(s) + SiO₂(s) → CaSiO₃(l)
CaSiO₃ – Calcium silicate
K₂O(s) + H₂O(l) → 2KOH(aq)
KOH – Potassium hydroxide

7. Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)
Mg(OH)₂ – Magnesium hydroxide
H₂ – Hydrogen gas

8. Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
ZnCl₂ – Zinc chloride
H₂ – Hydrogen gas

9. Ca(s) + Cl₂(g) → CaCl₂(s)
CaCl₂ – Calcium chloride

Question 3:

Answer the following questions :

Question 3.1:

Describe the method to remove impurity of iron from ore.

Solution :

The method of magnetic separation is used to remove the impurity of iron from an ore.
Finely powdered ore is allowed to fall on the leather belt of the separator.
A magnet is present at one end of the belt. This magnet attracts the iron particles from the ore. The iron particles fall near the magnet and the remaining particles of the concentrated ore fall away.
Thus, iron impurity can be removed from an ore due to its magnetic property using a magnetic separator.

Question 3.2:

Explain the method of concentration of copper pyrites – the ore of copper.

Solution :

Copper pyrite is an ore of copper in the sulphide form. Thus, for the concentration of copper pyrites, the froth flotation method is used.
In this method, finely powdered copper pyrite is mixed with water in a big vessel. Pine oil or turpentine oil is added to the vessel.
The sulphide particles of ore get wet by this oil and stick to it, whereas the impurities such as clay and sand do not stick to the oil.
In this mixture, air is passed with high pressure through a tube. This results in the formation of froth.
The froth contains light particles of sulphide ore and is collected.
The impurities get wet by water and settle at the bottom of the vessel.
The froth is passed through sieves and concentrated ore is collected.

Question 3.3:

By which method conversion of ZnS and ZnC0₃ into ZnO can be carried out ? Explain writing chemical equation.

Solution :

Question 3.4:

Explain the refining of copper by electrolysis.

Solution :

For the refining of copper by electrolysis, a rod of impure copper is arranged as the anode and a rod of pure copper is arranged as the cathode. The electrolyte used is an aqueous solution of copper sulphate. A small amount of dilute sulphuric acid is added to the electrolyte.
Electric current is allowed to pass through the circuit through which the cathode and the anode are connected.
Copper from the anode gets dissolved in the aqueous solution of copper sulphate and the same amount of copper from the electrolyte gets deposited at the cathode.
Thus, at the cathode, we get copper with almost 100% purity.
The following reactions are observed during electrolysis:
At the anode: Cu(s) (impure) → Cu²⁺ (aq) + 2e^– [oxidation]
At the cathode: Cu²⁺ (aq) + 2e^–  → Cu(s) (pure) [reduction]
Net reaction: Cu(s) (impure) → Cu(s) (pure)

Question 3.5:

Write four physical properties of metals.

Solution :

Four physical properties of metals are

• The surface of metals is shining and can be polished.
• Most metals are solid and heavy in weight. Metals such as mercury and gallium are liquid at room temperature. Sodium, potassium and aluminium are comparatively light metals.
• Metals are good conductors of heat and electricity. Metals such as lead and mercury are poor conductors of heat and electricity.
• Metals have high melting and boiling points.

Question 3.6:

Write chemical equations of the reaction of magnesium metal with dioxygen gas, water, dilute hydrochloric acid and dichlorine gas.

Solution :

Magnesium reacts with dioxygen to form magnesium oxide.
2Mg(s) + O₂(g) → 2MgO(s)

Magnesium reacts with hot water to form magnesium hydroxide and dihydrogen gas.
Mg(s) + 2H₂O(l) → Mg(OH)₂(aq) + H₂(g)

Magnesium reacts with dilute hydrochloric acid to form magnesium chloride and release hydrogen gas.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Magnesium reacts with dichlorine gas to form magnesium chloride.
Mg(s) + Cl₂(g) → MgCl₂(s)

Question 3.7:

Mention the components and uses of brass and bronze.

Solution :

Alloy	Components	Uses
Brass	Copper, zinc	In preparation of cooking utensils, parts of machines and musical instruments
Bronze	Copper, tin	In preparation of statues, coins and medals

Question 4:

Answer the following questions in detail :

Question 4.1:

Write a short note : Earth – Treasure of elements

Solution :

Earth is a treasure of elements. Different elements obtained from different parts of the Earth are

• Lithosphere:

It is the solid part of the Earth made of sand, clay and rocks. It contains different metal elements such as iron, aluminium, copper, zinc etc. in the oxide or sulphide form.

• Hydrosphere:

It is the liquid part of the Earth which comprises the water of seas, rivers, lakes and the ice of the polar regions. Non-metals such as chlorine and fluorine, and metals such as sodium and potassium in the combined form are obtained from this part.

• Atmosphere:

The atmosphere is the cover of gases present around the Earth. Non-metallic gases such as nitrogen, oxygen, carbon dioxide etc. are present here.

Question 4.2:

Explain different methods of concentration of ores.

Solution :

Concentration of ores can be carried out on the basis of types of impurities and form and proportion of metal to be extracted from the ore.
There are three methods for concentration of ores:
Centrifugation on the basis of difference in densities:
Centrifugation is used for the concentration of ores when there is a large difference between the densities of ores and impurities in them.
In this method, the finely powdered ore is placed on a moving table with slots and the table is moved rapidly. Due to the centrifugal force, the light particles of the metal separate out in the slot in the table. Thus, the concentrated ore of a metal can be obtained.

Froth floatation method:
This method is used for the concentration of ores of metals when metals are present in the sulphide form in the ores.
In this method, the finely powdered ore is mixed with water in a big vessel. Pine oil or turpentine oil is added to the vessel.
The sulphide particles of the ore get wet by this oil and stick to it, whereas the impurities such as clay and sand do not stick to the oil.
Air is then passed with high pressure through a tube into this mixture. This results in the formation of froth.
The froth contains light particles of sulphide ore and is collected.
The impurities get wet by water and settle at the bottom of the vessel.
The froth is passed through sieves and concentrated ore is collected.
By this method, ores such as copper pyrites are concentrated and impurities such as clay and sand are removed.

Magnetic separation:
The method of magnetic separation is used to remove the impurity of iron from an ore.
Finely powdered ore is allowed to fall on the leather belt of the separator.
A magnet is present at one end of the belt. This magnet attracts the iron particles from the ore. The iron particles fall near the magnet and the remaining particles of the concentrated ore fall away.
Thus, iron impurity can be removed from an ore due to its magnetic property using a magnetic separator.
In the same manner, other impurities from ores of iron can be separated using a magnetic separator.

Question 4.3:

Explain roasting, calcination and smelting.

Solution :

Roasting:
The method of conversion of sulphide-containing ore to metal oxides is called roasting. For converting sulphide-containing ores to metal oxides, the ore is heated for a long time in the presence of excess of air.
Examples:

Calcination:
The method of conversion of carbonate-containing ore to metal oxides is called calcination. For converting carbonate-containing ores to metal oxides, the ore is heated for a long time in the absence of air.
Examples:

Smelting:
If the ore is in the melted form, then it is called smelting.

Question 4.4:

Explain chemical reduction and electrochemical reduction.

Solution :

Reduction is the process carried out to convert metal oxides to their respective metals. Reduction can be carried out by the chemical reduction method or by the electrochemical method.

Chemical reduction:
In chemical reduction, the metal oxide can be heated in a blast furnace with carbon or carbon monoxide to obtain pure metal.

Oxides of metal such as chromium, iron and manganese can also be heated with aluminium powder to reduce oxides to obtain metal.

Electrochemical reduction:
All metals cannot be reduced by the chemical reduction method. Some metals such as sodium and aluminium possess more attraction towards oxygen than carbon and hence cannot by reduced by chemical reduction. For such metals, electrochemical reduction is used.
In electrochemical reduction, electrodes of inert metals such as graphite or platinum are used. Here, the cathode works as a reducing agent.
Example:
Electrochemical reduction of alumina yields molten aluminium at the cathode and oxygen gas at the anode.

At the cathode: 2Al³⁺(l) + 6e^– → 2Al(l)
At the anode: 6O^2-(l) → 3O₂(g) + 12e^–

Question 4.5:

Explain liquefaction and zone refining method for refining of metals.

Solution :

Metals obtained after reduction are further purified by refining methods. Two of the methods of refining are liquefaction and zone refining.

Liquefaction:
The liquefaction method for refining of metals is used for metals such as tin and lead which have low melting points.
In this method, a furnace with a slope is used. The temperature of the furnace is kept slightly higher than the melting point of the metal. As the impure metal is passed through the slope, the metal melts and is collected in the vessel below the slope.
The melting point of impurities is higher than that of metal; hence, it does not melt and solid impurities remain on the slope.

Zone refining:
Zone refining uses the principle of fractional crystallisation to remove trace impurities present in a metal.
The trace impurities are generally soluble in molten metal. Their solubility decreases as the metal gets cooled. These impurities separate from the metal in the form of crystals.

Question 5:

Answer the following questions pointwise :

Question 5.1:

Discuss the extraction of aluminium from bauxite.

Solution :

Bauxite is the most abundant ore of aluminium on Earth. Extraction of aluminium from bauxite is carried out in two steps: (1) Obtaining alumina from bauxite/Bayer’s process and (2) Obtaining aluminium from alumina.

1. Obtaining alumina from bauxite/Bayer’s process:
Bauxite is an ore of aluminium which also contains iron oxide (Fe₂O₃) and sand (SiO₂) as impurities. Bayer’s process is used to obtain pure aluminium oxide or alumina from bauxite.

In this process, bauxite powder and concentrated sodium hydroxide (45%) are heated in a closed vessel at 433 K temperature and 5-6 bar pressure for 6-8 hours. Aluminium oxide present in bauxite is converted into sodium aluminate which is soluble in water.
Al₂O₃(s) + 2NaOH(aq) →2NaAlO₂(aq) + H₂O(l)

Impurities of iron oxide are removed by filtration as it does not dissolve in sodium hydroxide. Silica forms sodium silicate which is soluble in water. Sodium aluminate and sodium silicate are present in the filtrate. Hydrolysis of sodium aluminate is brought about by adding excess of water and continuous stirring of filtrate. Aluminium hydroxide precipitates and sodium silicate remains as an impurity in the solution.

NaAlO₂(aq) + 2H₂O(l) → Al(OH)₃(s) + NaOH(aq)
The precipitate is washed properly and then dried at 1473 K to obtain pure alumina.

Al(OH)₃(s) →Al₂O₃(s) + 3H₂O(s)
Alumina obtained by this method is 99.5% pure.

2. Obtaining aluminium from alumina:
Alumina obtained by the Bayer’s process is then subjected to the electrochemical reduction method to obtain aluminium.

Electric current cannot pass through the solid form of alumina, and the melting point of alumina is also very high. Hence, cryolite is added to alumina in a small amount to carry out electrolysis easily. This mixture acts as a good conductor of electricity. Addition of feldspar also lowers the melting point of alumina.
Thus, a mixture of alumina, cryolite and feldspar is subjected to electrolysis to obtain pure aluminium.
For electrolysis, an iron vessel with an inner layer coated with carbon-layered graphite is used. This acts as a cathode. The carbon rods are joined with copper clamp and immersed in an electrolyte. These rods act as the anode.
On passing electric current, molten aluminium is deposited at the cathode and oxygen gas is released at the anode.

At the cathode: 2Al³⁺(l) + 6e^–→2Al(l)
At the anode: 6O^2-(l) → 3O₂(g) + 12e^–

Question 5.2:

Explain extraction of iron from haematite.

Solution :

Iron is extracted from haematite by the reduction of haematite in a blast furnace.
The blast furnace is narrow on its upper part, becomes wider in the middle and again becomes narrow in its lower part.
The inner side of the blast furnace is lined with bricks which are fireproof. There is arrangement to blow hot air in the blast furnace form its lower part.

Most of the impurities are removed from the haematite and concentrated ore with coke, sand and limestone is added to the furnace with the help of a cup and cone arrangement in the upper part.
Due to high temperature, the following series of reactions take place in the furnace and molten iron is obtained.

i. At first, the coke combines with the oxygen and forms carbon dioxide. The reaction, being exothermic, releases heat, and the temperature of the furnace increases from 1773 K to 1973 K.
C(s) + O₂(g) → CO₂ + Heat

ii. Due to the high temperature in the furnace, calcium oxide decomposes to form calcium oxide and carbon dioxide.
CaCO₃(s) → CaO(s) + CO₂(g)

iii. Hot carbon dioxide rises in the furnace and combines with coke to form carbon monoxide. This is an endothermic reaction; thus, it lowers the temperature of the furnace to 1173 K.
CO₂(g) + C(s) → 2CO(g)

iv. Iron oxide present in haematite is reduced to the liquid form as the temperature of the furnace goes to 673-973 K.
2Fe₂O₃(s) + CO(g) → 2Fe₃O₄(s) + CO₂(g)
Fe₃O₄(s) + CO(g) → 3FeO(s) + CO₂(g)
FeO(s) + CO(g) → Fe(l) + CO₂(g)

Calcium oxide formed by the decomposition of calcium carbonate combines with silica to form calcium silicate known as slag.
CaO(s) + SiO₂(s) → CaSiO₃(l)

Slag being lighter than molten iron does not mix with the iron and floats on it. In the blast furnace, there are separate outlets for the removal of molten iron and slag.
The molten iron is cooled and solid iron blocks are prepared.

Question 5.3:

Write the activity series of metals. Discuss the experiment for determination of activity series of Fe, Cu and Ag metals.

Solution :

The activity of different metals is different. Some metals lose electrons easily and are more active, whereas some metals do not lose electrons so easily and are less active. The following principle is taken into consideration while determining the activity of a metal: ‘A less active metal can be displaced from the solution of its salt by a more active metal’.

The following experiment can be carried out to determine the activity series of Fe, Cu and Ag metals:

• Take three test tubes 1, 2 and 3.
• In test tube 1, add 0.1 gm of ferrous sulphate (FeSO₄.7H₂O).
• In test tube 2, add 0.1 gm of ferrous sulphate (CuSO₄.5H₂O).
• In test tube 3, add 0.1 gm of silver nitrate (AgNO₃).
• Add 10 ml of distilled water in all the three test tubes.
• Add zinc metal in test tube 1, iron nail in test tube 2 and copper wire in test tube 3.

The following results can be observed after half an hour:

Test tube	Test tube contents	Observation (On metal)	Observation (in solution)
1	FeSO4.7H2O + 10 ml water + Zinc metal	Zinc metal shows colour of iron metal	Colour of ferrous sulphate solution changes from light green to colourless
2	CuSO4.5H2O + 10 ml water + Iron nail	Iron nail shows brown colour due to deposition of copper metal	Colour of copper sulphate changes from dark blue to light blue
3	AgNO3 + 10ml water + Copper wire	Shining white colour silver is deposited on copper wire	Colour of silver nitrate changes from colourless to light blue

Thus, at the end of the experiment, we can conclude that

• Zn is more active than Fe as Fe is displaced from FeSO₄ solution by Zn, i.e. Zn > Fe.
• Fe is more active than Cu as Cu is displaced from CuSO₄ solution by Fe, i.e. Fe > Cu.
• Cu is more active than Ag as Ag is displaced from AgNO₃ solution by Cu, i.e. Cu > Ag.
• Thus, the order of activity of these metals will be Zn > Fe > Cu > Ag.

The activity series of common metals is
K > Na > Ca > Mg > Al >  Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au

Question 5.4:

Discuss chemical properties of metals.

Solution :

The chemical properties of metals are

• Reaction with dioxygen

Metals can easily donate electrons to dioxygen. Hence, they react with oxygen to form metal oxides.
Metal + Dioxygen gas → Metal oxides
Example:
2Mg(s) + O₂(g) →2MgO(s)

Magnesium oxide
Reaction with water
Metals react with water to form metal hydroxide or oxide and release dihydrogen gas.
Metal + water → metal hydroxide/metal oxide + hydrogen gas.
However, not all metals show the same reactivity with water. Metals such as sodium and potassium react vigorously with cold water and release hydrogen gas. The reaction is so vigorous that it burns with an explosion.
2K(s) + H₂O(l) → 2KOH(aq) + H₂(g)
Potassium hydroxide Hydrogen gas
Magnesium metal does not react with cold water, but it reacts with hot water.
Mg(s) + 2H₂O(l) → Mg(OH)₂ + H₂(g)

Magnesium hydroxide    Hydrogen gas
Metals such as aluminium, zinc and copper do not react with hot or cold water, but they react with water vapour and form oxides of metals and release hydrogen gas.
2Al(s) + 3H₂O(g) → Al₂O₃(s) + H₂(g)

Aluminium oxide Hydrogen gas
Zn(s) + H₂O(g) → ZnO(s) + H₂(g)

Zinc oxide Hydrogen gas
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)

Iron oxide Hydrogen gas
Metals such as gold, silver and copper do not react with water.

• Reaction with acids

Not all metals react with dilute acids. Metals which react with dilute acids form corresponding salt and release hydrogen gas.
Metal + Dil. acid → Salt of corresponding metal + Hydrogen gas
Example:
Zn(s) + 2HCl → ZnCl₂(aq) + H₂(g)

Zinc chloride Hydrogen gas
Fe(s) + H₂SO₄ → FeSO₄(aq) + H₂(g)

Iron sulphate Hydrogen gas

• Reaction with dichlorine

Metals react with dichlorine to form metal chlorides.
Metal + Dichlorine → Metal chloride
Example:
Ca(s) + Cl₂(g) → CaCl₂(s)
Calcium chloride
Mg(s) + Cl₂(g) → MgCl₂(s)
Magnesium chloride

• Reaction with dihydrogen

Not all metals react with dihydrogen as it does not accept electrons which metals donate. Some active metals such as sodium, potassium and calcium react with dihydrogen and form metal hydrides.
Active metal + Dihydrogen → Metal hydride
Example:
2Na(s) + H₂(g) →2NaH
Sodium hydride
2K(s) + H₂(g) → 2KH
Potassium hydride
Ca(s) + H₂(g) → CaH₂
Calcium hydride

Question 5.5:

Mentioning reasons for metallic corrosion describe the remedies to prevent it.

Solution :

The erosion reaction of any metal with water or moisture when it comes in its contact is called metallic corrosion. Prolonged contact with moist air or water and air is the only reason for metallic corrosion.

Remedies to prevent metallic corrosion are

• Application of paint:

Applying paint on the iron surface is an easy and cheap method to prevent corrosion. Bodies of car, window grills or iron pillars can be painted and prevented from corrosion.

• Application of oil:

Small tools such as a hammer, gardener’s scissors, nails or the scissors for cutting metals can be protected from corrosion by applying oil on their surfaces. Oil prevents water and air from coming in contact with the surface of iron objects.

• Galvanisation:

Galvanisation is the method of applying a layer of zinc metal on the surface of iron to prevent corrosion. Iron sheets used in the roof of houses are galvanised sheets.

• Sacrificial anode:

In vehicles, such as a steamer, which continuously remains in water, iron plates are prevented from corrosion by using blocks of more active metals such as magnesium or zinc. By doing so, the iron plates act like an anode in sea water and corrosion of the zinc block takes place continuously. This is called sacrificial anode. These blocks need to be replaced at intervals.

• Use of inhibitors:

Suitable chemicals called inhibitors when combined with the surface of iron metals form chemical bonds with the metals and prevent their corrosion.

• Making alloys:

Making alloys of metals turns them to a harder, stronger substance and also prevents their corrosion.

Question 5.6:

What is an alloy ? Mention its advantages. Mention the name of three alloys and also mention the components in them, properties and uses.

Solution :

An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal.
Advantages of alloys are

• Alloys are strong and do not get easily corroded. Example: Iron alloys
• Electrical conductivity of an alloy is less than that of pure metal. Example: Alloys of mercury
• The melting point of alloy is less than that of component elements. Example: Lead and tin alloys

Name, components, properties and uses of some alloys are

Alloy	Components of Alloy	Properties of Alloy	Uses of Alloy
Steel	Iron, carbon	Hard and strong	Construction of buildings and bridges, manufacture of ships, manufacture of spare parts of motorcycles
Brass	Copper, zinc	Malleable, strong, resistant to corrosion	Preparation of cooking utensils, manufacture of parts of machines, making musical instruments
Bronze	Copper, tin	Strong and resistant to corrosion	Building of statues, making coins and medals