NCERT Solutions For Class 10 Chemistry: Metals and Non-Metals

Intext Questions (Page 41)

Question 1: Give an example of a metal which
(i) is a liquid at room temperature.
(ii) can be easily cut with a knife.
(iii) is the best conductor of heat.
(iv) is a poor conductor of heat.

Answer-
(i) The metal which is a liquid at room temperature is mercury.
(ii) Sodium (or lithium or potassium), which are alkali metals, are so soft that they can be cut with a knife.
(iii) The best conductor of heat is silver (and copper).
(iv) Lead (and mercury) are comparatively poor conductors of heat.

Question 2: Explain the meanings of malleable and ductile.

Answer-
Malleability is the property of metals that allows them to be beaten into thin sheets. Gold and silver are the most malleable metals.
Ductility is the ability of metals to be drawn into thin wires. Gold is cited as the most ductile metal.

Intext Questions (Page 46-47)

Question 1: Why is sodium kept immersed in kerosene oil?

Answer-
Sodium is kept immersed in kerosene oil because it is a highly reactive metal. Sodium reacts so vigorously with oxygen in the air that it catches fire if kept in the open. Kerosene oil is used to protect sodium and prevent accidental fires.

Question 2: Write equations for the reactions of
(i) iron with steam
(ii) calcium and potassium with water

Answer-
(i) Iron reacts with steam (gaseous water) to form iron oxide and hydrogen gas:
$3\text{Fe}(s) + 4\text{H}_2\text{O}(g) \rightarrow \text{Fe}_3\text{O}_4(s) + 4\text{H}_2(g)$

(ii) Calcium reacts with water (cold water) to form calcium hydroxide and hydrogen gas:
$\text{Ca}(s) + 2\text{H}_2\text{O}(l) \rightarrow \text{Ca}(\text{OH})_2(aq) + \text{H}_2(g)$

Potassium reacts violently with water (cold water) to form potassium hydroxide and hydrogen gas, releasing heat energy:
$2\text{K}(s) + 2\text{H}_2\text{O}(l) \rightarrow 2\text{KOH}(aq) + \text{H}_2(g) + \text{heat energy}$

Question 3: Samples of four metals A, B, C and D were taken and added to the following solution one by one. The results obtained have been tabulated.
(i) Which is the most reactive metal?
(ii) What would you observe if B is added to a solution of Copper(II) sulphate?
(iii) Arrange the metals A, B, C and D in the order of decreasing reactivity.

Answer-
(i) Metal B is the most reactive metal. This is concluded because Metal B causes a displacement reaction with Iron(II) sulphate solution. Since a more reactive metal displaces a less reactive metal, B is more reactive than Iron, placing it high on the activity series.

(ii) If metal B is added to a solution of Copper(II) sulphate, a displacement reaction will occur. Since B displaces iron (Fe), B must be more reactive than copper (Cu). Therefore, B will displace copper from the solution. The blue color of the Copper(II) sulphate solution will change/fade, and solid copper metal will be deposited on the surface of metal B.

(iii) The order of decreasing reactivity is: B > A > C > D.
(B displaces Iron; A displaces Copper but not Iron; C displaces Silver but not Copper; D shows no displacement with any of the salts).

Question 4: Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction when iron reacts with dilute $\text{H}_2\text{SO}_4$.

Answer-
When a reactive metal reacts with dilute hydrochloric acid, hydrogen gas is produced.
The chemical reaction when iron reacts with dilute sulphuric acid ($\text{H}_2\text{SO}_4$) is:
$\text{Fe}(s) + \text{H}_2\text{SO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{H}_2(g)$

Question 5: What would you observe when zinc is added to a solution of iron(II) sulphate? Write the chemical reaction that takes place.

Answer-
When zinc is added to a solution of iron(II) sulphate, a displacement reaction is observed. Since zinc is placed higher than iron in the activity series, zinc is more reactive and displaces iron from its salt solution. The green color of the iron(II) sulphate solution will fade or change color, and solid iron metal will be deposited on the zinc.
The chemical reaction that takes place is:
$\text{Zn}(s) + \text{FeSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Fe}(s)$

Intext Questions (Page 49)

Question 1: (i) Write the electron-dot structures for sodium, oxygen and magnesium.
(ii) Show the formation of $\text{Na}_2\text{O}$ and $\text{MgO}$ by the transfer of electrons.
(iii) What are the ions present in these compounds?

Answer-
(i) Based on electronic configurations:
Sodium ($\text{Na}$): 2, 8, 1. $\rightarrow$ $\text{Na} \cdot$ (One valence electron)
Oxygen ($\text{O}$): 2, 6. $\rightarrow$ $\colon\ddot{\text{O}}\colon$ (Six valence electrons)
Magnesium ($\text{Mg}$): 2, 8, 2. $\rightarrow$ $\cdot\text{Mg}\cdot$ (Two valence electrons)

(ii) Formation of $\text{Na}_2\text{O}$: Two sodium atoms each lose one electron to form two $\text{Na}^+$ cations, and one oxygen atom gains these two electrons to form an $\text{O}^{2-}$ anion.
$\text{Na} \cdot + \colon\ddot{\text{O}}\colon + \cdot \text{Na} \rightarrow [\text{Na}]^+ [\colon\ddot{\text{O}}\colon]^{2-} [\text{Na}]^+$

Formation of $\text{MgO}$: One magnesium atom loses its two valence electrons to form an $\text{Mg}^{2+}$ cation, and one oxygen atom gains these two electrons to form an $\text{O}^{2-}$ anion.
$\cdot\text{Mg}\cdot + \colon\ddot{\text{O}}\colon \rightarrow [\text{Mg}]^{2+} [\colon\ddot{\text{O}}\colon]^{2-}$

(iii) The ions present in these compounds are:
In $\text{Na}_2\text{O}$: Sodium cation ($\text{Na}^+$) and Oxide anion ($\text{O}^{2-}$).
In $\text{MgO}$: Magnesium cation ($\text{Mg}^{2+}$) and Oxide anion ($\text{O}^{2-}$).

Question 2: Why do ionic compounds have high melting points?

Answer-
Ionic compounds have high melting points because they are composed of oppositely charged ions which are held together by strong electrostatic forces of attraction. A considerable amount of energy is required to overcome or break these strong inter-ionic attractions, leading to high melting points.

Intext Questions (Page 53)

Question 1: Define the following terms.
(i) Mineral
(ii) Ore
(iii) Gangue

Answer-
(i) Mineral: The elements or compounds which occur naturally in the earth’s crust are known as minerals.
(ii) Ore: Minerals that contain a very high percentage of a particular metal and from which the metal can be profitably extracted are called ores.
(iii) Gangue: These are large amounts of unwanted impurities, such as soil and sand, that contaminate ores mined from the earth.

Question 2: Name two metals which are found in nature in the free state.

Answer-
Two metals which are found in nature in the free state (native state) are gold and silver. These metals are located at the bottom of the activity series and are least reactive.

Question 3: What chemical process is used for obtaining a metal from its oxide?

Answer-
The chemical process used for obtaining a metal from its oxide is reduction. This is achieved either by using reducing agents like carbon, or for highly reactive metals, by electrolytic reduction.

Intext Questions (Page 55)

Question 1: Metallic oxides of zinc, magnesium and copper were heated with the following metals. (In which cases will you find displacement reactions taking place?)

Answer-
A displacement reaction will take place only if the metal used for heating is more reactive than the metal present in the oxide.
Displacement reactions occur in these cases:

1. Zinc metal heated with Copper oxide ($\text{CuO}$) (since $\text{Zn} > \text{Cu}$).
2. Magnesium metal heated with Zinc oxide ($\text{ZnO}$) (since $\text{Mg} > \text{Zn}$).
3. Magnesium metal heated with Copper oxide ($\text{CuO}$) (since $\text{Mg} > \text{Cu}$).

Question 2: Which metals do not corrode easily?

Answer-
Metals that do not corrode easily are those at the bottom of the activity series, as they are the least reactive. Examples include gold ($\text{Au}$), silver ($\text{Ag}$), and platinum.

Question 3: What are alloys?

Answer-
An alloy is defined as a homogeneous mixture of two or more metals, or a metal and a non-metal. Alloys are prepared by melting the primary metal first and then dissolving the other elements in definite proportions, followed by cooling.

Exercise Questions

Question 1: Which of the following pairs will give displacement reactions?
(a) $\text{NaCl}$ solution and copper metal
(b) $\text{MgCl}_2$ solution and aluminium metal
(c) $\text{FeSO}_4$ solution and silver metal
(d) $\text{AgNO}_3$ solution and copper metal.

Answer-
The correct pair is (d) $\text{AgNO}_3$ solution and copper metal. Copper is a more reactive metal than silver ($\text{Ag}$), so copper will displace silver from its salt solution.

Question 2: 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-
The methods listed—applying grease, applying paint, and galvanising (applying a coating of zinc)—are all effective methods for preventing the rusting of iron.
The most comprehensive answer is (d) All of the above.

Question 3: 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-
The element is likely to be (a) calcium. Calcium (a metal) reacts with oxygen to form calcium oxide ($\text{CaO}$), which is an ionic compound having a high melting point and is soluble in water (forming alkali).

Question 4: 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-
Food cans are coated with tin because (c) zinc is more reactive than tin. Zinc is higher in the activity series than tin. If zinc were used, it could react with the acidic food components (since zinc is more reactive) and spoil the food.

Question 5: You are given a hammer, a battery, a bulb, wires and a switch.
(a) How could you use them to distinguish between samples of metals and non-metals?
(b) Assess the usefulness of these tests in distinguishing between metals and non-metals.

Answer-
(a) The materials can be used to distinguish samples based on two physical properties:

1. Malleability/Hardness Test (using the hammer): Metals are generally hard and malleable (they can be beaten into thin sheets). Non-metals are usually brittle and will break when struck by the hammer.
2. Electrical Conductivity Test (using the circuit): The battery, bulb, wires, and switch can be set up as a circuit. If the sample is a metal, the bulb will glow because metals are good conductors of electricity. If the sample is a non-metal, the bulb will generally not glow, as they are bad conductors.

(b) These tests are useful as metals and non-metals have generally opposite physical properties. However, there are exceptions:

1. Hardness/Malleability: Alkali metals (like sodium) are soft and can be cut with a knife. Diamond, an allotrope of carbon (a non-metal), is the hardest natural substance known.
2. Electrical Conductivity: Metals are excellent conductors. Graphite, an allotrope of carbon, is a non-metal but is a conductor of electricity.

Question 6: What are amphoteric oxides? Give two examples of amphoteric oxides.

Answer-
Amphoteric oxides are certain metal oxides that exhibit both acidic and basic behaviour. These oxides react with both acids and bases to produce salts and water.
Two examples of amphoteric oxides are aluminium oxide ($\text{Al}_2\text{O}_3$) and zinc oxide ($\text{ZnO}$).

Question 7: Name two metals which will displace hydrogen from dilute acids, and two metals which will not.

Answer-
Metals that displace hydrogen are those above hydrogen in the Activity Series:

1. Magnesium ($\text{Mg}$)
2. Zinc ($\text{Zn}$)
   (Magnesium reacts vigorously with dilute acid, showing a high rate of bubble formation).

Metals that will not displace hydrogen are those below hydrogen in the Activity Series:

1. Copper ($\text{Cu}$)
2. Silver ($\text{Ag}$)

Question 8: In the electrolytic refining of a metal M, what would you take as the anode, the cathode and the electrolyte?

Answer-
In the electrolytic refining process:

1. Anode: The impure metal M.
2. Cathode: A thin strip of the pure metal M.
3. Electrolyte: A solution of the metal salt of M (e.g., a solution of acidified metal M sulphate).

Question 9: Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it, as shown in figure below.
(a) What will be the action of gas on (i) dry litmus paper? (ii) moist litmus paper?
(b) Write a balanced chemical equation for the reaction taking place.

Answer-
(a) The gas evolved is sulphur dioxide ($\text{SO}_2$), formed when sulphur (a non-metal) burns in oxygen. Non-metal oxides are generally acidic.
(i) Dry litmus paper: There will be no action because the gas must dissolve in water to exhibit acidic properties.
(ii) Moist litmus paper: The gas will turn blue litmus paper red, indicating that the oxide formed is acidic.

(b) Balanced chemical equation for the reaction:
$\text{S}(s) + \text{O}_2(g) \rightarrow \text{SO}_2(g)$

Question 10: State two ways to prevent the rusting of iron.

Answer-
The rusting of iron requires exposure to both air and water. Two ways to prevent rusting are:

1. Galvanisation: Protecting the steel or iron by coating it with a thin layer of zinc.
2. Painting/Oiling/Greasing: Applying a protective coat that prevents moist air from contacting the iron surface.

Question 11: What type of oxides are formed when non-metals combine with oxygen?

Answer-
When non-metals combine with oxygen, they form oxides that are generally acidic or neutral.

Question 12: 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.

Answer-
(a) Platinum, gold, and silver are metals of low reactivity. They are found at the bottom of the activity series, meaning they do not react with oxygen or corrode easily, thus retaining their metallic lustre and shine over long periods.
(b) Sodium, potassium, and lithium are highly reactive alkali metals. They react so vigorously with oxygen in the air that they instantly catch fire if kept in the open. They are stored immersed in kerosene oil to protect them and prevent accidental fires.
(c) Although aluminium is a highly reactive metal, its surface develops a thin layer of aluminium oxide when exposed to air. This protective oxide layer is highly resistant to further corrosion and prevents the metal from reacting further. Aluminium is also a good conductor of heat, making it suitable for cooking utensils.
(d) Carbonate and sulphide ores must be converted into oxides because it is easier to obtain a metal from its oxide than from its sulphide or carbonate form. Sulphide ores are converted by roasting, and carbonate ores by calcination, both yielding the metal oxide prior to reduction.

Question 13: 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.

Answer-
Copper vessels tarnish because they react with moist carbon dioxide in the air, forming a green coating of basic copper carbonate. Lemon and tamarind juice contain acids. Since metal oxides are basic in nature, the acid in the sour substances reacts chemically with and dissolves the basic copper carbonate layer, removing the tarnish and cleaning the vessel.

Question 14: Differentiate between metal and non-metal on the basis of their chemical properties.

Answer-
The classification of elements as metals and non-metals can be clearly established based on their distinct chemical properties, which primarily stem from their tendency to achieve a completely filled valence shell.

1. Ion Formation (Tendency to Lose or Gain Electrons):

Metals, such as sodium, potassium, and magnesium, typically have one, two, or three electrons in their outermost shell.
Metals have a tendency to lose electrons from their outermost shell to attain a stable octet configuration. When they lose electrons, they form positive ions known as cations (e.g., $\text{Na}^+$).

Non-metals, such as oxygen, chlorine, and sulphur, generally have five, six, or seven electrons in their outermost shell.
Non-metals have a tendency to gain electrons when reacting with metals to complete their stable octet configuration. When they gain electrons, they form negative ions known as anions (e.g., $\text{Cl}^-$). The compounds formed by this transfer of electrons are known as ionic compounds.

2. Reaction with Oxygen and Nature of Oxides:

Almost all metals combine with oxygen to form metal oxides. Most metal oxides are basic in nature. For example, sodium oxide and potassium oxide dissolve in water to produce alkalis (hydroxides).
However, certain metal oxides, such as aluminium oxide ($\text{Al}_2\text{O}_3$) and zinc oxide ($\text{ZnO}$), show both acidic and basic behaviour; these are known as amphoteric oxides.

Non-metals combine with oxygen to form oxides that are generally acidic or neutral. Non-metal oxides typically produce acidic solutions when they dissolve in water.

3. Reaction with Dilute Acids:

Reactive metals react with dilute acids to produce a salt and hydrogen gas. For instance, magnesium reacts vigorously, and the reactivity decreases in the order $\text{Mg} > \text{Al} > \text{Zn} > \text{Fe}$.
Non-metals generally do not displace hydrogen from dilute acids.

4. Reaction with Hydrogen:

Non-metals react with hydrogen to form stable compounds known as hydrides.
Metals generally do not react directly with hydrogen under normal conditions, or they require specific high-temperature conditions to form metal hydrides.

Question 15: A man went door to door posing as a goldsmith. He promised to bring back the glitter of old and dull gold ornaments. An unsuspecting lady gave a set of gold bangles to him which he dipped in a particular solution. The bangles sparkled like new but their weight was reduced drastically. The lady was upset but after a futile argument the man beat a hasty retreat. Can you play the detective to find out the nature of the solution he had used?

Answer-
The solution used was Aqua Regia. Aqua regia is a freshly prepared, highly corrosive mixture of concentrated hydrochloric acid and concentrated nitric acid in a 3:1 ratio. Gold is one of the least reactive metals and does not dissolve in either acid alone. However, aqua regia is a powerful reagent capable of dissolving gold (and platinum). By dipping the ornaments, a thin outer layer of gold dissolved into the solution, removing the dullness and revealing the shiny surface underneath, but resulting in a reduction of the total weight.

Question 16: Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).

Answer-
Copper is preferred over steel (an alloy of iron) for making hot water tanks primarily due to their difference in reactivity with water and steam.

1. Copper is unreactive towards water: Copper does not react with water at all, even at high temperatures (hot water or steam).
2. Iron reacts with steam: Iron, though it does not react with cold or hot water, reacts with steam to form iron oxide ($\text{Fe}_3\text{O}_4$) and hydrogen. In a hot water tank, prolonged exposure to high temperatures may lead to corrosion of the steel due to its iron content reacting with steam.
   Therefore, copper is chosen for its superior corrosion resistance in this specific environment.