Elements, Compounds and Mixtures

Elements, Compounds & Mixtures

Question 1

Differentiate between the terms elements, compounds & mixtures with suitable examples.

Answer

Elements, compounds and mixtures can be differentiated as below:

Question 2

State which element exists in the highest percentage in -

(a) earth's crust

(b) atmosphere

(c) human body

Answer

(a) Oxygen [O] - 46.1% exists in the highest percentage in earth's crust.

(b) Nitrogen [N] - 78% exists in the highest percentage in atmosphere.

(c) Oxygen [O] - 65% exists in the highest percentage in human body.

Question 3

Elements are mainly classified into metals & non-metals. State six properties of metals which differ from non-metals.

Answer

Six properties of metals which differ from non-metals are as below:

Question 4

Give the symbols of the following elements - & state in each case whether they are metals, non-metals, metalloids or noble gases.

(a) potassium

(b) carbon

(c) oxygen

(d) mercury

(e) fluorine

(f) neon

(g) sulphur

(h) zinc

(i) boron

(j) copper

(k) calcium

(l) xenon

(m) arsenic

(n) mercury

(o) chlorine

(p) silicon

(q) neon

(r) lead

(s) antimony

(t) sodium

(u) iodine

(v) platinum

(w) radon

(x) phosphorus

(y) bromine

(z) aluminium

Answer

(a) potassium [K] — Metal

(b) carbon [C] — Non-metal

(c) oxygen [O] — Non-metal

(d) mercury [Hg] — Metal

(e) fluorine [Fl] — Non-Metal

(f) neon [Ne] — Noble Gas

(g) sulphur [S] — Non-Metal

(h) zinc [Zn] — Metal

(i) boron [B] — Metalloid

(j) copper [Cu] — Metal

(k) calcium [Ca] — Metal

(l) xenon [Xe] — Noble gas

(m) arsenic [As] — Metalloid

(n) mercury [Hg] — Metal

(o) chlorine [Cl] — Non-metal

(p) silicon [Si] — Metalloid

(q) neon [Ne] — Noble gas

(r) lead [Pb] — Metal

(s) antimony [Sb] — Metalloid

(t) sodium [Na] — Metal

(u) iodine [I] — Non-metal

(v) platinum [Pt] — Metal

(w) radon [Rn] — Noble Gas

(x) phosphorus [P] — Non-Metal

(y) bromine [Br] — Non-Metal

(z) aluminum [Al] — Metal

Question 5

State what is meant by the term 'activity series of metals'. State the most reactive & the least reactive metal from the following - Zn, Ag, Na, Fe, Cu, Pb.

Answer

It is a series of metals arranged according to their reactivity in which the most reactive metal i.e., potassium is at the top of the series and the least reactive metal i.e, gold is at the bottom.

Amongst the mentioned metals: Zn, Ag, Na, Fe, Cu, & Pb, Sodium (Na) is the most reactive metal and Silver (Ag) is the least reactive metal.

Question 6

Give three reasons why - carbon dioxide is considered a compound, while carbon - an element.

Answer

Carbon is an element, whereas carbon dioxide is considered a compound because:

1. Carbon is made up of only one kind of atom, Carbon dioxide is made up of two kinds of atoms i.e., Carbon and Oxygen [CO₂].

2. Carbon cannot be broken down into two or more simpler substances by physical or chemical means. Carbon dioxide can be broken down into simpler substances by chemical means only.

3. Carbon has its own set of properties. Properties of Carbon dioxide differ from the properties of carbon and oxygen.

Question 7

State what information is provided by the formula of calcium hydroxide - Ca(OH)₂.

Answer

The important information provided by the formula of calcium hydroxide [Ca(OH)₂] is:

1. The compound calcium hydroxide comprises of the elements calcium, oxygen, and hydrogen.
2. It has one atom of calcium, two atoms of oxygen and two atoms of hydrogen.

Question 8

Four atoms of hydrogen combine with one atom of carbon to give methane [CH₄]. State the valency of carbon.

Answer

Valency of carbon is 4 because four atoms of hydrogen combine with one atom of carbon to form CH₄.

Question 9

Write the symbols of the following elements & radicals along with their valencies.

(a) sodium

(b) magnesium

(c) chlorine [chloride]

(d) nitrate

(e) sulphite

(f) aluminium

(g) oxygen [oxide]

(h) bromine [bromide]

(i) zinc

(j) sulphur [sulphide]

(k) sulphate

(l) hydroxide

(m) calcium

(n) carbonate

(o) potassium

(p) phosphate

(q) iodine [iodide]

Valency list:

(i) 1⁺

(ii) 2⁺

(iii) 3⁺

(iv) 1^-

(v) 2^-

(vi) 3^-

Answer

Symbols along with valencies of mentioned elements and radicals is as follows :

(a) Sodium — Na¹⁺

(b) Magnesium — Mg²⁺

(c) chlorine [chloride] — Cl^1-

(d) Nitrate — NO₃^1-

(e) sulphite — SO₃^2-

(f) aluminium — Al³⁺

(g) oxygen [oxide] — O^2-

(h) bromine [bromide] —Br^1-

(i) zinc — Zn²⁺

(j) sulphur [sulphide] — S^2-

(k) sulphate —SO₄^2-

(l) hydroxide — OH^1-

(m) calcium — Ca²⁺

(n) carbonate — CO₃^2-

(o) potassium — K¹⁺

(p) phosphate — PO₄^3-

(q) iodine [iodide] — I^1-

Question 10

Write the chemical formula of the following -

(a) magnesium oxide

(b) sodium bromide

(c) calcium sulphide

(d) magnesium sulphate

(e) aluminium chloride

(f) zinc oxide

(g) calcium carbonate.

Answer

Please find below the chemical formulas of mentioned compounds:

(a) Magnesium oxide:

Step 1 — Write each symbol with its valency

$\text{Mg}^{2+} \phantom{\nearrow} \text{O}^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{1}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{O}} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Magnesium oxide : $\bold{MgO}$

(b) sodium bromide:

Step 1 — Write each symbol with its valency

$\text{Na}^{1+} \phantom{\nearrow} \text{Br}^{1-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{Br}} \Rightarrow \underset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{Br}} \\[0.5em]$

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of Sodium bromide : $\bold{NaBr}$

(c) calcium sulphide:

Step 1 — Write each symbol with its valency

$\text{Ca}^{2+} \phantom{\nearrow} \text{S}^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{S}} \Rightarrow \underset{\phantom{1}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{S}} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Calcium sulphide : $\bold{CaS}$

(d) magnesium sulphate:

Step 1 — Write each symbol with its valency

$\text{Mg}^{2+} \phantom{\nearrow} \text{SO}_4^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{1}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{SO}_4} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Magnesium sulphate : $\bold{MgSO_4}$

(e) aluminium chloride:

Step 1 — Write each symbol with its valency

$\text{Al}^{3+} \phantom{\nearrow} \text{Cl}^{1-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{3}}{\text{Al}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{Cl}} \Rightarrow \underset{\phantom{1}{1}}{\text{Al}} \space {\searrow}\mathllap{\swarrow} \space \underset{3}{\text{Cl}} \\[0.5em]$

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of Aluminium chloride : $\bold{AlCl_3}$

(f) zinc oxide:

Step 1 — Write each symbol with its valency

$\text{Zn}^{2+} \phantom{\nearrow} \text{O}^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{2}}{\text{Zn}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{O}} \Rightarrow \underset{\phantom{1}{2}}{\text{Zn}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{O}} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Zinc oxide : $\bold{ZnO}$

(g) calcium carbonate:

Step 1 — Write each symbol with its valency

$\text{Ca}^{2+} \phantom{\nearrow} \text{CO}_3^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{CO}_3} \Rightarrow \underset{\phantom{1}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{CO}_3} \\[0.5em]$

Step 3 — Write the interchanged number, ignore equal numbers & hence the formula

Therefore, we get

Formula of Calcium carbonate : $\bold{CaCO_3}$

Question 11

Give the formulas of the following acids:

(a) hydrochloric

(b) nitric

(c) sulphuric

(d) carbonic - acid.

Answer

Please find below the chemical formulas of mentioned acids:

(a) Hydrochloric acid:

Step 1 — Write each symbol with its valency

$\text{H}^{1+} \phantom{\nearrow} \text{Cl}^{1-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{Cl}} \Rightarrow \underset{\phantom{1}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{Cl}} \\[0.5em]$

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of hydrochloric acid: $\bold{HCl}$

(b) Nitric acid:

Step 1 — Write each symbol with its valency

$\text{H}^{1+} \phantom{\nearrow} \text{NO}_3^{1-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{1}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{NO}_3} \\[0.5em]$

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of nitric acid: $\bold{HNO}_3$

(c) Sulphuric acid:

Step 1 — Write each symbol with its valency

$\text{H}^{1+} \phantom{\nearrow} \text{SO}_4^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{1}{2}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{SO}_4} \\[0.5em]$

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of Sulphuric acid : $\bold{H}_2\bold{SO}_4$

(d) Carbonic acid:

Step 1 — Write each symbol with its valency

$\text{H}^{1+} \phantom{\nearrow} \text{CO}_3^{2-}$

Step 2 — Interchange the valencies

$\overset{\phantom{2}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{CO}_3} \Rightarrow \underset{\phantom{1}{2}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{CO}_3} \\[0.5em]$

Step 3 — Write the interchanged number & hence the formula

Therefore, we get

Formula of Carbonic acid : $\bold{H}_2\bold{CO}_3$

Question 12

Give the formulas of the following gases:

(a) hydrogen chloride

(b) ammonia

(c) carbon monoxide

(d) nitric oxide

(e) nitrous oxide

(f) nitrogen dioxide

(g) nitrogen

(h) sulphur dioxide.

Answer

(a) hydrogen chloride — HCl

(b) ammonia — NH₃

(c) carbon monoxide — CO

(d) nitric oxide — NO

(e) nitrous oxide — N₂O

(f) nitrogen dioxide — NO₂

(g) nitrogen — N₂

(g) sulphur dioxide — SO₂

Question 13

Explain the term mixture. Differentiate between a homogeneous & heterogeneous mixture with one example of each in the

(a) solid-solid

(b) solid-liquid

(c) liquid-liquid state of the mixture.

Answer

A mixture is an impure substance containing more than one element or compound, mechanically mixed together in any proportion. The mixture retains the properties of its constituents (elements or compounds) and it can be separated into its components by simple mechanical or physical means.

Question 14

State four differences between - elements, compounds & mixtures with suitable examples.

Answer

Question 15

State the correct technique for separation of the following mixtures.

(a) a sublimable solid & a non-sublimable solid.

(b) a liquid component from soluble impurities in the liquid component.

(c) a lighter liquid from a heavier liquid.

(e) a low boiling point liquid from a high boiling point liquid.

(f) solid constituents in a liquid constituent by adsorption.

Answer

(a) a sublimable solid & a non-sublimable solid — sublimation

(b) a liquid component from soluble impurities in the liquid component — distillation

(c) a lighter liquid from a heavier liquid — separating funnel

(d) a low boiling point liquid from a high boiling point liquid — fractional distillation

(e) solid constituents in a liquid constituent by adsorption — chromatography

Question 16

Give —

(a) the principle involved in separation of the mixture

(b) the technique of separation for each of the following mixtures.

(i) naphthalene & sodium chloride

(ii) common salt from a solution of common salt in water

(iii) pure water from impure water

(iv) kerosene & water

(v) methyl alcohol & water

(vi) dyes of an ink

Answer

(i) Naphthalene & sodium chloride:

(a) Principle involved: Based on the difference in sublimable and non-sublimable nature of solids.

(b) Technique of separation: The mixture of the sublimable solid and non-sublimable solid is heated in an evaporating dish covered with funnel plugged at one end with cotton.

Sublimable solids on heating sublimes and the vapours condense and collect in the pure form on the inner side of the funnel, from where the sublimable solid is scrapped off.

(ii) Common salt from a solution of common salt in water :

(a) Principle involved: Based on evaporation of the liquid component in a soluble solid-liquid mixture.

(b) Technique of separation: The soluble solid can be separated from its liquid component by allowing the liquid component to evaporate either on its own or by heating. On heating, water is lost to the atmosphere and common salt remains behind in the evaporating dish.

(iii) Pure water from impure water:

(a) Principle involved: Based on the distillation of the liquid component in a soluble solid-liquid mixture.

(b) Technique of separation: The soluble impurity can be separated from its liquid component or pure water from impure water by placing the mixture, i.e., impure water in the distillation flask. On heating the distillation flask, the solid or solid impurities in water remains behind in the distillation flask. The liquid or water vaporizes, condenses in the Leibig’s condenser and is collected in the receiver.

(iv) Kerosene & water:

(a) Principle involved: Based on the separation of two immiscible liquids (one heavy, one light) through separating funnel.

(b) Technique of separation: The kerosene & water mixture is added to the separating funnel and the funnel is kept aside for some time.

The heavier immiscible liquid i.e., water settles down in the separating funnel and is then removed from below on opening the tap and collected out separately.

The lighter immiscible liquid i.e., kerosene remains above the heavier liquid.

(v) Methyl alcohol & water:

(a) Principle involved: Based on the separation of mixture of two miscible liquids with different boiling points.

(b) Technique of separation: The mixture of two miscible liquids e.g., methyl alcohol and water having different boiling points is taken in the distillation flask and heated slowly till it boils. The component with the higher boiling point i.e., water remains behind in the distillation flask after condensation. The component with the lower boiling point i.e., methyl alcohol collects in the receiver after condensation in the Leibig's condenser.

(vi) Dyes of an ink:

(a) Principle involved: Chromatography is a method by which the various components of a mixture e.g., dyes in an ink, are separated due to their difference in the rate of flow over an adsorbent medium.

(b) Technique of separation:

• Stationary phase in chromatography, the adsorbent medium is known as the stationary phase e.g., filter paper made of cellulose.

• Mobile phase the solvent or the solution that moves due to capillary action is known as the mobile phase.

Separation of dyes in ink by paper chromatography. Ink prepared by dissolving different dyes e.g. A, B, C, D in a suitable solvent, is taken.

• A line is drawn with a pencil over a long strip of a filter paper.
• Place the ink spot containing the different dyes on the filter paper.
• The filter paper is then hung with its lower end dipped in a suitable solvent.
• The paper strip with the spot of ink at its end is dipped into the solvent in a way that the spot itself remains above the liquid surface.
• The solvent flows over the ink spot & the various components of the mixture, of dyes in the ink are separated.

Objective Type Questions

Question 1

Select the correct answer from A, B, C, D & E for each statement given below:

A: Chalk B: Oxygen C: Silicon D: Bromine E: Nitrous Oxide

1. An element present in the earth's crust, atmosphere & human body.

2. The chemical name for dinitrogen oxide [N₂O].

3. A compound containing carbon, oxygen & calcium.

4. A metalloid.

5. A non-metal which exists in the liquid state.

Answer

1. Oxygen.

2. Nitrous Oxide.

3. Chalk.

4. Silicon.

5. Bromine.

Question 2

Select the correct answer from the choice in bracket.

1. The least reactive metal. [magnesium/silver/copper].

2. The positively charged particle of an atom. [electron/proton/neutron].

3. The formula of caustic soda. [KOH/Ca(OH)₂/NaOH].

4. The ideal method to separate iodine & KCl. [sublimation/evaporation/distillation].

5. A homogeneous mixture. [brass / dust in air / chalk & water].

Answer

1. The least reactive metal — silver.

2. The positively charged particle of an atom — proton.

3. The formula of caustic soda — NaOH.

4. The ideal method to separate iodine & KCl — sublimation.

5. A homogeneous mixture — brass.

Question 3

Match the ideal method of separation of components in a mixture in List I with the appropriate process in List II.

Answer

Question 4

Give reasons for the following statements:

1. If fractional distillation is carried out using a liquid-liquid mixture, one liquid will remain in the flask & the other will be collected in the receiver.

2. Evaporation of a common salt solution or sea water, leaves behind common salt inside the evaporating dish after heating.

3. Components in a mixture are present in varying proportions and not in a fixed proportion.

4. Gunpowder is an example of a heterogeneous mixture.

5. In chromatography the absorbent medium e.g. Whatman filter paper is known as the - stationary phase.

Answer

1. Fractional distillation separates two miscible liquids with different boiling points. The fractionating column traps and condenses the higher boiling point component, allowing only the lower boiling point component to enter the condenser and be collected.

2. During evaporation of common salt solution or sea water, common salt is left behind inside the evaporating dish on heating because water, i.e., the liquid component is lost to the atmosphere as it gets converted into water vapour.

3. Mixture is an impure substance formed by mixing two or more elements or compounds in any proportion. Mixtures do not have a definite set of properties. If the components are mixed in a fixed proportion then it will form a compound and not a mixture.

4. Gunpowder is considered as a heterogeneous mixture because its components charcoal, sulphur, and potassium nitrate are not uniformly mixed throughout, and it has different properties and composition throughout the mixture.

5. In chromatography the absorbent medium e.g., Whatman filter paper is known as the stationary phase because the filter paper made up of cellulose binds the water tightly to the paper whereas in mobile phase, the solvent or the solution moves due to capillary action.

Question 5

Name the following:

1. The non-sublimable solid from a mixture of iodine & potassium nitrate.

2. The heavier liquid component from - mercury & water.

3. The lower boiling point component from methyl alcohol & water.

4. The compound containing one atom of sulphur & two atoms of oxygen.

5. An acid whose formula is 'H₂CO₃'.

Answer

1. Potassium nitrate.

2. Mercury.

3. Methyl alcohol.

4. Sulphur dioxide [SO₂].

5. Carbonic acid.