ELEMENTS, COMPOUNDS & MIXTURES - Questions & Answers

EXERCISE

Introduction Elements, Compounds & Mixtures

1. Represent with the help of a simple chart how matter is classified into pure or impure substances & further into elements, compounds & mixtures, with elements further segmented.

Matter is classified as follows:
MATTER
I. Pure Substances
   1. Elements
      a. Metals (e.g., Iron)
      b. Non-metals (e.g., Sulphur)
      c. Metalloids (e.g., Silicon)
      d. Noble gases (e.g., Helium)
   2. Compounds (e.g., Iron sulphide)
II. Impure Substances (Mixtures)
   1. Homogeneous (e.g., Air)
   2. Heterogeneous (e.g., Oil & water)

Elements

2. Define the terms elements, compounds & mixtures with a view to show their basic difference.

Element: A pure substance made up of only one kind of atoms that cannot be broken down into simpler substances by physical or chemical means.
Compound: A pure substance formed by the chemical combination of two or more different elements in a fixed proportion.
Mixture: An impure substance formed by mechanically mixing two or more substances (elements, compounds, or both) in any proportion, where constituents retain their original properties.

3. 'An atom is the basic unit of an element'. Draw a diagram of an atom - divisible as seen today.

An atom consists of two main parts:
1. Nucleus: The central part containing Protons (positively charged) and Neutrons (no charge).
2. Orbits: The paths surrounding the nucleus in which Electrons (negatively charged) revolve.
(Note: Schematic diagram shows a central cluster of protons/neutrons with electrons on circular rings/orbits around it).

4. The modern periodic table consists of elements arranged according to their increasing atomic numbers. With reference to elements with atomic numbers 1 to 20 only in the periodic table - differentiate them into - metallic elements, metalloids, non-metals & noble gases.

Based on atomic numbers 1 to 20:
Metallic elements: Lithium (3), Beryllium (4), Sodium (11), Magnesium (12), Aluminium (13), Potassium (19), Calcium (20).
Metalloids: Boron (5), Silicon (14).
Non-metals: Hydrogen (1), Carbon (6), Nitrogen (7), Oxygen (8), Fluorine (9), Phosphorus (15), Sulphur (16), Chlorine (17).
Noble gases: Helium (2), Neon (10), Argon (18).

5. Elements are broadly classified into metals & non-metals. State six general differences in physical properties of metals & non-metals. State two metals & two non-metals which contradict with the general physical properties - giving reasons. State one difference in property between metalloids & noble gases.

Differences:
1. Metals have lustre; Non-metals do not.
2. Metals have high melting/boiling points; Non-metals have low points.
3. Metals have high density; Non-metals have low density.
4. Metals are malleable; Non-metals are non-malleable.
5. Metals are ductile; Non-metals are non-ductile.
6. Metals are good conductors of heat/electricity; Non-metals are poor conductors.

Contradictions:
- Metals: Mercury (liquid at room temperature); Zinc (is non-malleable beyond certain temperatures).
- Non-metals: Graphite (conducts electricity and is lustrous); Iodine (is lustrous).

Difference: Metalloids show properties of both metals and non-metals and are chemically reactive, whereas noble gases are chemically inert and exist in traces in the atmosphere.

Molecules

6. With reference to elements - define the term 'molecule'. Give two examples each of a monoatomic, diatomic & polyatomic molecule.

Molecule: The smallest particle of a substance which can normally exist independently and retain the physical and chemical properties of that substance.
Monoatomic: Helium (He), Neon (Ne).
Diatomic: Hydrogen (H₂), Oxygen (O₂).
Polyatomic: Ozone (O₃), Phosphorus (P₄).

Compounds

7. Define the term 'compound'. In the compound carbon dioxide - the elements carbon & oxygen are combined in a fixed ratio. Explain.

A compound is a pure substance made up of two or more different elements combined chemically in a fixed proportion.
In Carbon dioxide (CO₂), one atom of carbon always combines with two atoms of oxygen. By mass, carbon and oxygen always combine in a fixed ratio of 3:8 to form carbon dioxide.

8. State five different characteristics of compounds. Give three differences between elements & compounds with relevant examples.

Characteristics:
1. Components are in a definite proportion.
2. Particles in a compound are of one kind.
3. They have a definite set of properties.
4. Elements in the compound do not retain their original properties.
5. Components can be separated by chemical means only.

Differences:
1. Atoms: Element contains one kind of atoms (e.g., Carbon); Compound contains two or more kinds (e.g., CO₂).
2. Separation: Elements cannot be broken down; Compounds can be broken down by chemical means.
3. Properties: Element (e.g., Oxygen) supports combustion; Compound (e.g., CO₂) is a non-supporter of combustion.

Mixtures

9. Explain the term 'mixture'. Differentiate between homogeneous & heterogeneous mixtures. State why brass is considered as a homogeneous mixture while a mixture of iron & sulphur - heterogeneous. Give an example of two liquids which form a] homogeneous b] heterogeneous - mixtures.

A mixture is made of two or more substances mechanically mixed together in any proportion, retaining their individual properties.
Homogeneous: Constituents are uniformly mixed (e.g., Salt + water).
Heterogeneous: Constituents are not uniformly mixed (e.g., Oil + water).
Brass is homogeneous because the copper and zinc are uniformly distributed. Iron and sulphur is heterogeneous because iron particles can be seen separately.
a] Homogeneous: Alcohol + water.
b] Heterogeneous: Oil + water.

10. Compare the properties of iron [II] sulphide with iron-sulphur mixture, considering iron [II] sulphide as a compound & particles of iron & sulphur mixed together as an example of a mixture.

1. Composition: Compound (FeS) has fixed proportions; Mixture (Fe+S) has varying proportions.
2. Magnetism: Iron in FeS is not attracted by a magnet; Iron in the mixture is attracted by a magnet.
3. Reaction with acid: FeS does not give Hydrogen gas with dil. acid; the mixture gives Hydrogen gas with dil. acid.
4. Separation: FeS components require chemical means; mixture components can be separated by physical means (magnet).

Separation of mixtures

11. State any one method - to separate the following mixtures -
a] Two solid mixtures one of which - directly changes into vapour on heating.
b] Two solid mixtures one of which - dissolves in a particular solvent and other does not
c] A solid-liquid mixture containing - an insoluble solid in the liquid component
d] A solid-liquid mixture containing - a soluble solid in the liquid component
e] A liquid-liquid mixture containing - two immiscible liquids having different densities
f] A liquid-liquid mixture containing - two miscible liquids having different boiling points.
g] A liquid-gas mixture containing - a gas dissolved in a liquid component.
h] A gas-gas mixture containing - two gases with different densities.
i] A mixture of different solid constituents - in a liquid constituent.

a] Sublimation
b] Solvent extraction
c] Filtration
d] Evaporation
e] Separating funnel
f] Fractional distillation
g] Boiling the liquid-gas mixture
h] Diffusion
i] Chromatography

12. Explain with diagrams the process used to - separate the following substances from the given mixtures.
a] Ammonium chloride from a mixture of - ammonium chloride & potassium chloride.
b] Iron from a mixture of - iron & copper.
c] Sulphur from a mixture of - sulphur & copper.
d] Potassium nitrate from a mixture of - potassium nitrate & potassium chlorate.
e] Lead carbonate [insoluble] from a mixture of - lead carbonate & water.
f] Lead nitrate [soluble] from a mixture of - lead nitrate & water i.e lead nitrate solution.
g] Carbon tetrachloride from a mixture of - carbon tetrachloride [heavier component] & water.
h] Benzene from a mixture of - benzene [b.p. 80°C] & toluene [b.p. 110°C].
i] Different dyes - in their liquid constituent ink.

a] Sublimation: Heat the mixture; Ammonium chloride sublimes and collects on the cool inner funnel walls.
b] Magnetic Separation: Use a magnet to attract iron filings from copper particles.
c] Solvent Extraction: Add carbon disulphide to dissolve sulphur; filter to remove copper.
d] Fractional Crystallisation: Based on difference in solubility in a particular solvent.
e] Filtration: Filter the mixture; lead carbonate remains as residue on filter paper.
f] Evaporation: Heat the solution; water escapes as vapour leaving lead nitrate behind.
g] Separating Funnel: Allow the liquids to stand; the heavier carbon tetrachloride is tapped out from below.
h] Fractional Distillation: Use a fractionating column; benzene distills over first due to lower b.p.
i] Chromatography: Dyes separate as the solvent flows over an ink spot on filter paper based on adsorption levels.

CRITICAL THINKING OBJECTIVE TYPE QUESTIONS

Elements, Compounds & Mixtures

Q.1 Select the correct answer from A, B, C, D & E for each statement given below:
A: Gunpowder B: Iodine C: Boron D: Helium E: Bromine

1. A diatomic molecule.
Answer: E: Bromine

2. A metalloid.
Answer: C: Boron

3. A non-metal which is lustrous.
Answer: B: Iodine

4. A mixture consisting of elements & a compound.
Answer: A: Gunpowder

5. A noble gas.
Answer: D: Helium

Q.2 Match the separation of components in List I with the most appropriate process in List II.
List I
1. Naphthalene from naphthalene & sodium chloride.
2. Cream from milk.
3. Kerosene oil from kerosene oil & water.
4. Lead nitrate from an aqueous solution of lead nitrate.
5. Ammonia from an aqueous solution of ammonia.

List II
A: Separating funnel
B: Sublimation
C: Boiling
D: Centrifugation
E: Distillation

Answer:
1. B: Sublimation
2. D: Centrifugation
3. A: Separating funnel
4. E: Distillation
5. C: Boiling

Q.3 The diagram represents fractional distillation for separation of mixtures. Answer the following:

1. Can two immiscible liquids be separated by this process.
Answer: No, this process is used for miscible liquids.

2. Separation of liquids by this process is based on which physical property.
Answer: Difference in boiling points of the two liquids.

3. If methyl alcohol & water are to be separated, which liquid would remain in flask 'X' after condensation.
Answer: Water.

4. Give a reason for the above answer.
Answer: Water has a higher boiling point (100°C) compared to methyl alcohol (64.7°C).

5. State the purpose of the fractionating column in the apparatus.
Answer: The fractionating column helps in better condensation and separation of vapours of different liquids.

Q.4 Select the correct answer from the choice in bracket to complete each sentence:-

1. Dust in air is an example of heterogeneous [heterogeneous/homogeneous] mixture.

2. A soluble solid is separated from an insoluble solid by solvent extraction [fractional crystallisation/solvent extraction].

3. The reactive element from the two monoatomic elements is silicon [neon/silicon].

4. Compounds are always homogeneous [homogeneous or heterogeneous/always homogeneous] in nature.

5. An example of a monoatomic molecule is helium [hydrogen/helium].

Q.5 Give reasons for the following statements:

1. Components in a mixture can be separated by physical methods only.
Answer: In a mixture, the components are mechanically mixed and not chemically combined, so they retain their original physical properties which can be exploited for separation.

2. Centrifugation can be used for separating an insoluble heavier solid, present in an insoluble solid-liquid mixture.
Answer: The centrifugal force causes the more dense component to migrate away from the axis of rotation, effectively separating it from the less dense liquid.

3. The filter paper made into a cone & placed in a funnel for filtering out the solid particles in a solid-liquid mixture, should be moistened before placing.
Answer: Moistening allows the filter paper to stick to the funnel walls properly, preventing the mixture from bypassing the paper.

4. Brass & bronze are examples of mixtures, while copper sulphate & lead nitrate are examples of compounds.
Answer: Brass and bronze are alloys where components are mixed in varying proportions and retain their properties, while copper sulphate and lead nitrate have elements chemically combined in fixed proportions.

5. Zinc is considered an element, while zinc sulphide is considered a compound.
Answer: Zinc is made of only one kind of atoms, whereas zinc sulphide is made of two different elements (Zinc and Sulphur) combined chemically in a fixed proportion.