Chapter 3 – Elements Compounds and Mixtures

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Book Name	: Concise Chemistry Middle School
Subject	: Chemistry
Class	: 7
Publisher	: Selina Publisher PVT Ltd
Chapter Name	: Elements Compounds and Mixtures

EXERCISE – I

Question 1.

Write the symbols of helium, silver, krypton, antimony, barium.

Answer

Element	Symbol
Helium	He
Silver	Ag
Krypton	Kr
Antimony	Sb
Barium	Ba

Question 2.

Write the names of following elements Na, C, Kr, U, Ra, Fe, Co.

Answer

Symbol	Element
Na	Sodium
C	Carbon
Kr	Krypton
U	Uranium
Ra	Radium
Fe	Iron
Co	Cobalt

Question 3.

Define :

1. Elements
2. Compound

Answer

1. Elements: An element is the basic form of matter that cannot be broken down into simpler substances by chemical reactions.
2. Compounds: A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass.

Question 4.

Name the main metal present in the following :

1. Haemoglobin
2. Chalk
3. Chlorophyll
4. Chocolate

Answer

Substance	Element
Haemoglobin Iron	Iron
Chalk Calcium	Calcium
Chlorophyll	Magnesium
Chocolate wrappers	Aluminium

Question 5.

Give four examples of non-metallic elements.

Answer

1. Oxygen (O)
2. Nitrogen (N)
3. Chlorine (Cl)
4. Carbon (C)

Question 6.

What do you understand by :

1. Metalloids
2. Noble gases

Answer

1. Metalloids: Metalloids are those substances which have some properties of metals and some of non-metals e.g. boron, silicon.
2. Noble gases: Noble gases are those which do not react chemically with other elements or compounds e.g. helium, neon, etc.

Question 7.

Select elements and compounds from the following list:

Iron, plaster of Paris, chalk, common salt, copper, aluminium, calcium oxide, cane sugar, carbon, silica, sodium sulphate, uranium, potassium carbonate, silver, and carbon dioxide.

Answer

Elements	Compounds
Iron	Plaster of Paris
Copper	Common salt
Aluminium	Calcium oxide
Uranium	Cane sugar
Silver	Sodium sulphate
Carbon	Silica
	Potassium carbonate
	Carbon dioxide

 

EXERCISE – II

Question 1.

State four differences between compounds and mixtures.

Answer

Compound	Mixture
Pure substance	Impure substance
Homogeneous	Homogeneous or Heterogeneous
Fixed composition	No fixed composition
Involves change in energy	Does not involve a change in energy
The specific set of properties	No specific set of properties
Components can be separated by simple physical methods	Components can only be separated by complex chemical processes

Question 2.

What are the characteristic properties of a pure substance? Why do we need them?

Answer

The characteristic properties of a pure substance are:

1. Definite set of properties: Pure substances have a definite set of properties such as boiling point, melting point, density, etc. These properties remain constant regardless of the size of the sample.
2. Homogeneous: Pure substances are homogeneous, meaning that their composition is uniform throughout the bulk.

We need pure substances for various reasons:

1. Manufacture medicines: Pure substances are required to manufacture medicines. Impurities in a substance can cause negative side effects and can be harmful to human health.
2. Prepare chemicals in the industry: In the chemical industry, pure substances are needed to prepare chemicals with specific properties and to ensure consistency in the manufacturing process.
3. For scientific purposes: Pure substances are necessary for scientific research, as they allow for accurate and reproducible results.
4. To maintain good health: Many pure substances are essential for maintaining good health, such as vitamins and minerals. Ensuring that these substances are pure and free from impurities is crucial to prevent any adverse health effects.

Question 3.

Give two examples for each of the following :

1. Solid + Solid mixture
2. Solid + Liquid mixture
3. Liquid + Liquid mixture

Answer

Solid + Solid mixture:

1. Salt and sugar mixture
2. Iron and sulfur mixture

Solid + Liquid mixture:

1. Salt and water solution
2. Sugar and water solution

Liquid + Liquid mixture:

1. Water and ethanol mixture
2. Oil and vinegar mixture

Question 4.

Define :

1. Evaporation
2. Filtration
3. Sublimation
4. Distillation
5. Miscible liquids
6. Immiscible liquids

Answer

1. Evaporation: The process by which a liquid changes into its gaseous state at a temperature below its boiling point is called evaporation. During evaporation, the liquid absorbs heat from the surroundings, which causes its particles to gain energy and escape from the surface of the liquid.
2. Filtration: Filtration is a separation process used to separate insoluble solids from a liquid or a mixture of liquids and solids. It involves passing the mixture through a porous material, called a filter, which allows the liquid to pass through while retaining the solid.
3. Sublimation: Sublimation is the process in which a solid directly changes into its gaseous state without passing through the liquid state. This occurs when the vapor pressure of the solid is greater than the atmospheric pressure.
4. Distillation: Distillation is a separation process used to separate two or more liquids based on their differences in boiling points. The mixture is heated to boil and the vapors are condensed and collected separately.
5. Miscible liquids: Miscible liquids are liquids that can mix in any proportion to form a homogeneous mixture. This occurs because the molecules of the two liquids are attracted to each other.
6. Immiscible liquids: Immiscible liquids are liquids that cannot mix with each other to form a homogeneous mixture. Instead, they form separate layers when mixed together. This occurs because the molecules of the two liquids are not attracted to each other.

Question 5.

Name the process by which the components of the following mixtures can be separated.

1. Iron and sulphur
2. Ammonium chloride and sand
3. Common salt from seawater
4. Chaff and grain
5. Water and mustard oil
6. Sugar and water
7. Cream from milk

Answer

1. Iron and sulphur:
   Magnetic separation.
   Since iron is magnetic, a magnet can be used to separate the iron from the sulphur.
2. Ammonium chloride and sand:
   Sublimation.
   Ammonium chloride can be separated from sand by sublimation, as it sublimes on heating while sand does not.
3. Common salt from seawater:
   Evaporation.
   Seawater can be evaporated to obtain salt crystals.
4. Chaff and grain:
   Winnowing.
   The mixture is allowed to fall from a height, and the lighter chaff is carried away by the wind while the heavier grains fall straight down.
5. Water and mustard oil:
   Separation using a separating funnel.
   Since water and oil are immiscible, they can be separated using a separating funnel.
6. Sugar and water:
   Evaporation.
   The water can be evaporated from the solution to obtain the sugar.
7. Cream from milk:
   Centrifugation.
   The milk can be spun in a centrifuge, which causes the cream to separate from the milk due to the difference in densities.

Question 6.

How will you separate a mixture of common salt, chalk powder and powdered camphor? Explain.

Answer

The mixture of common salt, chalk powder, and powdered camphor can be separated using a combination of filtration, sublimation, and evaporation. First, water is added to dissolve the salt and then the mixture is filtered to remove the chalk and camphor. The camphor is then separated from the chalk powder by sublimation, while the salt is obtained by evaporating the water.

Question 7.

How is distillation more advantageous than evaporation?

Answer

Distillation is more advantageous than evaporation as it can separate liquids with different boiling points, while evaporation can only separate solids from liquids or obtain solids from solutions.

Question 8.

1. What is chromatography?
2. Why is it named so?
3. What are the advantages of chromatography?
4. Name the simplest type of chromatography.
5. On what principle is this method based?
6. What is meant by stationary phase and mobile phase in chromatography?

Answer

1. The process of separating different dissolved constituents of a mixture by their absorption on an appropriate material is called chromatography.
2. It is named so, because earlier it was used to separate mixtures containing coloured components only but these days this technique is applied to colourless substances too.
3. Advantages of chromatography :(i) A very small quantity of the substance can be separated.
   (ii) Components with very similar physical and chemical properties can be separated.
   (iii) It identifies the different constituents of a mixture.
4. The simplest type of chromatography is “Paper chromatography”.
5. Chromotography is based on differential affinities of compounds towards two phases i.e. stationary and mobile phase.
6. The filter paper acts as a “stationary phase” while the solvent act as a “mobile phase”.

Question 9.

On what principle are the following methods of separation based? Give one example of a mixture for each of the methods mentioned in which they are used

1. Sublimation
2. Filtration
3. Sedimentation and decantation
4. Solvent extraction method
5. Magnetic separation
6. By using a separating funnel
7. By using a separating funnel
8. Fractional distillation

Answer

1. Sublimation: Sublimation is based on the principle that some solids can directly change into gas upon heating without going through the liquid state. An example of a mixture where sublimation is used is a mixture of iodine and sand. Iodine can be separated from sand by sublimation as it sublimes on heating while sand does not.
2. Filtration: Filtration is based on the principle that a porous material can separate a solid from a liquid or a mixture of solids and liquids. An example of a mixture where filtration is used is a mixture of sand and water. Sand can be separated from water by filtration as it does not dissolve in water.
3. Sedimentation and decantation: Sedimentation and decantation are based on the principle that solids settle down in a liquid when left undisturbed. An example of a mixture where sedimentation and decantation are used is muddy water. The mud particles settle down to the bottom, and the clear water can be decanted from the top.
4. Solvent extraction method: The solvent extraction method is based on the principle that a substance can be separated from a mixture by selectively dissolving it in a suitable solvent. An example of a mixture where the solvent extraction method is used is a mixture of oil and water. Oil can be separated from water by dissolving it in a suitable solvent such as ether or benzene.
5. Magnetic separation: Magnetic separation is based on the principle that some materials are attracted to magnets. An example of a mixture where magnetic separation is used is a mixture of iron filings and sand. Iron filings can be separated from the sand by using a magnet.
6. Separating funnel: Separating funnel method is based on the principle of differential solubility and density of the components. An example of a mixture where the separating funnel is used is a mixture of oil and water. The immiscible oil and water can be separated by using a separating funnel.
7. Fractional distillation: Fractional distillation is based on the principle of differences in boiling points of the components of a mixture. An example of a mixture where fractional distillation is used is crude oil. Crude oil is a mixture of different hydrocarbons, and fractional distillation is used to separate them into different fractions based on their boiling points.

OBJECTIVE TYPE QUESTIONS

Question 1.

Fill in the blanks:

1. ____ are made up of the same kind of atoms.
2. ____ and ____ are pure substances.
3. In a ____ the substances are not combined chemically.
4. Clay is separated from water by the method called ____.
5. ____ is a process to obtain a very pure form of a solid dissolved in a liquid.
6. Camphor and ammonium chloride can ____.

Answer

1. Elements
2. Elements and compounds
3. mixture
4. loading and decantation.
5. Crystallisation
6. sublimate

Question 2.

Give one-word answers for the following :

1. The solid particles which remain on the filter paper after the filtration ____
2. The liquid which evaporates and then condenses during the process of distillation ____
3. The process of transferring the clean liquid after the solid settles at the bottom of the ____
4. The process by which two miscible liquids are separated ____

Answer

1. residue
2. distillate
3. decantation
4. distillation

MULTIPLE CHOICE QUESTIONS

Select the correct alternative from the choices given for the following statements:

Question 1.

A pure liquid is obtained from a solution by :

1. evaporation
2. distillation
3. Alteration
4. crystallisation

Answer

distillation

Explanation

A pure liquid can be obtained from a solution by distillation, which involves heating the solution to its boiling point and collecting the resulting vapour in a separate container. As the vapour condenses, it forms a pure liquid with a higher degree of purity than the original solution.

Question 2.

Components of crude petroleum can be separated by :

1. distillation
2. evaporation
3. filtration
4. fractional distillation

Answer

fractional distillation

Explanation

Components of crude petroleum can be separated by fractional distillation. Crude petroleum is a complex mixture of hydrocarbons with different boiling points, and fractional distillation allows for the separation of these components based on their boiling points.

Question 3.

An example of a homogeneous mixture is :

1. tap water
2. distilled water
3. sand and water
4. water and oil

Answer

tap water

Explanation

Distilled water is a compound, not a mixture, since it is exactly pure H₂O Sand and water and water in oil, are examples of heterogeneous mixtures. Tap water contains some minerals as well as metallic impurities distributed uniformly throughout the sample. Hence option (a) is correct.

Question 4.

In chromatography, the filter paper is :

1. stationary phase
2. mobile phase
3. mixture
4. none of the above

Answer

stationary phase

Explanation

In chromatography, the filter paper is the stationary phase. In chromatography, the mixture is applied to the stationary phase, which does not move, while the mobile phase (usually a liquid or gas) moves through the stationary phase, carrying the components of the mixture with it.

Question 5.

A set of mixtures is :

1. ink, honey, ice cream, milk
2. tapwater, gold, common salt, alloy
3. milk, brass, silver, honey
4. butter, petroleum, tapwater, iron

Answer

ink, honey, icecream, milk