Selina Concise Solutions for Chapter 3 Elements, Compounds and Mixtures Class 8 Chemistry ICSE

Exercise 3(A)

1. Define: (a) Elements (b) Compounds
 Answer
(a) Elements: Element is a substance which cannot be broken further into simpler substances and has a definite set of properties. Elements are made up of only one kind of atoms.
(b) Compounds: Compounds are pure substances composed of two or more elements in definite proportion by mass and has properties, entirely different from those of its constituents elements.
Compound, are made up of different types of atoms combined chemically.

2. Give two examples for each of the following:
(a) Metals (b) Non-metals
(c) Metalloids (d) Inert gases
 Answer
(a) Metals: Iron, silver, gold.
(b) Non-metals: Carbon, sulphur, oxygen.
(c) Metalloids: Antimony, silicon, boron.
(d) Inert gases: Helium, argon, neon.

3. (a) Pure and impure substances
(b) Homogenous and heterogeneous substances
 Answer
(a) Difference between Pure substances and Impure Substances:

Pure substances

Impure substances

1. Pure substances have definite composition and definite physical and chemical properties.

1. Impure substances are made up of two or more pure substances mixed together in any proportion.

2. They are all homogeneous i.e. their composition is uniform throughout the bulk.

2. They may be homogeneous or heterogeneous i.e. their composition is not uniform throughout the bulk.

Examples: Elements and compounds.

Examples: air, sea water, petroleum, a solution of sugar in water are all impure substances.

(b) Difference between Homogeneous mixture and Heterogeneous Mixtures

Homogenous mixture

Heterogeneous mixture

A homogenous mixture is a mixture where the components that make up the mixture are uniformly distributed throughout the mixture.

A heterogeneous mixture is a mixture, where the components of the mixture are not uniform or have localized regions with different properties.

Components of homogeneous mixture cannot be seen through naked eyes.

Components of heterogeneous mixtures can be seen through naked eyes.

In this mixture the component sizes are atomic/molecular level.

Here, in this mixture, the size of particles are large.

Components of homogeneous mixture cannot be separated easily.

Components of heterogeneous mixture can be separated easily.

The constituent particles in a homogeneous mixture possess the same physical properties.

The constituent particles in heterogeneous mixture possess different physical properties.

Example: air, sugar water, rain water etc.

Example: Cereal in milk, vegetable soup etc.


4. Write the chemical name of the following and also give their molecular formulae:
(a) Baking soda 
(b) Vinegar
(c) Marble 
(d) Sand
 Answer
(a) Sodium bicarbonate (Baking soda) — NaHCO3
(b) Acetic acid (Vinegar) — CH3COOH
(c) Calcium carbonate (Marble) — CaCO3
(d) Silicon dioxide (Sand) — SiO2

5. Name:
(a) a soft metal
(b) a metal which is brittle
(c) a non-metal which is lustrous
(d) a liquid metal
(e) a metal which is a poor conductor of electricity.
(f) a non-metal which is a good conductor of electricity.
(g) a liquid non-metal
(h) the hardest naturally occurring substance
(i) an inert gas
 Answer
(a) Gold
(b) Zinc
(c) Iodine
(d) Mercury
(e) Tungsten
(f) Graphite
(g) Bromine
(h) Diamond
(i) Neon, helium

6. How is sodium chloride different from its constituent elements?
 Answer
The properties of sodium chloride are completely different from those of sodium and chlorine. Sodium is a soft, highly reactive metal. Chlorine is a poisonous non-metallic gas while sodium chloride is a very useful non poisonous compound which is added to our food to get minerals and also to add taste to it.

7. Why is iron sulphide a compound?
 Answer
Iron sulphide is a compound which can be broken into the elements iron and sulphur they both have different properties. The properties of compound are entirely different from there of its constituents elements.

Exercise 3(B)


1. Classify the following substances into compounds and mixtures:
 Answer
Carbon dioxide, air, water, milk, common, salt, blood, fruit juice, iron sulphide.
  • Carbon dioxide — (Compound)
  • air — (Mixture)
  • water — (Compound)
  • milk — (Mixture)
  • common salt — (Compound)
  • blood — (Mixture)
  • fruit juice — (Mixture)
  • iron sulphide — (Compound)

2. Give one example for each of the following types of mixtures
(a) solid-solid homogenous mixture
(b) solid-liquid heterogeneous mixture
(c) miscible liquids
(d) liquid-gas homogenous mixture
 Answer
(a) Solid-solid homogenous mixture — Alloys of metals e.g. brass, bronze stainless steel etc.
(b) Solid-liquid heterogeneous mixture — Sand and water, mud and water, sugar and oil.
(c) Miscible liquids — water and ethanol.
(d) Liquid-gas homogenous mixture — Air

3. Suggest a suitable technique to separate the constituents of the following mixtures. Also give the reason for selecting the particular method.
(a) Salt from sea water
(b) Ammonium chloride from sand
(c) Chalk powder from water
(d) Iron from sulphur
(e) Water and alcohol
(f) Sodium chloride and potassium nitrate
(g) Calcium carbonate and sodium chloride
 Answer
(a) The technique used to separate the salt from seawater is Evaporation.
Reason – Because this method is used to separate the components of the homogeneous solid-liquid mixture. In this method, sea water is collected in a shallow bed and allowed to evaporate in the sun. When all the water is evaporated, salt is left behind. By this method, we only get solid and liquid is evaporated in its vapour form.

 (b) Technique used to separate Ammonium chloride from sand is sublimation.
Because this method is used for solid mixtures in which one of the components can sublime on heating. In this method, Ammonium chloride changes into vapours on heating and salt is left behind.

 (c) Technique used to separate chalk powder from water is filtration.
Reason – Because this process is used to separate the components of a heterogeneous solid-liquid mixture in which solids are lights and insoluble in liquids. Substances used as filters are sand filter paper at C. These filters allows the liquid to pass through them, but not solids.

 (d) Technique to separate iron from sulphur is magnetic separation.
Because, this method is used when one of the component of mixture is Iron. Iron gets attracted towards the magnet and hence get separated.

 (e) Technique used to separate water and Alcohol is Fractional Distillation.
Because in this method, the vapours of water is left behind in the original vessel as the alcohol boils at lower temperature than water. Thus these two liquids can be separated.

 (f) Technique used is Fractional-crystallisation.
Because: This method is used when solubility of solid components of mixture and different in the same solvent. Here, sodium chloride and potassium nitrate. Both are soluble in water but solubility of potassium nitrate is more.

 (g) Technique used is Solvent Extraction Method: Because, by this method, salts get dissolve in water while calcium carbonate being insoluble in water settles down in the container. And hence get separated about.

4. (a) Define mixture.
(b) Why is it necessary to separate the constituents of a mixture.
(c) State four differences between compounds and mixtures.
 Answer
(a) “Mixtures can be defined as. a kind of matter which is formed by mixing two or more pure substances (elements and compounds) in any proportion, such that they do not undergo any chemical change and retain their individual properties. Therefore they are impure substances.

 (b) Because: The mixtures contain unwanted substances which may be harmful and may degrade the properties of mixtures. So we, need to separated them and extract useful substances.
This is necessary because
  1. It removes unwanted and harmful substances
  2. to obtain pure and useful substances them.
Example: Sea water is rich in common salt which is an important ingredient of our food to add taste and nutrients. But sea water, cannot be directly used to get the salt.
Hence, it is necessary to separate both.

 (c) Difference between Compound and Mixtures

Compound

Mixtures

1. A compound is formed from its constituent elements as a result of chemical reaction.

1. A mixture is obtained form its (elements, compounds) components as a result of physical change.

2. A compound is always homogeneous in nature.

2. The mixtures can be homogeneous or heterogeneous.

3. In a compound the elements are present in a fixed ratio by weight.

3. In a mixture the components can be present in any ratio.

4. The components of a compound can’t be separated by physical methods but can be separated by chemical methods only.

4. The components of a mixture can be separated by physical methods only.

5. The properties of a compound are different from those of its elements.

5. The properties of a mixture lie between those of-its components.

6. The formation of a compound from its elements is accompanied by energy changes.

6. The formation of a mixture from its constituents is not accompanied by energy changes.

5. (a) What is chromatography? For which type of mixture is it used?
(b) What are the advantages of chromatography.
 Answer
(a) This is one of the latest techniques to separate the coloured components of a mixture when all the components are very similar in their properties. Example: Components of ink are separated by this method. Ink is a mixture of different dyes, which are separated by chromatography because some of the dyes are less soluble and some are more soluble in a solvent.

 (b) Advantages of chromatography:
  1. A very small quantity of the substance can be separated.
  2. Components with very similar physical and chemical properties can be separated.
  3. It identifies the different constitutes of a mixture.
  4. It also helps in quantitive estimation of components of a mixture.

6. Choose the most appropriate answer from the options given below:

 (a) a mixture of sand and ammonium chloride can be separated by
 1. filtration
2. distillation
3. sublimation
4. crystallisation
Answer
3. sublimation

(b) A pair of metalloids are
 1. Na and Mg
2. B and Si
3. C and P
4. He and Ar
Answer
2. B and Si

(c) Which of the following property is not shown by compounds?
1. They are heterogeneous.
2. They are homogeneous.
3. They have definite molecular formulae.
4. They have fixed melting and boiling points.
Answer
1. They are heterogeneous.

(d) A solvent of Iodine is
1. Water
2. Kerosene oil
3. Alcohol
4. Petrol
Answer
3. Alcohol

(e) Which of the gas is highly soluble in water?
1. Ammonia
2. Nitrogen
3. Carbon monoxide
4. Oxygen
Answer
1. Ammonia

Additional Questions


Check Your Progress 1


Fill in the blanks.
1. A pure substance has definite          and constant         .
2. Mixtures can be            or          .
3. Use of a          to separate the components of a mixture of solids is based on the difference in the size of the components.
4.            is used when the constituents of a mixture of solids have difference in their weights.
5. Iodine, camphor, naphthalene, ammonium chloride and dry ice are some substances that          .

Answer

1. A pure substance has definite composition and constant properties.
2. Mixtures can be heterogeneous or homogenous.
3. Use of a sieve to separate the components of a mixture of solids is based on the difference in the size of the components.
4. Winnowing is used when the constituents of a mixture of solids have difference in their weights.
5. Iodine, camphor, naphthalene, ammonium chloride and dry ice are some substances that sublimate.


Check Your Progress 2


1. Filtraction is a method used to separate fine particles of EEE.. solid from a liquid.
Answer
Filtraction is a method used to separate fine particles of insoluble solid from a liquid.

2. Loading is basically speeding up EEEE
Answer
Loading is basically speeding up sedimentation.

3. Name the techniques used to separate a solid from its solution.
Answer
Sedimentation and Decantation

4. A separating funnel can be used for separating a mixture of immiscible liquids. True or false ?
Answer
True

5. During centrifugation, solid particles of the mixture move towards the bottom. True or false ?
Answer
True

Exercises


Tick the most appropriate answer.


(a) The constituents of a mixture are present in
1. a fixed ratio
2. a variable ratio,
3. the ratio of 2: 1
4. none of these
Answer
2. a variable ratio

(b) Solutions are
1. heterogeneous mixtures.
2. compounds
3. homogeneous mixtures.
4. elements.
Answer
3. homogeneous mixtures.


(c) The methods of separating components of a given mixture are based on the
1. physical properties and state of the components.
2. colour of the components only.
3. state of the components.
4. none of these.
Answer
1. physical properties and state of the components.


(d) Winnowing is the method used to separate
1. chaff from grain.
2. stones from rice
3. oil from water
4. salt from sand
Answer
1. chaff from grain.


(e) During filtration the substance left behind on the filter paper is called
1. distillate
2. filtrate
3. sublimate.
4. residue.
Answer
4. residue.


(f) Loading is a process in which
1. impurities become heavy and sink to the bottom.
2. impurities float on the top.
3. impurities vaporize.
4. none of these
Answer
1. impurities become heavy and sink to the bottom.


(g) Fractional distillation is used to separate liquids having an appreciable difference in their
1. size and shape
2. solubility
3. boiling points.
4. none of these
Answer
3. boiling points.


(h) The different constituents of an ink are separated by
1. handpicking.
2. paper chromatography
3. filtration.
4. magnetic separation.
Answer
2. paper chromatography


B. Fill in the blanks.

1. A        substance has only one kind of matter.
2. The composition and properties of a          mixture is uniform throughout.
3. An          is a homogeneous mixture of two or more metals.
4. Chalk powder dissolved in water is an example of a Chalk powder dissolved in water is an example of a          .
5. An         is formed when one liquid is dispersed as tiny droplets throughout another liquid
6. Immiscible liquids are separated by using a         .
Answer
1. A pure substance has only one kind of matter.
2. The composition and properties of a homogenous mixture is uniform throughout.
3. An alloy is a homogeneous mixture of two or more metals.
4. Chalk powder dissolved in water is an example of a Chalk powder dissolved in water is an example of a suspension.
5. An emulsion is formed when one liquid is dispersed as tiny droplets throughout another liquid
6. Immiscible liquids are separated by using a separating funnel.

C. Write true or false for each statement. Rewrite the false statements correctly.

1. All pure substances have characteristic melting and boiling points.
Answer
True

2. Milk is an emulsion.
Answer
True



3. A heterogeneous mixture has a uniform composition throughout the mixture.
Answer
False.
A heterogeneous mixture has a non-uniform composition through out the mixture.

4. The constituents of a mixture can only be separated by chemical means.
Answer
False.
The constituents of a mixture can be separated by chemical means and physical means.

5. Handpicking can be used as a separation technique if the particle size of the constituents of the mixture is the same.
Answer
False.
Handpicking can be used as a separation technique if the particle size of the constituents of the mixture is the not the same.

D. Match the columns.


E. Differentiate between the following.
1. solution and suspension
2. Supernatant liquid and Fill rate
3. Filter Paper and Alum
Answer
1. Difference between Solution and Suspension

Solution

Suspension

1. It is an example of homogeneous mixture.

1. It is an example of heterogeneous mixture.

2. It is formed when a solid dissolves in liquid.

2. It is formed when an insoluble solid is added to solvent.

3. For example – sugar dissolved in water.

3. For example-chalk dissolved in water.

2. Difference between Supernatant liquid and Fill rate

Supernatant liquid

Filtrate

1. It is obtained during sedimentation.

1. It is obtained during filtration.

2. It is used to separate mixture of liquid and insoluble solid heavier than liquid.

2. It is used to separate fine particles of insoluble solids from liquids.

3. No filter paper is used.

3. filter paper is used.

3. Difference between Filter Paper and Alum

Filter Paper

Alum

1. It is a special paper fitted in funnel during filtration.

1. It is a solid which is used to load suspended mud particles in water.

2. It does not dissolve in water.

2. It dissolves easily in water.

3. It does not speeds up-loading.

3. It speeds up loading

Write short answers.

5. Name the substance you will add to speed up sedimentation.
Answer
Alum

6. Name the different types of chromatographic techniques.
Answer
  1. Paper Chromatography
  2. Column Chromatography
  3. Thin Layer Chromatography
  4. Gas Chromatography.

Answer in detail


1. What is the difference between a pure substance and a mixture ?
Answer

Pure Substance

Mixture

A pure substance consists of only one kind of matter that is all the particles are same.

A Mixture contains two or more substances in any proportion which can be separated by physical methods.

It has a definite composition and constant properties.

Mixtures are generally of two types: Homogeneous and Heterogeneous.

It cannot be split into simpler substances by physical method.

In a mixture, the constituents can be present in any ratio.

All pure substances have characteristic melting and boiling points.

 

They do not have characteristic melting and boiling points. In a mixture each constituent retain its original properties.

A pure substance is either a compound or an element.

These can be separated by physical means.

2. What are the different types of mixtures ?
Answer
Mixtures are basically of two types:
(i) Homogeneous
(ii) Heterogeneous
(i) Homogeneous Mixtures: The composition of these mixtures is uniform throughout the mixture. Besides the properties of the mixture are the same in all the parts of the mixture, e.g. sugar dissolved in water.
(ii) Heterogeneous Mixtures: A heterogeneous mixture composition is not uniform throughout the mixture. The properties of the mixture are different in different parts of mixture e.g. chalk dissolved in water.

Answer in detail.

3. Why is filtration a better technique than sedimentation and decantation ?
Answer
Filtration is a better technique than sedimentation and decantation because it can separate very Fine insoluble particles as compared to the latter. Besides the filtrate obtained is generally pure solvent. In decantation when we pour out supernatant liquid there are chances of their getting mixed up with solute particles.

4. How is distillation method different from evaporation?
Answer
In distillation and evaporation both the solution is allowed to boil by giving heat. Evaporation does not involves the collection of vapour thus obtained. Distillation involves the condensation of the vapour obtained to obtain pure solvent. This is done in specially designed condensation chamber.

5. What is centrifugation ? Also write the principle on which it works.
Answer
Centrifugation: It is a technique used to speed up sedimentation of fine particles suspended in a solid liquid mixture.
Principle: The principle of centrifugation is that an object, when spun at high speed, experiences an outward force away from the centre of rotation.

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