Elements, Compounds and Mixtures

Atoms of different kinds combine to form molecules of :

1. an element
2. a compound
3. a mixture
4. none of the above

Answer

a compound

Reason — Atoms of different kinds (i.e., different elements) chemically combine to form compounds.
An element is made of only one kind of atom. A mixture is just a physical combination, not chemical bonding.

Which of the following is an inert gas ?

1. H₂
2. Cl₂
3. He
4. S

Answer

He

Reason — He (Helium) is a noble gas, also known as an inert gas. It is chemically non-reactive due to its full outer electron shell.
Hydrogen is a reactive gas from Group 1. Chlorine is a halogen. Sulphur is a solid element from Group 16.

The molecular formula of common salt is :

1. MgO
2. NaCl
3. CO₂
4. Na₂SO₄

Answer

NaCl

Reason — The molecular formula of common salt is NaCl.

Which of the following is a metalloid ?

1. Argon
2. Silicon
3. Beryllium
4. Phosphorus

Answer

Silicon

Reason — Metalloids are elements that show some properties of metals and some properties of non-metals.
Argon is a noble gas. Beryllium is a metal. Phosphorus is a non-metal.

Quicklime is a compound of :

1. Calcium and oxygen
2. Magnesium and oxygen
3. Calcium and chlorine
4. Magnesium and sulphur

Answer

Calcium and oxygen

Reason — Quicklime is the common name for calcium oxide, which has the chemical formula CaO.

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, carbon dioxide.

Answer

Write the symbols of : Helium, Silver, Krypton, Antimony, Barium.

Answer

Helium (He)

Silver (Ag)

Krypton (Kr)

Antimony (Sb)

Barium (Ba)

Write the names of the following elements:

Na, C, Kr, U, Ra, Fe, Co

Answer

Na — Sodium

C — Carbon

Kr — Krypton

U — Uranium

Ra — Radium

Fe — Iron

Co — Cobalt

Name the main metal present in the following:

(a) Haemoglobin

(b) Chalk

(c) Chlorophyll

(d) Chocolate wrappers

Answer

(a) Iron

(b) Calcium

(c) Magnesium

(d) Aluminium

Name the particles of which :

(a) an element is made up of

(b) a compound is made up of

Answer

(a) Atoms

(b) Molecules

Define:

(a) Elements

(b) Compounds

Answer

(a) An element is a pure substance that cannot be converted further into anything simpler than itself by any physical or chemical process. It is made up of only one kind of atom.

(b) A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass.

What do you understand by:

(a) Metalloids

(b) Noble gases

Give two examples of each.

Answer

(a) Metalloids are those elements that show some properties of metals and some properties of non-metals. They are hard solids. Examples — Boron(B), Silicon(Si).

(b) Noble gases or inert gases are those elements that do not react chemically with other elements or compounds and they are found in air in traces. Examples — Helium(He), Neon(Ne).

Give four examples of non-metallic elements.

Answer

Hydrogen(H), Chlorine(Cl), Sulphur(S), Phosphorus(P)

A pure liquid is obtained from a solution by:

1. evaporation
2. distillation
3. filteration
4. sublimation

Answer

distillation

Reason — Distillation is a process that separates components of a solution based on their different boiling points. It allows us to obtain a pure liquid from a solution, especially if one of the components has a significantly different boiling point than the others.
Evaporation removes the solvent and leaves the solid behind, so the pure liquid is not recovered. Filtration separates insoluble solids from liquids. Sublimation separates substances that directly change from solid to gas.

The components of crude petroleum can be separated by:

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

Answer

fractional distillation

Reason — Fractional distillation is the process used to separate the components of crude petroleum. Since crude oil is a mixture of many different hydrocarbons with varying boiling points, fractional distillation allows these components to be separated into different fractions (like gasoline, kerosene, diesel, etc.).
Distillation separates only a pure liquid from a solution. Evaporation removes solvent, not suitable for separating many liquids. Filtration separates insoluble solids from liquids.

An example of a homogeneous mixture is:

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

Answer

tap water

Reason — Homogeneous mixtures are those in which the components are uniformly distributed and can't be easily distinguished from one another.
Distilled water is actually a pure substance, not a mixture. Sand and water is heterogeneous (non-uniform, sand settles). Water and oil is heterogeneous (form separate layers).

In chromatography, the filter paper is :

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

Answer

stationary phase

Reason — In chromatography, the filter paper acts as the stationary phase. The stationary phase is the part of the system that stays fixed while the other component (the mobile phase) moves through it.

A set of mixtures is :

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

Answer

ink, honey, ice cream, milk

Reason — A set of mixtures is ink, honey, ice cream, milk. Gold, Silver, and Iron are pure metals, and not mixtures.

The substance which can be separated by sublimation from a mixture is:

1. salt
2. camphor
3. petrol
4. sand

Answer

camphor

Reason — Sublimation is the process in which a solid changes directly into a gas without passing through the liquid phase. Its vapours can later be cooled down to get camphor back.
Salt does not sublime. Petrol is a liquid mixture, not separated by sublimation. Sand does not sublime.

A solid-solid mixture is:

1. Salt and water
2. Mustard oil and water
3. Honey
4. Alloys

Answer

Alloys

Reason — An alloy is a solid-solid mixture of two or more metals (or a metal and a non-metal).
Examples — brass, bronze and stainless steel.
Salt and water is solid and liquid solution. Mustard oil and water is liquid and liquid (immiscible). Honey is liquid mixture.

Assertion - The molecules of elements and compounds are represented by formulae, formed using atomic symbols.

Reason - Mixtures are impure substances.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true but R is not the correct explanation of A.

Reason — A is true as molecules of elements and compounds are represented by chemical formulae, which are made using atomic symbols.
R is true as mixtures are indeed impure substances because they contain two or more substances physically combined.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

Assertion - Kerosene oil and water can be separated by a separating funnel.

Reason - Kerosene oil and water are immiscible liquids.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

Both A and R are true and R is the correct explanation of A.

Reason — A is true as a separating funnel is a laboratory apparatus used to separate two liquids that do not mix.
R is true as immiscible liquids are liquids that do not dissolve in each other and form separate layers. Kerosene and water always form two distinct layers because they have different nature and density.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Assertion - Mixtures are either homogeneous or heterogeneous in nature.

Reason - A salt solution in water is a heterogeneous mixture.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is true but R is false.

Reason — A is true as mixtures are either homogeneous or heterogeneous in nature.
R is false as salt dissolves in water to form a homogeneous mixture, not a heterogeneous mixture.

Therefore, A is true but R is false is the correct option.

Assertion - When the first letter of more than one element is the same, their symbol is denoted by the first two letters of their name.

Reason - The symbol of calcium is Ca and that of chlorine is Cl.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.

Answer

A is false but R is true.

Reason — A is false because chemical symbols are not always formed by taking the first two letters of the element’s name.
R is true as because Ca (calcium) and Cl (chlorine) are correct symbols.

Therefore, A is false but R is true is the correct option.

Fill in the blanks:

(a) ............... are made up of same kind of atoms.

(b) ............... and ............... are pure substances.

(c) In a ..............., the substances are not combined chemically.

(d) Clay is separated from water by a method called ............... .

(e) ............... is a process to obtain a very pure form of a solid dissolved in a liquid.

(f) Camphor and ammonium chloride can ............... .

Answer

(a) Elements

(b) Elements, compounds

(c) mixture

(d) filtration

(e) Crystallization

(f) sublime

Write true or false.

(a) The components of mixtures retain their individual properties.

(b) All elements are made up of the same kind of atoms.

(c) Compounds can be homogeneous or heterogeneous.

(d) Air can be represented by a chemical formula.

(e) In a homogeneous mixture, the components are uniformly distributed throughout its bulk.

Answer

(a) True

(b) False
Correct Statement — All atoms in an element are identical, but they differ from the atoms of other elements.

(c) False
Correct Statement — Mixtures can be homogeneous or heterogeneous.

(d) False
Correct Statement — Air cannot be represented by a single chemical formula.

(e) True

Match the following :

Answer

The matched columns are given below:

Give an appropriate term for the following :

(a) The solid particles which remain on the filter paper after filtration.

(b) The liquid which evaporates and then condenses during the process of distillation.

(c) The process of transferring the clean liquid after the solid settles at the bottom of the container.

(d) The process by which two miscible liquids are separated.

Answer

(a) Residue

(b) Distillate

(c) Decantation

(d) Fractional Distillation

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

(a) Iron and sulphur

(b) Ammonium chloride and sand

(c) Common salt from sea water

(d) Chaff and grain

(e) Water and mustard oil

(f) Sugar and water

Answer

(a) Magnetic separation

(b) Sublimation

(c) Evaporation

(d) Winnowing

(e) Separating funnel

(f) Evaporation

Give one example for each of the following :

(a) A solid — solid heterogeneous mixture

(b) A solid — liquid heterogeneous mixture

(c) A liquid — liquid heterogeneous mixture

(d) A solid — solid homogeneous mixture

(e) A solid — liquid homogeneous mixture

(f) A liquid — liquid homogeneous mixture

Answer

(a) Sand and stone

(b) Sand in water

(c) Oil and water

(d) Brass

(e) Sugar solution

(f) Alcohol and water

Define:

(a) Evaporation

(b) Filtration

(c) Sublimation

(d) Distillation

(e) Miscible liquids

(f) Immiscible liquids

Answer

(a) Evaporation is the process of converting a liquid into its vapour state either by exposing it to air or by heating.

(b) The process of separating insoluble solid particles from a liquid by allowing it to pass through a filter is called filtration.

(c) The process in which a solid substance directly changes into its vapour state on heating without passing through its liquid state is called sublimation.

(d) Distillation is the method of getting a pure liquid from a solution by evaporating and then condensing the vapours.

(e) Liquids which dissolve in each other completely in all proportions are called miscible liquids.

(f) Liquids which do not dissolve in each other are called immiscible liquids.

How is distillation more advantageous than evaporation?

Answer

Distillation is more advantageous than evaporation because both the components of the solid-liquid mixtures are obtained in distillation process but in evaporation process only solid is obtained.

Differentiate between a homogeneous and a heterogeneous mixture.

Answer

State four differences between compounds and mixtures.

Answer

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

Answer

The characteristic properties of a pure substance are:

1. They have a definite composition and a definite set of properties like boiling point, melting point, density etc.
2. They are all homogeneous i.e. their composition is uniform throughout the bulk.

We need pure substances to:

1. Prepare medicines.
2. Maintain good health of human beings.
3. Prepare chemicals in laboratories.

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

Answer

The mixture of common salt and chalk powder is mixed with water. Common salt gets dissolved in water while chalk powder does not. The mixture is then filtered; chalk powder remains on the filter paper as residue and the salt solution is obtained as filtrate. Common salt can be recovered from the filtrate by evaporation.

(a) What is chromatography?

(b) Why is it named so?

(c) What are the advantages of chromatography?

(d) Name the simplest type of chromatography?

(e) On what principle is this method based?

(f) What is meant by stationary phase and mobile phase in chromatography?

Answer

(a) The process of separating different dissolved constituents of a mixture by their adsorption (the process by which particles get attached to the surface of a solid material) on an appropriate material is called chromatography.

(b) The word chromatography means colour writing. It is named so because earlier it was used to separate mixtures containing coloured components only.

(c) The advantages of chromatography are:

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 constituents of a mixture.

(d) Paper chromatography.

(e) Chromatography separates the components of a mixture on the basis of differences between two phases, one of which is stationary while the other is mobile.

(f) The stationary phase is made up of common adsorbents like filter paper, silica gel as they are stationary and the mobile phase is made up of common solvents like water, ethyl alcohol, acetic acid etc. which moves through or spreads on the stationary phase.

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.

(a) sublimation

(b) filtration

(c) sedimentation and decantation

(d) solvent extraction method

(e) magnetic separation

(f) by using a separating funnel

(g) fractional distillation

Answer

(a) This method is used for solid mixtures in which one of the components can sublime on heating. It is used to separate a mixture of common salt and ammonium chloride.

(b) This method is used for separating the components of a heterogeneous solid-liquid mixture in which solids are insoluble in liquids. A mixture of chalk and water can be separated by this method.

(c) This method is used when the solid component is insoluble and heavier than the liquid component. A mixture of sand and water can be separated by this method.

(d) This method is used when one component is soluble in a particular liquid while the other component is insoluble. A mixture of sand and salt can be separated by this method.

(e) This method is used when one of the components of the mixture is magnetic in nature. A mixture of iron and sulphur can be separated by this method.

(f) It is used to separate components of liquid-liquid heterogeneous mixture when one of the liquid components is heavier than the other. Kerosene oil and water can be separated by this method.

(g) It is used to separate components of homogeneous liquid-liquid mixture on the basis of their difference in boiling points. A mixture of water and alcohol can be separated by this method.

Draw a well-labelled diagram to show the separation of a mixture of kerosene oil and water.

Answer

The below diagram shows the separation of a mixture of kerosene oil and water:

Study the figure given below and answer the following questions :

(a) Label A and B.

(b) Name the process shown.

(c) For what type of mixtures is this method used ?

(d) What is the substance left on the filter paper called ?

Answer

(a) 'A' and 'B' denote:
A → Funnel with filter paper cone
B → Filtrate

(b) The process shown is known as 'Filtration'.

(c) This method is used to separate insoluble solid particles from a liquid by allowing it to pass through a filter.

(d) Residue.

A chemistry teacher had taught the concept of elements, compounds and mixtures to her students in the class. Aaliya, one of the students, was watching her father cook food one day after she was taught about homogeneous and heterogeneous mixtures. He mixed flour, water and common salt and kneaded them into a dough to prepare breads, all of which tasted the same throughout. She also observed her mother serving fruit cream after the dinner which had different kinds of fruits mixed with cream.

(a) What kind of substance is the dough ?

(b) Give the chemical name of common salt. Name a method by which common salt is separated from its salt solution.

(c) Why is fruit cream a heterogeneous mixture ?

(d) Define an 'element'. Name the elements from which sugar is made up of ?

Answer

(a) The dough is a homogeneous mixture because salt, flour, and water are mixed uniformly, so its composition is the same throughout.

(b) Chemical name of common salt is sodium chloride.
A method by which common salt is separated from its salt solution is evaporation.

(c) Fruit cream is a heterogeneous mixture because the fruits and cream are not uniformly mixed. We can easily see different components (pieces of fruits and cream) separately.

(d) An element is a pure substance that cannot be broken down into simpler substances by chemical methods.

Sugar is made up of the elements — carbon, hydrogen and oxygen.