Elements, Compounds And Mixtures

Question 1

What are pure substances?

Answer 1

Pure substances have a definite chemical composition and definite physical and chemical properties. They are all homogeneous. Elements and compounds are pure substances.

Question 2

What are impure substances?

Answer 2

Mixtures are made up of two or more pure substances in any proportion. They do not have any definite set of properties. They may be homogenous or heterogeneous.

Question 3

What are elements?

Answer 3

An element is a pure substance which cannot be converted into anything simpler than itself by any physical or chemical process.

Question 4

How many elements are known? How many are artificial and how many are natural?

Answer 4

118 elements are known, out of which 92 are naturally occurring, while the rest 26 are artificially created.

Question 5

Characteristics of elements

Answer 5

• Simplest type of pure and homogeneous substances.
• Cannot be broken down into more simpler substances.
• have a fixed melting and boiling points
• consists of only one kind of atoms
• can react chemically with other elements and compounds
to form new substances.

Question 6

Classification of elements:-?

Answer 6

• Metals • non-metals • metalloids • noble or inert gases

Question 7

There are only __ non metals known to us.

Answer 7

11

Question 8

Define metalloids

Answer 8

Metalloids are elements showing some properties of metals and some properties of non metals.

Question 9

Define noble gases

Answer 9

These are monoatomic gaseous elements which do not react chemically with other elements or compounds, so they are known as noble or inert gases. They are only 6 in number.

Question 10

The most abundant element present in the earth’s crust is _____

Answer 10

Oxygen

Question 11

Name the element which has the highest percentage in air

Answer 11

Nitrogen

Question 12

Name two soft metals

Answer 12

Sodium, potassium

Question 13

Example of liquid metals and nonmetals

Answer 13

Metals - mercury
Non metals - bromine

Question 14

Example of a brittle metal

Answer 14

Zinc

Question 15

Define atomicity

Answer 15

The number of atoms present in a molecule of an element is called its atomicity.

Question 16

Define compound

Answer 16

A compound is a pure substance composed of two or more elements combined chemically in a definite proportion by mass.

Question 17

Characteristics of compounds:-?

Answer 17

• pure homogeneous substances
•have a identical set of definite physical and chemical properties
• have definite melting and boiling points
• definite chemical formulae
• can be separated only by chemical methods
• during the formation of a compound, energy is either absorbed or liberated.

Question 18

Define mixtures

Answer 18

Mixtures can be defined as a kind of matter which is formed by mixing two or more pure substances in any proportion, such that they do not undergo any chemical change and retain their individual properties.

Question 19

Characteristics of mixtures:-?

Answer 19

• MIXTURE CONSISTS IF TWO OR MORE PURE SUBSTANCES THAT EXIST TOGETHER WITHOUT AHY CHEMICAL COMBINATION BETWEEN THEM.
• MAY BE HOMOGENEOUS OR HETEROGENEOUS.
• COMPONENTS OF MIXTURES VARY IN THEIR PROPORTIONS.
• THE COMPONENTS ARE LOOSELY HELD TOGETHER
• DO NOT HAVE FIXED BOILING OR MELTING POINT
• CAN BE SEPARATED BY SIMPLE PHYSICAL METHODS.
• NO ENERGY CHANGE TAKES PLACE DURING THE FORMATION OF MIXTURES
• cannot be represented by any Chemical FORMULA

Question 20

what is the need for the separation of components of mixtures?

Answer 20

• to obtain useful substances
• to remove unwanted and harmful substances
• to obtain pure substances for preparing other useful substances.

Question 21

Define solvent extraction method

Answer 21

The process by which a soluble solid component of a mixture is separated from an insoluble solid component by using a suitable solvent is called solvent extraction method.

Question 22

Define fractional crystallisation

Answer 22

The process of separating two or more solids based on the difference in their solubilities in the same solvent is called fractional crystallisation.

Question 23

Filtration

Answer 23

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

Question 24

Crystallisation

Answer 24

The process of formation of solid crystals from a hot super-saturated solution by cooling is called crystallisation.

Question 25

Distillation

Answer 25

Distillation is the process of converting a liquid into vapour by heating and the subsequent condensation of the vapour back into liquid.

Question 26

Centrifugation

Answer 26

Centrifugation is the method of separating suspended solids from liquids where the mixture is homogeneous. This is also called churning.

Question 27

Fractional distillation

Answer 27

Fractional distillation is the process which involves distillation and the collection of fractions of different liquids which are boiling at different temperatures.

Question 28

Chromatography

Answer 28

The process of separating different dissolved constituents of a mixture by their absorption on an appropriate material is called chromatography.

Question 29

Advantages of chromatography

Answer 29

• A very small quantity of the substance can be seperated.
• components with very similar physical and chemical properties can be separated.
• it identifies the different constituents of a mixture.
• also helps in quantitative estimation of the components of a mixture.