Class 9 Science Chapter 2 Important Extra Questions

A pure substance is one which made up of only one kind of particle either atoms or molecules. For example, oxygen, carbon etc.

Mass of sodium chloride = 36g
Mass of water = 100g
Total mass of the solution = (36 + 100) g = 136g
Concentration (mass percentage) of solution = (Mass of NaCl )/(Mass of solution ) × 100
= 36g/(136g ) × 100
= 26. 47g

The miscible mixture of kerosene and petrol can be separated by fractional distillation. On heating the distillation flask, petrol having lower boiling points than kerosene, distils our first.

(i) Centrifugation
(ii) Evaporation
(iii) Sublimation

By heating the saturated solution, it becomes unsaturated.

It is a pure substance because chemical composition of sugar crystals is same irrespective of its source.

A solution is said to be diluted, concentrated or saturated on the basis of the amount of solute dissolved in the solution.

Iodine is the solute and alcohol is the solvent.

It is defined as the mass in grams of the solute present in one hundred grams of a solution.

The two components of a colloidal solution are dispersed phase and dispersing medium.

A true solution is homogeneous whereas a colloidal solution is heterogenous.

It is a process that separate a pure solid in the form of its crystals from a solution.

It is a process that separate a pure solid in the form of its crystals from a solution.

Crystallisation is considered better than simply evaporation because
(i) some solids decompose or get charred on heating to dryness.
(ii) some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.

Water is known as universal solvent because it has the ability to dissolve wide variety of substances.

Melting of iron metal, bending of iron rod and drawing a wire of iron metal are physical changes because there is no change in the chemical composition of iron, only its form is changing.

Gallium and mercury exist in liquid state at room temperature.

Unknown substance A should be a compound because elements and mixture do not decompose.

The elements are sulphur and neon.

Sea water is a mixture of salts and water which cannot be separated except by evaporation. Therefore, sea water is considered homogenous.
Sea water also contains mud, decayed plants, etc., other than salts and water, so it is heterogeneous also.

A true solution is homogenous in nature. The solute and solvent particles are very small. They cannot be distinguishing even under a microscope.

Particle size in a suspension is large than those in a colloidal solution. Also molecular interaction in a suspension is not strong enough to keep the particles suspended and hence they settle down.

True solution are vinegar and glucose solution. Colloids are frog, aluminium paint.

(i) Aerosol: Clouds smoke
(ii) Emulsion: Milk, face cream

Both fog and smoke have gas as the dispersion medium. The only difference is that the dispersed phase in fog is liquid and in smoke it is a solid.

Distillation is used to get back acetone. We know that boiling point of water is 1000C and since acetone is more volatile it will separate our first.

Chromatography is a technique used for the separation of a mixture of solutes brought about by distribution of dissolved material between two immiscible phases, one of which is mobile phase and the other part is stationary phase. It is useful in forensic science to detect and identify trace amounts of substances in the contents of bladder or stomach.

Water is a compound because of the following reasons:
(i) It is compound of two different elements, hydrogen and oxygen which cannot be separated by physical methods. They can be separated only by electrolysis.
(ii) The physical and chemical properties of hydrogen and oxygen are entirely different from the properties of water.

(a) Separation by using separating funnel.
(b) Sublimation
(c) Filtration to separate sand followed by evaporation/distillation.
(d) Separation by using separating funnel to separate kerosene oil followed by evaporation or distillation.

(a) Solid potassium chloride will separate out.
(b) Initially the water will evaporate ad then sugar will get charred.
(c) Iron suplhide will be formed.

(a) Chemical change.
(b) Acidic and basic solution can be prepared by dissolving the products of the above process water.
CaO + H₂O → Ca(OH)₂ (basic solution)
CO₂ + H₂O → H₂CO₃ (acidic solution)

Metal – Copper
Non Metals – Diamond, iodine and helium
Metalloids – Germanium, boron.

Physical change:
(i) In a physical change, one physical properties such as colour, physical state, density, volume, etc. change; chemical properties remain unchanged.
(ii) No new substance is formed in a physical change.
(iii) Very little or no energy in the form of heat, light or sound is usually absorbed or given out in a physical change.
(iv) A physical change is a temporary change.
(v) The original form of a substance can be regained by simply physical methods.
(vi) A physical change is reversible.
Chemical change:
(i) In a chemical change, the chemical composition and chemical properties undergo a changes.
(ii) A new substance is formed in a chemical change.
(iii) A chemical change is always accompanied by absorption or evolution of energy.
(iv) A chemical change is a permanent change.
(v) Original substance cannot be obtained by simply physical methods.
(vi) A chemical change is irreversible.

(a) Metals: (i) They have lustre (sheen).
(ii) They are malleable and ductile.
(iii) They have high density and high melting and boiling points.
(iv) Except mercury and gallium all other metals are solid at room temperature.
(v) They are sonorous.
(vi) They are good conductors of heat and electricity.
(vii) They are generally hard (except sodium or potassium).
(viii) They have high tensile strength.
(b) Non-metal: (i) They are non-lustrous.
(ii)The are neither malleable nor ductile.
(iii) They have low density and low melting and boiling points.
(iv) Non- metals may exist in solid, liquid or gaseous states at room temperature.
(v) They are not sonorous.
(vi) They are poor conductors of heat and electricity.
(vii) Non-metals are generally soft. (except diamond)
(viii) They have low tensile strength.

(a) Compound: (i) Compounds are formed as a result of chemical reactions between two or more elements or compounds.
(ii) The components of a compound are always present in a definite ratio by the mass.
(iii) The properties of a compound are entirely different from its constituents.
(iv) Compounds are always homogenous in nature.
(v) Compound formation is accompanied by absorption or evolution of light, heat or electrical energy.
(vii)Melting and boiling points of a compound are usually sharp are fixed.
(viii) The constituents of a compound cannot be separated by physical by physical or mechanical means. They can however be separated by chemical methods.
(b) Mixtures: (i) Mixtures are formed by simply mixing two or more constituents. There are no chemical reactions between the constituents.
(ii) The properties of a mixture are same as those of its constituents.
(iii) The components of a mixture may be present in any ratio.
(iv) Mixture are usually heterogeneous (except in solution)
(v) Heat, light or electrical energy may not be evolved or absorbed during the formation of a mixture.
(vii) The components of a mixture can be easily separated by physical methods.

Robert Boyle discovered element in 1661 an element is a basic form of matter that cannot be broken down into simpler substance by chemical reaction.

The scattering of a beam of light is called Tyndall effect this happens due to the scattering of light by the particles of dust and seen when the light passes through a dense forest.

The principle of evaporation method is that a volatile component is separated by its non-volatile component.

The method which is used to separate the component of a mixture in which the components have difference less than 25K in their boiling points is called fractional distillation method. It is used to separate different gases from air.

The concentration of a solution is the amount of a solute that present in the given amount of a solution.
Concentration of solution = (amount of solute )/(amount of solution)