notes of chapter 2 in chemistry of class 9 ncert
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We do know that matter is made up of particles which are arranged in a certain way. Gases particles are well separated and can freely move about, solids, on the other hand, have particles that are tightly packed, usually with no scope to move around.
Purity
Pure substances
Pure substances are elements or compounds.
They are made up of only one kind of entity.
They cannot be broken down into simpler entities by chemical or physical methods.
They have a fixed composition.
Example: Diamond, carbon dioxide.
Mixtures
Mixtures are formed by just mixing two or more pure substances (components) such that each substance retains its own chemical identity.
Types of mixtures
Types:
1. Homogeneous mixture
A mixture which has a uniform composition throughout is called a homogeneous mixture or solution.
Examples: sugar in water, salt in water.
Heterogeneous mixture
A mixture which contains physically distinct parts and has a non-uniform composition is called a heterogeneous mixture.
Examples: Mixture of salt and iron filings, sand and sugar.
Physical vs Chemical Changes
Physical and chemical changes
A substance is said to undergo a physical change when only the physical properties such as the shape, size, colour or state of the substance change. No new substance is formed.
Example: Melting of ice, boiling water.
A substance is said to undergo a chemical change when a new substance with completely new properties (physical and chemical) is formed.
Example: Burning of wood or paper, souring of milk.
Solutions
Solutions and their properties
A solution is a homogeneous mixture of two or more substances.
Properties:
Its particles are too tiny and have a diameter less than 1 nm.
The particles are not visible to naked eyes.
Particles do not scatter a beam of light passing through it and hence do not show the Tyndall effect.
The solute particles never settle down on keeping undisturbed.
The components of a solution cannot be separated using filtration.
Alloys
Alloys are homogeneous mixtures of metals or a mixture of a metal and another element that cannot be separated into their components by physical methods.
Examples:
Steel, a combination of iron (metal) and carbon (non-metal).
Bronze, a combination of copper (metal) and tin (metal).
Brass, a mixture of copper (metal) and zinc (metal).
Concentration of Solutions
Solubility
Solubility is the property showing the ability of a given substance, which is the solute, to dissolve in a solvent.
It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium.
The resulting solution is called a saturated solution.
Factors Affecting Solubility:
Temperature – Solubility increases with temperature. The situation is different for gases. With the increase in temperature, they became less soluble in each other and in water, but more soluble in organic solvents.
Pressure – For the majority of solid and liquid solutes, pressure does not affect solubility. The solubility of gas is directly proportional to the pressure of this gas.
Types of solutions based on the concentration of the solution
Three types of solutions exist based on the concentration of the solution:
Dilute
Concentrated
saturated solution.
Ways of representing the concentration of a solution
The concentration of a solution can be represented in many ways
(i) Mass by the mass percentage of a solution = (Mass of solute / Mass of solution) × 100
(ii) Mass by volume percentage of a solution = (Mass of solute/ volume of solution)×100
Suspensions
Suspension and its properties
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.
The solute particles settle down when a suspension is left undisturbed.
They can be separated from the mixture by filtration.
Colloids
Types of mixtures based on particle size
Classified into:
Solution
Suspension.
Colloidal solution.
Colloidal Solutions
A colloidal solution is a mixture in which the substances are regularly suspended in a fluid.
Classified into: Foam, Emulsion, Sol
Tyndall Effect
Tyndall effect is the scattering of light by particles in a colloid or else particles in a very fine suspension.
e.g.It can be observed when sunlight passes through the canopy of a dense forest.
Dispersed phase
The solute-like component of the dispersed particles in a colloid form the dispersed phase.
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