- A pure substance is a homogeneous material with definite, invariable chemical composition and physical and chemical properties.
- A pure substance consists of only one type of atoms or molecules.
- On the basis of their chemical composition, pure substances are classified into elements and compounds.
- Impure substances are mixtures of two or more elements, compounds or both, and they generally have different compositions and properties in their different parts.
What is a Mixture?
- A mixture contains more than one substance mixed in any random proportion. For example: milk, soil, lemon juice etc.
- Mixtures are constituted by more than one kind of pure form of matter known as a substance.
- A substance cannot be separated into other kinds of matter by any physical process.
Example: Dissolved sodium chloride can be separated from water by the physical process of evaporation. However sodium chloride itself is a substance and cannot be separated by physical processes into its chemical constituents.
Properties of a Mixture
- In a mixture, two or more elements or compounds are not chemically combined together.
- The constituents of a mixture retain their original properties.
- The constituents of a mixture can be separated by using a physical process such as hand picking, filtration, holding a magnet etc.
Types of Mixtures
A homogeneous mixture of two or more substances which are chemically non-reacting, whose composition can be varied within certain limits, is called a solution.
Solution = Solute + Solvent
Solute: A substance which gets dissolved in a solvent is called a solute.
Solvent: A substance in which a solute gets dissolved is called a solvent.
Concentration of a Solution
- The properties of a solution depend upon the nature of the solute and the solvent, and also on the proportion of the dissolved solute.
- A solution which has a high quantity of solute is said to be a concentrated solution, and a solution which has comparatively lesser quantity of solute is said to be a dilute solution.
- The concentration of a solution is the amount of solute present in a given amount (mass or volume) of solution or the amount of solute dissolved in a given mass or volume of solvent.
Methods of Expressing the Concentration of a Solution
A solution, in which more solute cannot be dissolved at that temperature, is called a saturated solution.
A solution, in which more quantity of solute can be dissolved without raising its temperature, is called an unsaturated solution.
The maximum amount of a solute which can be dissolved in 100 grams of a solvent at a specified temperature is known as the solubility of that solute in that solvent at that temperature.
Effect of Temperature and Pressure on Solubility
The effect of temperature and pressure on the solubility of a substance is as follows:
- The solubility of solids in liquids usually increases on increasing the temperature and decreases on decreasing the temperature.
- The solubility of solids in liquids remains unaffected by changes in pressure.
- The solubility of gases in liquids usually decreases on increasing the temperature and increases on decreasing the temperature.
- The solubility of gases in liquids increases on increasing the pressure and decreases on decreasing the pressure.
Distinguishing Properties of Solution, Suspension and Colloidal Solution
Dispersion System in Colloids
- A system consisting of a substance distributed as very small particles of a solid, droplets of liquids or tiny bubbles of a gas in a suitable medium is called as dispersion system.
- The distributed substance in the solution is called as dispersed phase.
- The medium in which the distributed substance is dispersed is referred to as the dispersion medium.
- Tyndall effect can be defined as the scattering of a beam of light by colloidal particles present in a colloidal solution.
- This effect can be observed when a fine beam of light passes through a small hole in a dark room. This effect occurs due to the scattering of light by particles of dust or smoke present in the air.
- The Tyndall effect can also be observed when sunlight passes through the canopy of a dense forest. In the forest, the mist contains tiny droplets of water which act as colloidal particles dispersed in the air.
Separating the Components of a Mixture
To obtain the coloured component of a dye from blue/black ink
Separation of Cream from Milk
To separate a mixture of two Immiscible liquids
Separate a mixture of Salt and Ammonium chloride
Separation of Components of Dye
To separate a mixture of two miscible liquids
Separate a mixture of two miscible liquids having the temperature difference less than 25oC.
Obtain different gases from air
Obtain Pure Copper sulphate Crystals From An Impure Sample
To Separate The Mixture of Iron Filings and Sulphur Powder
Purification of Drinking Water
Purification of drinking water is done at the following four stages:
- Water from a river or lake is brought through canals or long pipes to the water work, where it is mixed with the required quantities of alum and soda lime solutions. These substances react with one another to form aluminium hydroxide, a jelly-like, sticky solid.
- It is then pumped into big settling tanks, where most of the suspended impurities settle down in two or three days.
- The clear water still containing some suspended matter is passed through successive filters of boulders, gravel, coarse sand and fine sand.
- The clear water from the filters is chlorinated and then passed to the reservoirs for distribution in the city.
Physical and Chemical Changes
An element can be defined as a basic form of matter which cannot be broken down into simpler substances by any physical or chemical means.
Characteristics of an Element
- An element is made up of only a single type of atoms.
- It is a pure and homogeneous substance.
- It has a fixed melting and boiling point.
- An atom is the smallest particle of an element which takes part in a chemical reaction.
- An element may chemically react with other elements or compounds.
- An element can occur in the solid, liquid or gaseous state.
Classification of Elements
- A compound is a pure substance composed of two or more elements combined chemically in a fixed proportion by mass.
- The properties of compounds are different from the properties of their constituent elements. Example: H2O, CO2 etc.
- The smallest part of a compound is a molecule. All the molecules of a compound are alike and have properties similar to that of the compound.
Characteristics of Compounds
- Components in a compound are present in a definite proportion.
- A compound has a homogeneous composition.
- Particles in a compound are of one type.
- A compound is made up of one or more atoms of the same or different elements.
- In a compound the elements are present in a fixed ratio by mass.
- A compound can be divided into simpler substances by a chemical process.
- The physical and chemical properties of a compound are completely different from those of its constituents.