Pure Substances & Mixtures in Chemistry

Pure Substances & Mixtures in Chemistry

Chemistry: The Branch of Science

Chemistry is the branch of science which deals with the composition of matter and also the physical and chemical characteristics associated with different material objects. A French chemist, Lavoisier (1743-1793), is regarded as the father of modern chemistry.

Substance and its Nature:

Anything that occupies space, possesses mass, and can be felt by any one or more of our senses is called matter.

States of Matter:

1. Solid State:

• A solid possesses a definite shape and definite volume, meaning it cannot be compressed by applying pressure. Solids are generally hard and rigid.

Examples: Metals, wood, bricks, copper, etc.

2. Liquid State:

• A liquid possesses definite volume but no definite shape. This means that the liquid can take up the shape of the container in which it is placed.

Examples: Water, milk, oil, alcohol, etc.

3. Gaseous State:

• A gas does not have either a definite volume or a definite shape. It can be compressed to a large extent by applying pressure and also takes the shape of the container in which it is enclosed.

Examples: Air, oxygen, nitrogen, ammonia, carbon dioxide, etc.

4. Pure Substances:

• A single substance (or matter) that cannot be separated into other kinds of matter by any physical process is called a pure substance.

• Pure substances have been classified as elements and compounds.

Elements:

The simple form of a pure substance which can neither be broken into nor built from simpler substances by ordinary physical and chemical methods is called an element.

Elements are further classified into three types:

1. Metals:

• Metals are normally hard solids (exception: mercury, which is liquid at room temperature).

• They have lustre, high melting points (mp) and boiling points (bp), and are good conductors of electricity and heat.

• The conductivity of metal decreases with an increase in temperature due to the vibration of positive ions at their lattice points.

Examples: Iron, copper, silver, gold, aluminium, zinc, etc.

2. Non-metals:

• Non-metals are elements with properties opposite to those of metals. They are found in all states of matter. They do not possess lustre (exception: iodine), they are poor conductors of electricity (exception: graphite), and they are not malleable or ductile.

Examples: Hydrogen, carbon, oxygen, nitrogen, sulfur, phosphorus, etc.

3. Metalloids:

• Metalloids are elements that have common properties of both metals and non-metals.

Examples: Arsenic, antimony, bismuth, etc.

Compounds:

Compounds are pure substances that are composed of two or more different elements in a fixed proportion by mass. The properties of a compound are entirely different from those of the elements from which it is made.

Examples: Water, sugar, salt, chloroform, alcohol, ether, etc.

Compounds are classified into two types:

1. Organic Compounds:

• The compounds obtained from living sources are called organic compounds.

• The term "organic" is now applied to hydrocarbons and their derivatives.

Examples: Carbohydrates, proteins, oils, fats, etc.

2. Inorganic Compounds:

• The compounds obtained from non-living sources, such as rocks and minerals, are called inorganic compounds.

Examples: Common salt, marble, washing soda, etc.

Mixtures:

A material obtained by mixing two or more substances in any indefinite proportion is called a mixture. The properties of the components in a mixture remain unchanged.

Examples: Milk, seawater, petrol, paint, grass, cement, wood, etc.

There are two types of mixtures:

1. Homogeneous Mixture:

• A mixture is said to be homogeneous if it has a uniform composition throughout and there are no visible boundaries of separation between constituents.

• Moreover, the constituents cannot be seen even by a microscope.

Examples: Common salt dissolved in water, sugar dissolved in water, iodine dissolved in CCl₄, benzene in toluene, methyl alcohol in water.

2. Heterogeneous Mixture:

• A mixture is said to be heterogeneous if it does not have a uniform composition throughout and has visible boundaries of separation between the various constituents.

• The different constituents of a heterogeneous mixture can be seen even with the naked eye.

Examples: A mixture of sulfur and sand, a mixture of iron filings and sand, etc.

Separation of Mixtures:

Some methods of separation of mixtures are given below:

1. Sublimation:

• In this process, a solid substance passes directly into its vapours on the application of heat.

• The vapours, when cooled, give back the original substance.

• This method can be used for substances that sublime in their separation from non-sublimate materials.

Examples: Naphthalene, iodine, ammonium chloride, etc.

2. Filtration:

• This is a process for the quick and complete removal of suspended solid particles from a liquid by passing the suspension through a filter paper.

Examples:

• Removal of solid particles from engine oil in car engines.

• Filtration of tea from tea leaves during tea preparation.

3. Evaporation:

If a solution of a solid substance in a liquid is heated, the liquid gets converted into its vapours and slowly evaporates completely. This process is called evaporation.

Examples: 

• Evaporation of water in summer from ponds, wells, and lakes.

• Preparation of common salt from seawater by evaporation of water.

4. Crystallisation:

• This method is mostly used for the separation and purification of solid substances.

• The impure solid or mixture is heated with a suitable solvent to its boiling point, and the hot solution is filtered.

• The clear filtrate is then cooled slowly to room temperature, where pure solid crystallises out.

For more complex mixtures, fractional crystallisation is used, in which the components crystallise out at different intervals of time.

5. Distillation:

• It is a process of converting a liquid into its vapour by heating and then condensing the vapour again into the same liquid by cooling.

• This method is employed to separate liquids with different boiling points or a liquid from a non-volatile solid or solids in solution or suspension.

Examples: A mixture of copper sulphate and water, or a mixture of water (boiling point 100°C) and methyl alcohol (boiling point 45°C).

6. Fractional Distillation:

This process is similar to distillation, except that a fractionating column is used to separate two or more volatile liquids with different boiling points.
Examples:

• Separation of methyl alcohol (bp = 338K) and acetone (bp = 329K).

• Separation of petrol, diesel oil, kerosene oil, and heavy oil from crude petroleum.

• Separation of oxygen, nitrogen, inert gases, and carbon dioxide from liquid air.

7. Chromatography:

Chromatography is based on the difference in the rates at which the components of a mixture are absorbed by a suitable absorbent.

There are many types of chromatography:

• Column chromatography

• Thin-layer chromatography

• Paper chromatography

• High-pressure liquid chromatography

• Ion-exchange chromatography

• Gas chromatography

8. Sedimentation and Decantation:

• This method is used when one component is a liquid and the other is an insoluble solid heavier than the liquid, e.g., mud and water.

• If muddy water is allowed to stand undisturbed, the earth particles (clay and sand) settle at the bottom.

• This process is called sedimentation. The clear liquid at the top can be gently transferred into another beaker, known as decantation.

Concept of Change in State:

1. Melting Point:

• The temperature at which solid and liquid forms of a substance exist at equilibrium, or when both forms have the same vapour pressure, is called the melting point.

2. Boiling Point:

• The temperature at which the vapour pressure of the liquid is equal to atmospheric pressure is called the boiling point.

• Boiling Points:

  • Water: 373K

  • Ethanol: 349K

  • Chloroform: 334K

  • Acetone: 329K

3. Freezing Point:

• The temperature at which the vapour pressure of its liquid is equal to the vapour pressure of the corresponding solid is called the freezing point.

4. Evaporation:

• The process of conversion of a liquid into its vapours at room temperature is called evaporation.

• Evaporation causes cooling because, during the process, molecules with higher kinetic energy escape from the surface of the liquid.

Evaporation is affected by:

• Nature of liquid

• Temperature

• Surface area

5. Vapour Pressure:

The pressure exerted by the vapours of a liquid in equilibrium with the liquid at a given temperature is called vapour pressure.

Vapour pressure depends on:

• Nature of the liquid

• Temperature

  • The higher the vapour pressure, the lesser the intermolecular forces present in the liquid.

  • Vapour pressure increases with an increase in temperature.