Laws of chemical combinations

Based on the quantitative results of chemical reactions dealing with the weight – weight and weight – volume relationships, few laws formulated which are:

1.       Law of conservation of mass: It was proposed by Lavoisier and verified by Landolt. According to this law, in a chemical reaction total mass of the products is equal to the total mass of the reactants or mass 

Physical and Chemical changes

In a physical change, the composition of a substance remains the same, while in a chemical change the composition and structure is changed. A physical change is temporary while a chemical change is permanent.

Example of physical change: Melting of ice, boiling of water, magnetization of iron, heating of sulphur, sublimation of iodine, heating of gold etc.

Example of chemical change: Corrosion of metals, electrolysis curdling of milk, ripening of fruit, digestion of food, burning of candle, burning of metal or non metal, respiration, etching of glass with HF, radioactivity, setting of mortar or cement, heating of NH₄Cl above 400⁰ C, explosion of dynamite etc.

Atom and Molecule

• Atom is the smallest particle of an element that may or may not exist independently. However, it is the smallest particle of matter that takes part in a chemical reaction.

• Molecule is the smallest particle of matter that has independent existence. Molecule of the element is homoatomic whereas molecule of the compound is heteroatomic. The number of atoms present in a homoatomic molecule is called atomicity of the element.

Elements, Compounds and mixtures

•       An element is a substance which can neither be broken nor built from two or more simple substances by any known physical or chemical methods. The elements are building block of matter i.e. any matter is made up of one or more types of elements. Substances built of only one kind of atoms are called elements.
•      Compounds is a pure substance which contains two or more than two elements combined together in some fixed proportion by weight and which can be decomposed into two or more elements by a suitable method. It is always homogeneous.
•           Mixture is a material obtained by mixing two or more than two substance (elements or compounds) in any proportion. In it, the properties of constituents remain unchanged. It may be homogeneous or heterogeneous. 

1.                 Separation of components of the mixture depends upon (i) the physical state of the substance in the mixture (i.e. solid, liquid, gases or solution) (ii) physical properties (i.e. melting point, boiling point, solubility) of each component. The choice of the method used depends on (a) the type of the mixture to be separated. (b) The nature of each component to be separated.
2.               Types of mixtures:  Common types of mixtures are (i) solid – solid mixture (ii) solid – liquid mixture (iii) liquid – liquid mixture (iv) gas – liquid mixture (v) gas – gas mixture (vi) gas – solid mixture.

Matter

Matter is defined as anything which occupies space, possesses mass and can be judged by one or more of our senses. The matter may be classified as under:

1. Physical classification: Depending upon the rigidity, volume and shape, i.e., physical nature, matter may be classified into three states- solids, liquids and gases.
2. Chemical classification: Depending upon the chemical composition of the substance, matter may be classified into (a) pure substance i.e., elements and compounds (homogenous) (b) mixtures which may be homogenous (solutions) or heterogeneous.