chemistry class 9th notes chapter 2

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    CHEMICAL COMBINATIONSAND CHEMICAL EQUATION

    Laws of Chemical Combinations

    There are four laws of chemical combinations these laws explained the general feature of

    chemical change. These laws are:

    1.Law of Conservation of Mass

    2.Law of Definite Proportions

    3.Law of Multiple Proportions

    4.Law Reciprocal Proportions

    Antoine Lavoiser has rejected the worn out ideas about the changes that take place during

    a chemical reaction. He made careful quantitative measurements in chemical reactions and

    established that mass is neither created nor nor destroyed in a chemical change. Law ofConservation of Mass

    Statement

    It is presented by Lavoiser. It is defined as:

    "Mass is neither created nor destroyed during a chemical reaction but it only changes

    from one form to another form."

    In a chemical reaction, reactants are converted to products. But the total mass of the

    reactants and products remains the same. The following experiment easily proves law of

    conservation of mass.Practical Verification (Landolt Experiment)

    German chemist H. Landolt, studied about fifteen different chemical reactions with a

    great skill, to test the validity of the law of conservation of mass. For this, he took

    H.shaped tube and filled the two limbs A and B, with silver nitrate (AgNO3) in limb A and

    Hydrochloric Acid (HCl) in limb B. The tube was sealed so that material could not escape

    outside. The tube was weighed initially in a vertical position so that the solution should not

    intermix with each other. The reactant were mixed by inverting and shaking the tube. The

    tube was weighed after mixing (on the formation of white precipitate of AgCl). He

    observed that weight remains same.HCl + AgNO3 ----------> AgCl + NaNO3

    Law of Definite Proportions

    Statement

    It is presented by Proust. It is defined as:

    "When different elements combine to give a pure compound, the ratio between the

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    masses of these elements will always remain the same."

    Proust proved experimentally that compound obtained from difference source will always

    contain same elements combined together in fixed proportions.

    Example

    Water can be obtained from different sources such as river, ocean, well, canal, tube well,rain or by the chemical combination of hydrogen and oxygen. If different samples of water

    are analyzed, it will have two elements, hydrogen and oxygen and the ratio between their

    mass is 1:8.

    Law of Multiple Proportions

    Statement

    This law is defined as:

    "When two elements combine to give more than one compounds, the different masses

    of one element, which will combine with the fixed mass of other element, will be in

    simple whole number ratio."

    Two different elements can combine to form more than one compound. They can do so by

    combining in different ratios to give different compounds.

    Example

    Hydrogen and oxygen combine with one another to form water (H2O) and hydrogen

    peroxide (H2O2). In water and hydrogen oxide 2 g of hydrogen combine with 16g and 32g

    of oxygen respectively. According to law of multiple proportions, the different masses of

    oxygen (16g and 32g) which have reacted with fixed mass (2g) of hydrogen will have a

    simple ratio between each other i.e. 16:32 or 1:2. It means that hydrogen peroxidecontains double the number of oxygen atoms than water. This law proves this point of

    Dalton's Atomic Theory that atoms do not break in a chemical reaction.

    Law of Reciprocal Proportions

    Statement

    This law is defined as:

    "When two element A, B combine separately, with the mixed mass of the third

    element E, the ratio in which these elements combine with E is either the same or

    simple multiple of the ratio in which A and B combine with each other."

    Example

    Hydrogen and Nitrogen separately combine to form ammonia (NH3) and dinitrogen oxide

    (N2O), in these compounds, fixed mass of nitrogen is 14g and combines with 8 g of oxygen

    and 3 g of hydrogen. The ratio between the mass of oxygen and hydrogen is 8:3. Hydrogen

    and oxygen also combine with one another to form water (H2O). The ratio between

    hydrogen and oxygen in water is 16:2. These ratios are not same. Let us observe whether

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    Examples of Molecules of the elements are Hydrogen (H2). Nitrogen (N2), Sulphur (S8)

    etc.

    Molecules of different elements are called compounds. For example HCl, H2O, CH4 etc.

    Valency

    The combining capacity of all elements with other elements is called valency.Example

    H = 1

    C = 4

    Al = 3

    Mg = 2

    Na = 1