by • 14/07/2011 • 9th class ChemistryComments (4)1329

Q1: State law of conservation of mass (matter)? Give examples.
Ans.: During any process, mass is neither created nor destroyed this law was put forward by a French chemist, lavoiser in 1785.
This law states that:
Matter is neither created nor destroyed during a chemical reaction.
During a chemical reaction, the total mass of the product is equal to the total mass of the reactants.
Example no 1:
When a piece of iron is left in moist air its surface gradually turns brown, the object gets rusted and gains mass.
The increase in mass is just the mass of oxygen.
Example no 2
When coal burns it leaves behind ash. The ash is lighter than coal. But the mass of coal will be equal to that of ash and the liberated carbon dioxide.



Q2: state exothermic and endothermic reaction with examples?
Ans: exothermic reaction:-Reaction in which heat energy is evolved is known as exothermic reaction in such reaction the system become warmer and neat potential energy of substances decreases. The enthalpy of product is less than the enthalpy of
When unslaked calcium oxide is added to water. the water becomes warm with formation of
Slaked time . in this reaction heat energy is released.

Endothermic reaction:-Reaction in which heat energy is absorbed is known as endothermic reaction . in such reaction enthalpy of reactants is lower than those of product. Since energy
Is absorbed during reaction the temperature of reaction decreases. (example are)
Heat is absorbed in the decomposition of calcium carbonate. In this reaction heating must be continuous to decompose calcium carbonate.






Q3: What are different types of reaction? Give chemical reaction of each type?
Ans: Chemical reaction are of following types:
1: Simple decomposition :-In this reaction a single compound is decomposed into two or more simple substance.
2: Synthesis:-In this reaction two or more substance combine to from a single compound it is also known as addition reaction.
3: Displacement:-In this reaction an atom or radical displaced by another in a compound. Such reaction depend upon the electropositive or electronegative nature of an atom or radical to displaced another.
4: Double decomposition / displacement :-In this reaction the two reactants decomposed  to form two new substances by exchanging their radicals.
5: Neutralization:-In this reaction , the hydrogen ion of an acid neutralized by the hydroxide ion of a base to from salt and water.
6: Hydrolysis:-In this reaction the salt of either weak acid and base dissolve in water to from acid and base.

Q4:Explain Laws of Chemical Combinations with example.


Ans: 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 of Conservation of Mass


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


It is presented by Proust. It is defined as:

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


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


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.


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 peroxide contains 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


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.”


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 these ratios are simple multiple to each other or not following mathematical operation is carried out.

8:3 ::16:2

8/3 : 16/2


8/3 x 2/16


1/3 => 1:3

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  1. none says:

    numericals to h i ni

  2. mahmood says:


  3. A Rehman says:

    good.but only laws?

  4. ilyas_mehmood says:


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