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In a chemical reaction mass is neither created nor destroyed. This is the law of conservation of mass. This law was proposed by the scientist Antoine Lavoisier. That is, the total mass of the reactants will be equal to the total mass of the product.

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Antoin Lavoisier

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It is generally accepted that Lavoisier’s great accomplishments in chemistry stem largely from his changing the science from a qualitative to a quantitative one. Lavoisier is most noted for his discovery of the role oxygen plays in combustion. He recognized and named oxygen (1778) and hydrogen (1783). He wrote the first extensive list of elements, and helped to reform chemical nomenclature. He predicted the existence of silicon (1787) and discovered that, although matter may change its form or shape, its mass always remains the same.

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Given below is a table showing the mass of the reactants and the products when hydrogen combines with chlorine to form hydrogen chloride in two situations.

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Write down the total mass of the reactants and also the total mass of product in the above experiment.
a) Situation 1: …………………
b) Situation 2: ……………….
Answer:
a) Total mass of reactants 73g, products 73g
b) Reactants 146g, products 146 g

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Given below is a table showing the mass of the reactants and the products when hydrogen combines with oxygen to form water, in two situations.

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Write down the total mass of the reactants and the total mass of product in the above experiment.
a) Situation 1:
b) Situation 2:
Answer:
a) Reactants 18 g, Product 18 g
b) Reactants 36 g, Products 36 g

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Joseph Priestley (24 March 1733 – 6 February 1804) was an English chemist, natural philosopher, separatist theologian, grammarian, multi-subject educator, and liberal political theorist who published over 150 works. He has historically been credited with the discovery of oxygen, having isolated it in its gaseous state, although Carl Wilhelm Scheele and Antoine Lavoisier also have strong claims to the discovery, Scheele having discovered it in 1772, two years before Priestley.

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Equalizing the number of atoms of each element in molecules in the reactant side and that in the product side is called balancing of equation.

Zn + HCl → ZnCl2 + H2

Tabulate the total number of atoms of each elements in the reactant side and that in the product side of the above reaction.

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Q1. How many Zn atoms are there in the reactant side and in the product side?
Ans:          1 each

Q2. Which are the atoms showing a difference in their number?
Ans:          H and Cl

Q3. In order to make them equal on both sides how many molecules of HCI should be taken as reactant.
Ans:          2 molecules

So the balanced equation is,
Zn +  2HCl → ZnCl2 + H2
Note:
Chemical equation is balanced inorder to obey the law of conservation of mass.

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Oxidation is the process of loss of electrons. Reduction is the process of gain of electrons. The atom which loss electron is called the reducing agent and the atom which gains electron is called the oxidizing agent.

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Here in the above example sodium atom loses one electron so we say that sodium atom get oxidised and chlorine atom gains one electron so we say that chlorine atom get reduced. Therefore sodium is the reducing agent and chlorine is the oxidising agent.
Balanced equation for above reaction is,

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Above reaction is a combination of reduction and oxidation reaction.

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If oxidation and reduction takes place simultaneously during a chemical reaction such reactions are called redox reactions. The formation of sodium chloride is an example for redox reaction.

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Oxidation state is the formal charge carried by an element in its compound. Oxidation state is calculated by considering all the bonds in a molecule is ionic. The sum of oxidation states of all elements in a molecule will be zero.
Note:
Oxidation state of an element can
be calculated by knowing the oxidation states of other elements.

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1. A piece of magnesium is burned in air. What do you observe?
2. Name the reactants in the above reaction.
3. Write the balanced chemical equation for the above reaction.
4. Which is the oxidizing agent and reducing agent in the above reaction?
5. Some chemical equations are given below. Note down the number of reactant atoms and that of product atoms. Balance the unbalanced equation.

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6. The electronic configuration of magnesium and chlorine are 2, 8, 2 and 2, 8, 7 respectively. How many electrons does a magnesium atom donate? What charge will it get?
7. What are oxidation and reduction?
8.

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9. Find out the oxidation state of cr in K2Cr2O7.
10. Find out the oxidation state of Mn in KMnO4.
11. Find out the oxidation state of Fe in FeCl3.

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https://youtu.be/j5M9_qoGKXY
https://youtu.be/epDGHnwaVJY
https://youtu.be/epDGHnwaVJY
https://youtu.be/zmdxMlb88Fs
https://youtu.be/iSAwDJTLIKY