Chemical Reactions and Equations

Introduction

A chemical reaction is a process in which one or more substances (called reactants) change to form new substances (called products). These changes may involve color change, gas formation, temperature change, or formation of a solid.

1. Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction using chemical formulas.

Example:
Magnesium + Oxygen → Magnesium oxide
Mg + O₂ → MgO

This is a chemical equation showing the reactants and the product.

2. Balanced Chemical Equation
The law of conservation of mass says that mass is neither created nor destroyed in a chemical reaction.
So, the number of atoms of each element must be the same on both sides of the equation.

Balanced example:
2Mg + O₂ → 2MgO

3. Types of Chemical Reactions
Combination Reaction:--
Two substances combine to form one compound.
Example: CaO + H₂O → Ca(OH)₂

Decomposition Reaction:--
A single compound breaks down into simpler substances.
Example: 2H₂O₂ → 2H₂O + O₂

Displacement Reaction:--
One element replaces another in a compound.
Example: Zn + CuSO₄ → ZnSO₄ + Cu

Double Displacement Reaction:
Two compounds exchange their ions.
Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

4. Effects of a Chemical Reaction
Chemical reactions may show:

Change in color
Evolution of gas
Formation of a precipitate
Change in temperature

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Questions on Chemical Reactions and Equations

Showing 121–150 of 167

121.A student observed that on mixing solutions of two salts, a precipitate was formed. What are the common characteristics of such reactions? 122.Which of the following is a balanced equation? 123.Why is photosynthesis considered an endothermic reaction? 124.In the decomposition of lead nitrate powder, what is the color of the gas evolved and the residue left? 125.A student noted that a strip of aluminum metal was placed in a beaker containing a solution of copper chloride. After some time, the blue color of the solution faded, and a reddish-brown deposit was observed on the aluminum strip. What is the reason for this observation? 126.Which of the following reactions is used for the preparation of oxygen? 127.Define a redox reaction. Give one example. 128.Why are chips manufacturers flush bags of chips with nitrogen gas? 129.Consider the reaction: CuO(s) + H2(g) → Cu(s) + H2O(l). Identify the substance undergoing reduction. 130.When a zinc granule is added to a test tube containing dilute sulphuric acid, a gas is evolved. What is the gas, and how would you test for it? 131.Which of the following is not a characteristic of a chemical reaction? 132.Balance the following chemical equation: KMnO4 + HCl → KCl + MnCl2 + H2O + Cl2 133.In an experiment, when a student added a few drops of universal indicator to a solution of sodium hydroxide, the color changed to blue. What kind of reaction would occur if hydrochloric acid was then slowly added to this solution? 134.Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions. 135.Which of the following statements about the given reaction are correct? 3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g) (i) Iron metal is rusting. (ii) Water is getting reduced. (iii) Water is acting as a reducing agent. (iv) Iron is getting oxidized. 136.A solution of a substance 'X' is used for whitewashing. i) Name the substance 'X' and write its formula. ii) Write the reaction of the substance 'X' with water. 137.Identify the substance oxidized in the following reaction: ZnO + C → Zn + CO 138.Why does a brown layer form on iron articles when left exposed to air and moisture for some time? 139.A student dropped a small piece of marble into a test tube containing dilute hydrochloric acid. A gas evolved which turned lime water milky. What type of reaction occurred? 140.Which of the following is an exothermic process? 141.Explain why iron articles are often painted. 142.When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide is formed and sulphuric acid is formed. The reaction is an example of: 143.Which of the following processes involves a chemical change? 144.A shiny brown element X on heating in air becomes black. Name the element X and the black-colored compound formed. 145.When a student mixes solution A (containing lead(II) nitrate) with solution B (containing potassium iodide), a yellow precipitate forms. What type of chemical reaction is this, and write the balanced chemical equation? 146.What do you mean by a precipitation reaction? Explain by giving examples. 147.In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved. 148.What is the difference between displacement and double displacement reactions? Write equations for these reactions. 149.Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity. 150.Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions