Chapter 1 of Class 10 CBSE Science, Chemical Reactions and Equations, serves as the fundamental stepping stone for all future chemistry studies. A chemical reaction is a process where the original chemical substances lose their identity and form new products with distinct chemical and physical properties. Observable indicators that confirm a chemical reaction has taken place include a change in color, evolution of a gas, change in temperature (exothermic or endothermic), or formation of an insoluble solid precipitate.
The Law of Conservation of Mass dictates that mass can neither be created nor destroyed in a chemical transformation. Therefore, the total count of atoms of each element on the reactant side must equal the count on the product side. To balance a chemical equation systematically: begin with the compound that contains the largest number of atoms, balance non-oxygen and non-hydrogen elements first, balance oxygen next, and balance hydrogen last. For example, in the reaction between iron and steam: Fe + H2O gives Fe3O4 + H2. Balancing iron gives 3Fe; balancing oxygen in Fe3O4 requires 4H2O; balancing hydrogen yields 4H2, producing the balanced equation: 3Fe(s) + 4H2O(g) -> Fe3O4(s) + 4H2(g).
Chemical reactions are classified into five primary types in Class 10 CBSE. Combination reactions occur when two or more reactants unite to yield a single product, such as the slaking of quicklime: CaO(s) + H2O(l) -> Ca(OH)2(aq) with immense heat release. Decomposition reactions occur when a single compound splits into multiple simpler products upon supplying energy. These are subdivided into thermal decomposition (heating ferrous sulphate crystals FeSO4.7H2O or lead nitrate Pb(NO3)2 releasing brown NO2 fumes), electrolytic decomposition (electrolysis of water giving H2 and O2 in 2:1 volume ratio), and photolytic decomposition (decomposition of white silver chloride AgCl into grey silver under sunlight, utilized in black and white photography).
Displacement reactions involve a more reactive metal displacing a less reactive metal from its aqueous salt solution, governed strictly by the Reactivity Series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au. For instance, placing an iron nail in blue copper sulphate solution causes blue color to fade to light green FeSO4 while reddish-brown copper deposits on the nail. Double displacement reactions involve an exchange of ions between two aqueous reactants, typically yielding an insoluble precipitate (such as mixing colorless sodium sulphate Na2SO4 and barium chloride BaCl2 solutions to form a white precipitate of barium sulphate BaSO4).
Redox reactions involve simultaneous oxidation (gain of oxygen or loss of hydrogen/electrons) and reduction (loss of oxygen or gain of hydrogen/electrons). When black copper oxide is heated with hydrogen gas: CuO + H2 -> Cu + H2O, CuO loses oxygen (reduced to Cu) and acts as the oxidizing agent, while H2 gains oxygen (oxidized to H2O) and acts as the reducing agent. Real-world consequences of redox include corrosion of metals (such as rust Fe2O3.xH2O on iron) and rancidity of fat-containing foods prevented by nitrogen packaging and antioxidants.
Key Concept Takeaways
- Every chemical equation must be balanced to satisfy the fundamental Law of Conservation of Mass.
- Decomposition reactions require energy in three forms: heat (thermal), electricity (electrolytic), or light (photolytic).
- Electrolysis of water produces hydrogen gas at the cathode and oxygen gas at the anode in a strict 2:1 volume ratio.
- Displacement feasibility is determined by the reactivity series: a higher metal displaces a lower metal from its salt solution.
- In redox reactions: the substance that loses oxygen is reduced (oxidizing agent); the substance that gains oxygen is oxidized (reducing agent).
Authored by Dr. Aarzoo Saini
Founder & Lead Educator at We-Gyaan Classes Roorkee, with over 20 years of teaching excellence in Science and Chemistry for Board Exams, NEET, JEE, and CUET.