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Chemical Reactions and Equations — Question 20

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Question 20

Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Answer

(a) Corrosion — When a metal is attacked by substances around it such as moisture, acids, etc, it is said to corrode and this process is called corrosion.
For example, iron articles are shiny when new, but get coated with a reddish brown powder when left for some time due to rusting.

(b) Rancidity — When taste and smell of food items containing fat and oil change due to oxidation, then this condition is called rancidity.
For example, when butter is left exposed to air for a long time, it becomes rancid, making it unsuitable for consumption and cooking.

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Science | Chapter 1: Chemical Reactions and EquationsWeb Content

Chapter 1: Chemical Reactions and Equations — Quick Revision Guide

Introduction

Chemical reactions are processes where substances interact to form new substances with different properties. This chapter covers the writing and balancing of chemical equations, types of chemical reactions, and the real-world effects of oxidation.

Key Points at a Glance

  1. A chemical equation represents a reaction using symbols and formulae with state symbols (s), (l), (g), (aq)
  2. Equations must be balanced — same number of atoms of each element on both sides (Law of Conservation of Mass)
  3. Combination: Two or more substances combine into one product. Example: CaO + H2O → Ca(OH)2
  4. Decomposition: One substance breaks into two or more. Types: thermal, electrolytic, photolytic. Example: 2FeSO4 → Fe2O3 + SO2 + SO3
  5. Displacement: More reactive element displaces less reactive from its compound. Example: Fe + CuSO4 → FeSO4 + Cu
  6. Double displacement: Exchange of ions between two compounds. Example: Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl
  7. Redox: Simultaneous oxidation (gain of O / loss of H) and reduction (loss of O / gain of H). Example: CuO + H2 → Cu + H2O
  8. Corrosion: Slow destruction of metals by environment — rusting of iron (Fe2O3·xH2O); prevention: painting, oiling, galvanising, alloying
  9. Rancidity: Oxidation of fats/oils giving bad taste/smell; prevention: antioxidants, airtight containers, refrigeration, nitrogen flushing

Real-World Connections

Respiration is oxidation of glucose; digestion involves decomposition; formation of curd from milk is a chemical change; fireworks involve combination reactions; electroplating is a redox process.

Quick Self-Test (5 Questions)

  1. What is the most important concept you learned from this chapter?
  2. Can you write three key equations/formulae from this chapter from memory?
  3. Draw a labelled diagram relevant to this chapter without looking at your notes.
  4. Explain one real-world application of a concept from this chapter.
  5. What is one common mistake students make in this chapter, and how can you avoid it?

Further Study

  • NCERT Textbook Chapter 1
  • NCERT Exemplar Problems
  • Bright Tutorials Detailed Notes: ch01-chemical-reactions.html
  • Bright Tutorials Practice Questions: ch01-chemical-reactions.html
  • Previous Year CBSE Board Papers

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