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Acids, Bases and Salts — Question 12

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Question 12

A milkman adds a very small amount of baking soda to fresh milk.

(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?

(b) Why does this milk take a long time to set as curd?

Answer

(a) To prevent the milk from turning sour and spoiling due to lactic acid formation, the milkman shifts its pH from 6 to slightly alkaline by adding baking soda.

(b) For this milk to set as curd, the lactic acid formed during curdling process needs to first neutralise the baking soda added. Hence, it takes longer time to set as curd.

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Science | Chapter 2: Acids, Bases and SaltsWeb Content

Chapter 2: Acids, Bases and Salts — Quick Revision Guide

Introduction

Acids and bases are fundamental chemical substances found everywhere in daily life. This chapter explores their properties, the pH scale, neutralisation reactions, and the preparation of important salts.

Key Points at a Glance

  1. Acids produce H+ (H3O+) ions in water; bases produce OH− ions
  2. Indicators: Litmus (red/blue), methyl orange, phenolphthalein, turmeric; olfactory indicators change smell
  3. Acid reactions: with metals (salt + H2), carbonates (salt + H2O + CO2), metal oxides (salt + H2O), bases (neutralisation)
  4. pH scale: 0–14; pH < 7 acidic, pH = 7 neutral, pH > 7 basic; measured by universal indicator
  5. pH importance: stomach (1–2), blood (7.35–7.45), tooth decay below pH 5.5, soil pH for agriculture
  6. Always add acid to water (dilution is exothermic); dry HCl gas does not affect litmus
  7. Chlor-alkali process: electrolysis of brine gives NaOH + Cl2 + H2
  8. Important salts: Bleaching powder (CaOCl2), Baking soda (NaHCO3), Washing soda (Na2CO3·10H2O), Plaster of Paris (CaSO4·½H2O)
  9. Water of crystallisation: CuSO4·5H2O (blue) loses water on heating → CuSO4 (white)

Real-World Connections

Antacids neutralise excess stomach acid; baking soda makes bread fluffy by releasing CO2; washing soda softens hard water; bleaching powder purifies drinking water; Plaster of Paris is used in fracture casts.

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 2
  • NCERT Exemplar Problems
  • Bright Tutorials Detailed Notes: ch02-acids-bases-salts.html
  • Bright Tutorials Practice Questions: ch02-acids-bases-salts.html
  • Previous Year CBSE Board Papers

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