Acids, Bases and Salts

Introduction

This chapter introduces the fundamental concepts of acids, bases, and salts, which are essential chemical substances encountered in everyday life. You have already learnt that the sour taste in food is due to acids, while the bitter taste is due to bases. Acids and bases are two classes of compounds that exhibit characteristic properties and react in specific ways. For example, acids turn blue litmus paper red, whereas bases turn red litmus paper blue. Natural indicators such as litmus (extracted from lichen), turmeric, and synthetic indicators like methyl orange and phenolphthalein help identify whether a substance is acidic or basic. The chapter explores how acids and bases neutralize each other, their reactions with metals and carbonates, their electrical conductivity, and the pH scale that measures acidity or alkalinity. It also discusses salts formed from acids and bases, water of crystallisation in salts, and the practical uses of these substances in daily life and industry.

• Acids taste sour and turn blue litmus red.
• Bases taste bitter and turn red litmus blue.
• Indicators such as litmus, turmeric, methyl orange, and phenolphthalein detect acids and bases.
• Acids and bases neutralize each other's effects.
• Salts are formed from acid-base reactions.
• Water of crystallisation is water chemically bound in salt crystals.
• 📌 Acid: Substance that produces H+ ions in solution and tastes sour.
• 📌 Base: Substance that produces OH- ions in solution and tastes bitter.
• 📌 Indicator: Substance that changes color in presence of acid or base.

2.1 Understanding the Chemical Properties of Acids and Bases

This section explores the chemical properties of acids and bases through various laboratory activities. Acids such as hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3), and acetic acid (CH3COOH) and bases like sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)2], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)2], and ammonium hydroxide (NH4OH) are tested with indicators. Indicators like red and blue litmus, phenolphthalein, and methyl orange change color depending on the acidic or basic nature of the solution. Olfactory indicators, substances whose odour changes in acidic or basic media, such as onion, vanilla essence, and clove oil, are also explored. Acids react with metals like zinc to produce hydrogen gas and salts, demonstrating their acidic nature. The reaction is summarized as Acid + Metal → Salt + Hydrogen gas. Bases also react with metals, for example, zinc reacts with sodium hydroxide to form sodium zincate and hydrogen gas. Metal carbonates and hydrogencarbonates react with acids to produce salt, carbon dioxide, and water. The reaction is Metal carbonate/hydrogencarbonate + Acid → Salt + CO2 + Water. Acids and bases neutralize each other to form salt and water, a reaction called neutralization. Metallic oxides react with acids to form salt and water, indicating metallic oxides are basic oxides. Conversely, non-metallic oxides like carbon dioxide react with bases to form salt and water, showing non-metallic oxides are acidic oxides.

• Indicators show characteristic color changes with acids and bases.
• Olfactory indicators change smell in acidic or basic media.
• Acids react with metals to produce hydrogen gas and salts.
• Bases can also react with metals producing hydrogen and complex salts.
• Metal carbonates/hydrogencarbonates react with acids to produce CO2, salt, and water.
• Neutralization reaction: Acid + Base → Salt + Water.
• Metallic oxides are basic oxides reacting with acids to form salt and water.
• Non-metallic oxides are acidic oxides reacting with bases to form salt and water.