Acids, Bases and Salts Class 10 Notes: Quick Revision Sheet

Essential Exam Terminology

Acid

A substance that produces hydrogen ions (H+) or hydronium ions (H3O+) when dissolved in water. Acids have a sour taste and turn blue litmus paper red.

Base

A substance that produces hydroxide ions (OH-) when dissolved in water. Bases have a bitter taste, soapy touch, and turn red litmus paper blue.

Alkali

A base that is soluble in water. Examples include Sodium Hydroxide (NaOH) and Potassium Hydroxide (KOH).

pH Scale

A logarithmic scale used to measure the hydrogen ion concentration in a solution, indicating its acidity or alkalinity on a scale from 0 to 14.

Neutralisation Reaction

The reaction between an acid and a base to produce salt and water, accompanied by the evolution of heat.

Olfactory Indicators

Substances whose odor changes in acidic or basic media, such as onion, vanilla essence, and clove oil.

Water of Crystallisation

The fixed number of water molecules chemically combined in each formula unit of a salt in its crystalline form (e.g., 5 molecules of H2O in CuSO4·5H2O).

Chemical Properties of Acids and Bases

Understanding how acids and bases react with other substances is key to scoring full marks in CBSE exams. When an acid reacts with a metal, it typically produces salt and hydrogen gas (tested using the classic 'pop test'). Similarly, bases react with metals (like Zinc or Aluminium) to yield hydrogen gas, though these reactions do not occur with all metals. Another crucial area is the reaction of acids with metal carbonates and metal hydrogen carbonates, which produces salt, water, and carbon dioxide gas (which turns lime water milky). Additionally, the dilution of an acid must always be done by adding acid to water slowly with constant stirring, as adding water to acid is a highly exothermic reaction that can cause splashing and severe burns.

Core Differences: Acids vs. Bases

The Chlor-Alkali Process

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Must-Remember Revision Points

• The strength of an acid or base depends on the degree of ionisation (the number of H+ or OH- ions produced).
• pH of acidic solution is always < 7; pH of a neutral solution is 7; pH of a basic solution is > 7.
• Our stomach produces Hydrochloric Acid (HCl) of pH ~1.5 to 3.0 to facilitate food digestion without harming stomach walls.
• Tooth decay starts when the pH of the mouth falls below 5.5 due to acid produced by bacteria digesting sugar.
• Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$) should always be stored in moisture-proof containers to prevent its conversion to Gypsum.
• Common salt (NaCl) serves as an essential raw material for producing Bleaching Powder, Baking Soda, Washing Soda, and Sodium Hydroxide.
• Mixing an acid or base with water results in a decrease in the concentration of ions ($H_3O^+ / OH^-$) per unit volume; this process is called dilution.
• Bee-sting contains methanoic acid, which causes pain and irritation; applying a mild base like baking soda provides relief.

Solved Exemplar Problems

• {"title":"Reaction of Zinc with Sodium Hydroxide","description":"Write a balanced chemical equation for the reaction when Zinc metal is heated with Sodium Hydroxide.","formula":"2NaOH(aq) + Zn(s) -> Na2ZnO2(aq) + H2(g)","explanation":"In this reaction, Zinc reacts with a strong alkali to form Sodium Zincate (a salt) and liberates Hydrogen gas."}
• {"title":"Heating of Baking Soda","description":"What happens when Sodium Hydrogen Carbonate is heated during the cooking of food?","formula":"2NaHCO3 -(Heat)-> Na2CO3 + H2O + CO2","explanation":"Sodium Hydrogen Carbonate decomposes into Sodium Carbonate, water vapour, and Carbon Dioxide. The CO2 gas bubbles out, causing bread or cake to rise."}

Common Board Mistakes & Exam Tips

• The Dilution Rule: Always write that acid must be added to water slowly, and never the other way around. Adding water directly to concentrated acid produces sudden, intense heat that can shatter glass containers or cause acid burns.
• Water of Crystallisation: Don't forget that Plaster of Paris has half a water molecule per calcium sulphate formula unit ($CaSO_4 \cdot \frac{1}{2}H_2O$). The formula represents two formula units of $CaSO_4$ sharing one molecule of water.
• Indicator Colors: Make a small chart on your study desk. Phenolphthalein turns pink in basic solutions and remains colorless in acidic solutions.

Practice Questions with Solutions

• Why does distilled water not conduct electricity, whereas rainwater does? Distilled water is pure and does not contain dissolved ionic compounds. Rainwater contains dissolved gases like carbon dioxide and nitrogen oxides, which form acids (like carbonic acid) and dissociate into free ions that conduct electricity.
• Write the chemical equation for the plastering of fractured bones using Plaster of Paris. $CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$ (Gypsum, a hard solid mass).
• What happens when carbon dioxide gas is passed through lime water in excess? Initially, lime water turns milky due to insoluble Calcium Carbonate ($CaCO_3$). On passing excess carbon dioxide, the milkiness disappears due to the formation of soluble Calcium Hydrogen Carbonate [$Ca(HCO_3)_2$].
• Why is Sodium Hydrogen Carbonate used as an ingredient in antacids? It is a mild, non-corrosive basic salt that safely neutralizes excess hydrochloric acid in the stomach, providing rapid relief from acidity.

Frequently Asked Questions