Acids, Bases, and Salts: CBSE Class 10 Science Chapter Guide

Acids and Bases: Understanding Their Nature and Properties

Acids and bases are two fundamental classes of chemical compounds with distinct characteristics. Historically, acids were known for their sour taste and corrosive nature, while bases (also called alkalis if soluble in water) were recognized by their bitter taste and soapy feel. For CBSE Class 10 Science, we primarily focus on their definitions based on their ability to produce ions in water, and their observable chemical properties.

Acids are substances that release hydrogen ions (H⁺) when dissolved in water. These H⁺ ions combine with water molecules to form hydronium ions (H₃O⁺). Examples include hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃). Their key properties include:

• Taste: Sour (e.g., lemon, vinegar). Never taste chemicals in the lab!
• Effect on Indicators: Turn blue litmus red; turn methyl orange red; keep phenolphthalein colourless.
• Reaction with Metals: Acids react with active metals (like zinc, magnesium, iron) to produce hydrogen gas (H₂) and a salt. For example, Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
• Reaction with Metal Carbonates and Metal Hydrogen Carbonates: They react to produce a salt, water, and carbon dioxide gas (CO₂). For instance, Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g).
• Reaction with Bases: They neutralize bases to form salt and water.

Bases are substances that release hydroxide ions (OH⁻) when dissolved in water. Examples include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂). Their key properties are:

• Taste: Bitter (e.g., baking soda solution). Again, never taste in the lab!
• Feel: Soapy or slippery to touch.
• Effect on Indicators: Turn red litmus blue; turn methyl orange yellow; turn phenolphthalein pink.
• Reaction with Non-metal Oxides: Bases react with non-metal oxides (which are acidic in nature) to form salt and water. For example, Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).
• Reaction with Acids: They neutralize acids to form salt and water.

Understanding these basic properties is crucial for identifying and classifying different chemical substances in experiments and daily life, forming the bedrock for advanced chemistry concepts.

The pH Scale: Measuring Acidity and Basicity

Neutralisation Reaction and Formation of Salts

1. Understanding Neutralisation — Neutralisation is a chemical reaction in which an acid and a base react quantitatively with each other to form a salt and water. The general equation is: Acid + Base → Salt + Water. This reaction essentially involves the combination of H⁺ ions from the acid and OH⁻ ions from the base to form water (H₂O), thereby 'neutralizing' their acidic and basic properties.
2. Performing a Neutralisation Experiment — To demonstrate neutralisation, take a small amount of dilute HCl in a beaker. Add a few drops of phenolphthalein indicator. The solution remains colourless. Now, add dilute NaOH solution drop by drop using a dropper, stirring continuously. You will observe a slight pink colour appearing. This indicates that the solution has become basic. Add one more drop of HCl; the pink colour disappears, indicating it's acidic again. The point where the colour just changes from colourless to light pink (or vice-versa) is the endpoint, indicating neutralisation has occurred. The reaction is: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).
3. Formation and Types of Salts — Salts are ionic compounds formed when an acid reacts with a base. Depending on the strength of the acid and base involved, salts can be: Neutral Salts: Formed from a strong acid and a strong base (e.g., NaCl from HCl + NaOH). Acidic Salts: Formed from a strong acid and a weak base (e.g., NH₄Cl from HCl + NH₄OH). * Basic Salts: Formed from a weak acid and a strong base (e.g., Na₂CO₃ from H₂CO₃ + NaOH). Common salts like Sodium Chloride (NaCl), Sodium Carbonate (Na₂CO₃), Bleaching Powder (CaOCl₂), Baking Soda (NaHCO₃), Washing Soda (Na₂CO₃·10H₂O), and Plaster of Paris (CaSO₄·½H₂O) have various applications in daily life and industries, forming a significant part of this chapter.

Applying Concepts: Worked Examples

• Example 1: Identifying a Gas Produced A student adds zinc granules to dilute hydrochloric acid. A gas is produced which burns with a 'pop' sound. Identify the gas and write the balanced chemical equation for the reaction. Analysis: Metals reacting with acids typically produce hydrogen gas. Hydrogen gas is known for burning with a 'pop' sound. Identification: The gas is Hydrogen (H₂). Equation: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) Explanation: Zinc is a reactive metal, and it displaces hydrogen from hydrochloric acid to form zinc chloride and hydrogen gas.
• Example 2: pH and Solution Nature Solution 'A' has a pH of 2, and solution 'B' has a pH of 10. Which solution is strongly acidic, and which is strongly basic? What happens if you mix equal volumes of these solutions? Analysis: pH values below 7 are acidic, and values above 7 are basic. The further from 7, the stronger the nature. Identification: Solution 'A' (pH 2) is strongly acidic. Solution 'B' (pH 10) is strongly basic. * Mixing: When equal volumes of a strong acid (pH 2) and a strong base (pH 10) are mixed, a neutralisation reaction will occur. The resulting solution will likely be close to neutral (pH 7) if their concentrations are similar, forming a salt and water.
• Example 3: Reaction with Carbonates When quicklime (calcium oxide) reacts with water, a solution 'X' is formed. What is solution 'X'? Write the balanced chemical equation. What happens when carbon dioxide gas is passed through solution 'X'? Analysis: Quicklime (CaO) is a basic oxide. Its reaction with water forms a base. Passing CO₂ (an acidic oxide) through a base will cause a neutralisation reaction. Solution 'X': Calcium hydroxide, Ca(OH)₂ (also known as slaked lime). Equation for formation of X: CaO(s) + H₂O(l) → Ca(OH)₂(aq) Reaction with CO₂: When CO₂ gas is passed through Ca(OH)₂ solution, calcium carbonate (CaCO₃), a white precipitate, and water are formed. This is the test for CO₂ gas (turns limewater milky). * Equation: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

YoLearn's Exam Tips: Mastering Acids, Bases, and Salts

To excel in the Acids Bases and Salts Class 10 NCERT chapter, pay attention to these crucial points:

1. Memorise Indicator Changes: Know precisely how litmus, methyl orange, and phenolphthalein change colour in acidic, basic, and neutral solutions. Create a small table for quick revision.
2. Balance Chemical Equations: Always ensure your chemical equations are correctly balanced, especially for reactions involving acids with metals, carbonates, and bases. Practice writing equations for various neutralisation reactions.
3. Distinguish pH Concepts: Understand that pH values relate to the strength of an acid or base (how much it ionises), not just its concentration. A dilute strong acid can have the same pH as a concentrated weak acid.
4. Applications of Salts: Familiarize yourself with the preparation, properties, and uses of important salts like Sodium Chloride, Sodium Hydroxide, Bleaching Powder, Baking Soda, Washing Soda, and Plaster of Paris. Focus on the chemical formulas and key reactions involved in their preparation.
5. Reactions with Water: Remember that for an acid or base to show its properties, it must dissolve in water to produce H⁺ or OH⁻ ions, respectively. Dry HCl gas, for instance, does not turn blue litmus red.

Practice Questions with Solutions

• Q: What happens when dilute hydrochloric acid is added to iron filings? Write the balanced chemical equation. A: Step 1: Identify the reactants: dilute hydrochloric acid (HCl) and iron filings (Fe). Step 2: Recall the reaction of an acid with a metal: Acid + Metal → Salt + Hydrogen gas. Step 3: Determine the products: Iron (Fe) will displace hydrogen from HCl to form iron(II) chloride (FeCl₂) and hydrogen gas (H₂). Step 4: Write and balance the equation: Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g). Final answer: When dilute hydrochloric acid is added to iron filings, hydrogen gas is produced, which can be identified by its 'pop' sound when brought near a burning splinter. The balanced equation is Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g).
• Q: A solution turns red litmus blue. Its pH is likely to be: (a) 1 (b) 4 (c) 7 (d) 10 A: Step 1: Analyze the indicator change. Red litmus turning blue indicates that the solution is basic. Step 2: Recall the pH scale for basic solutions. Basic solutions have a pH greater than 7. Step 3: Evaluate the given options. Options (a) and (b) are acidic, (c) is neutral, and (d) is basic. Final answer: (d) 10. A solution with pH 10 is basic, which turns red litmus blue.
• Q: Why should curd and sour substances not be kept in brass and copper vessels? A: Step 1: Identify the nature of curd and sour substances. They contain acids (e.g., lactic acid in curd). Step 2: Recall the reaction of acids with metals. Acids react with metals to form hydrogen gas and a corresponding salt. Step 3: Consider the materials of the vessels. Brass and copper are alloys containing metals. Step 4: Conclude the implication. The acids in curd and sour substances would react with the metals (copper, zinc in brass) of the vessels. This reaction produces toxic metal salts, which can contaminate the food and make it unfit for consumption, potentially causing food poisoning. Final answer: Curd and sour substances are acidic. When kept in brass or copper vessels, the acids react with the metals present in these vessels. This reaction can produce harmful or poisonous compounds (metal salts) that contaminate the food, making it unsafe to eat.
• Q: How is Plaster of Paris prepared? Write the chemical equation for its preparation. A: Step 1: Recall the raw material for Plaster of Paris. It is prepared from Gypsum. Step 2: Understand the chemical process. Gypsum is heated to a specific temperature. Step 3: Identify the chemical names and formulas. Gypsum is calcium sulfate dihydrate (CaSO₄·2H₂O). Plaster of Paris is calcium sulfate hemihydrate (CaSO₄·½H₂O). Step 4: Write the balanced chemical equation, ensuring the correct temperature conditions. Final answer: Plaster of Paris is prepared by heating gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) to a temperature of 373 K (100°C). At this temperature, it loses three-fourths of its water of crystallisation to form calcium sulfate hemihydrate (Plaster of Paris). Equation: CaSO₄·2H₂O (s) → CaSO₄·½H₂O (s) + 1½H₂O (g) (at 373 K)

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