NCERT Class 10 Science: Acid, Bases and Salts Complete Guide

Chemical Characterization of Acids and Bases

Acids and bases are defined chemically by their behavior in aqueous solutions. According to the Arrhenius theory, an acid is a substance that dissociates in water to yield hydrogen ions ($H^+$ or hydronium ions, $H_3O^+$), whereas a base is a substance that yields hydroxyl ions ($OH^-$). Acids show a sour taste, change blue litmus paper to red, and react with active metals to liberate hydrogen gas ($H_2$). Bases, which are soapy to touch, turn red litmus blue and have a bitter taste. Strong acids and bases ionize completely in water (e.g., $HCl$, $NaOH$), whereas weak ones undergo partial ionization (e.g., $CH_3COOH$, $NH_4OH$). Chemical indicators such as phenolphthalein (colorless in acid, pink in base) and methyl orange (red in acid, yellow in base) are used to detect their presence dynamically.

Key Properties: Acids vs. Bases

Understanding the Neutralization Reaction Mechanism

1. Ion Generation — When an acid (e.g., HCl) and a base (e.g., NaOH) are dissolved in water, they separate into their respective constituent ions: H+, Cl-, Na+, and OH-.
2. Proton and Hydroxyl Combination — The hydronium ions (H+) from the acid and hydroxyl ions (OH-) from the base combine rapidly to form neutral water molecules: H+ + OH- -> H2O.
3. Salt Crystallization — The remaining spectator ions (Na+ and Cl-) stay dissolved in the aqueous medium. Upon evaporation of water, they form solid sodium chloride crystal lattice (NaCl).

Board Exam Caution: The Dilution of Acids

A frequent question in the CBSE Class 10 board exam is: 'Why is it recommended that the acid should be added to water, and not water to the acid?'

• The Reason: Mixing concentrated acid with water is highly exothermic. If you add water to a concentrated acid, the heat generated is so intense that the mixture can violently splash out, causing severe chemical burns on your face and hands. The local heating can also shatter the glass container.
• The Safe Way: Always add acid to water in a slow, controlled drop-by-drop manner while stirring the solution continuously. This distributes the heat throughout the larger volume of water safely.

Practice Questions with Solutions

• Q: Why does dry HCl gas not change the color of dry litmus paper? A: Step 1: Analyze the chemical condition. Dry HCl gas is in molecular form and does not contain free hydronium ions ($H^+$). Step 2: Understand indicator behavior. Litmus paper changes color only in the presence of $H^+$ or $H_3O^+$ ions, which are produced when acids dissociate in water. Step 3: Connect the facts. Since both the gas and litmus paper are dry, no water is present to initiate ionization. Final answer: Dry HCl gas does not ionize to produce $H^+$ ions, and thus cannot show acidic properties to change dry litmus paper's color.
• Q: Five solutions A, B, C, D, and E when tested with a universal indicator showed pH values of 4, 1, 11, 7, and 9 respectively. Which solution is neutral, strongly alkaline, strongly acidic, weakly acidic, and weakly alkaline? Arrange in increasing order of hydrogen ion concentration. A: Step 1: Classify each pH value. Neutral is pH 7 (Solution D). Strongly alkaline is high pH 11 (Solution C). Strongly acidic is low pH 1 (Solution B). Weakly acidic is pH 4 (Solution A). Weakly alkaline is pH 9 (Solution E). Step 2: Understand the relation between pH and hydrogen ion concentration. Lower pH corresponds to a higher concentration of hydrogen ions ($H^+$). Step 3: Arrange in increasing order of $H^+$ concentration (from lowest concentration to highest, i.e., highest pH to lowest pH): C < E < D < A < B. Final answer: Neutral = D, Strongly alkaline = C, Strongly acidic = B, Weakly acidic = A, Weakly alkaline = E. Increasing order of hydrogen concentration: C < E < D < A < B.
• Q: Write the chemical equation for the preparation of Plaster of Paris from Gypsum. What precaution should be taken during this reaction? A: Step 1: Identify reactants and conditions. Gypsum ($CaSO_4 \cdot 2H_2O$) must be heated carefully to a specific temperature of $373\text{ K}$ ($100^\circ\text{C}$). Step 2: Write the balanced chemical equation: $CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O$ Step 3: State the critical precaution. The temperature must not exceed $373\text{ K}$. If heated above this, all water of crystallization is lost, turning it into anhydrous calcium sulfate ($CaSO_4$), known as 'dead burnt plaster', which lacks setting properties when mixed with water. Final answer: Gypsum heated at $373\text{ K}$ yields Plaster of Paris ($CaSO_4 \cdot \frac{1}{2}H_2O$). Heating above this temperature must be avoided to prevent dead burnt plaster.
• Q: A compound X of sodium is commonly used in kitchen recipes to make crispy pakoras. It is also used to reduce acidity in the stomach. Identify X, write its chemical name, and write the balanced equation when it is heated. A: Step 1: Match compound properties to known substances. The sodium compound used for crispy baking and as an antacid is Baking Soda. Step 2: Identify chemical name and formula. Compound X is Sodium Hydrogen Carbonate ($NaHCO_3$). Step 3: Formulate reaction on heating. When heated, sodium hydrogen carbonate decomposes to produce sodium carbonate, water vapor, and carbon dioxide gas: $2NaHCO_3 \xrightarrow{\Delta} Na_2CO_3 + H_2O + CO_2\uparrow$ Final answer: X is Sodium Hydrogen Carbonate (Baking Soda, $NaHCO_3$). Heating equation: $2NaHCO_3 \rightarrow Na_2CO_3 + H_2O + CO_2$.

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