Acids Bases and Salts – Concept Booster | Class 10 Science CBSE

Q: Look at the formula for blue Copper Sulphate crystals: $\mathrm{CuSO_4 \cdot 5H_2O}$. Does this mean it’s wet? No! Water of crystallization is the fixed number of water molecules chemically trapped inside the crystalline framework of a salt. It gives the crystal its shape and color. If you heat blue copper sulphate crystals strongly, they lose this water and turn into a white, powdery “anhydrous” salt. Add a few drops of water, and the blue color instantly returns!

The compound ‘X’ is Baking Soda. Its chemical formula is $\mathrm{NaHCO_3}$ (Sodium hydrogen carbonate or Sodium bicarbonate). When it is heated during cooking, it undergoes thermal decomposition to release carbon dioxide gas, which makes the food fluffy and crispy. Reaction: $\mathrm{2NaHCO_3 \xrightarrow{Heat} Na_2CO_3 + H_2O + CO_2 \uparrow}$

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Home Concept Boosters CBSE CBSE Class 10 Science Acids Bases & Salts

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How to Use This Page

Read each concept carefully, then check the formula, common mistake, and exam tip before moving to the next. This page completely covers Acids, Bases and Salts for CBSE Class 10 Science, explaining how these crucial chemicals interact in our bodies and industries.

Key Concepts

Class 10 · Science · Chemistry

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Acids, Bases and Salts

The chemistry of everyday liquids and solids

Class 10 · Ch 2

Indicators Definition

Substances that change color (or odor) in acidic and basic solutions.
Litmus: Acid turns blue to red; Base turns red to blue.
Phenolphthalein: Colorless in acid, pink in base.
Olfactory Indicators: Change smell (e.g., onion, vanilla).

$$\text{Acid } \rightarrow \text{ Red (Danger) } | \text{ Base } \rightarrow \text{ Blue (Bitter)}$$

Reaction with Metals Formula

Acids react with most active metals to displace hydrogen gas, forming a metal salt. Bases also react with certain metals (like Zinc and Aluminum) to produce hydrogen gas.

$$\mathrm{Acid + Metal \rightarrow Salt + H_2(g)}$$

$$\mathrm{Zn + H_2SO_4 \rightarrow ZnSO_4 + H_2 \uparrow}$$

Reaction with Metal Carbonates Formula

Acids react with metal carbonates and metal hydrogen carbonates (bicarbonates) to produce brisk effervescence of carbon dioxide gas.

$$\mathrm{Acid + Metal~Carbonate \rightarrow Salt + H_2O + CO_2(g)}$$

$$\mathrm{Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2 \uparrow}$$

Neutralization Reaction Formula

The reaction between an acid and a base to give a salt and water. The effect of a base is nullified by an acid and vice versa.

$$\mathrm{Base + Acid \rightarrow Salt + Water}$$

$$\mathrm{NaOH + HCl \rightarrow NaCl + H_2O}$$

What happens in Water? Concept

Acids produce hydrogen ions, which attach to water to form Hydronium ions ($\mathrm{H_3O^+}$). Bases produce Hydroxide ions ($\mathrm{OH^-}$) when dissolved in water. Alkalis are bases that are soluble in water.

$$\mathrm{HCl + H_2O \rightarrow H_3O^+ + Cl^-}$$

The pH Scale Scale

A scale for measuring hydrogen ion concentration. Ranges from 0 (very acidic) to 14 (very alkaline). A pH of 7 is exactly neutral (pure water).

$$\text{Higher } \mathrm{H^+} \text{ concentration} = \text{Lower pH value}$$

Chlor-alkali Process Formula

When electricity is passed through an aqueous solution of sodium chloride (brine), it decomposes to form sodium hydroxide, chlorine gas, and hydrogen gas.

$$\mathrm{2NaCl_{(aq)} + 2H_2O_{(l)} \xrightarrow{Electricity} 2NaOH_{(aq)} + Cl_{2(g)} + H_{2(g)}}$$

Baking Soda & Washing Soda Chemical Formulas

Baking Soda ($\mathrm{NaHCO_3}$) is used in cooking and as an antacid. Heating it produces Washing Soda ($\mathrm{Na_2CO_3 \cdot 10H_2O}$), used in cleaning and removing permanent hardness of water.

$$\mathrm{2NaHCO_3 \xrightarrow{Heat} Na_2CO_3 + H_2O + CO_2}$$

Bleaching Powder Formula

Produced by the action of chlorine on dry slaked lime. Used for bleaching cotton/linen, wood pulp, and for disinfecting drinking water.

$$\mathrm{Ca(OH)_2 + Cl_2 \rightarrow CaOCl_2 + H_2O}$$

Plaster of Paris (PoP) Formula

On heating Gypsum at exactly 373 K, it loses water molecules and becomes Calcium Sulphate Hemihydrate. Used by doctors for setting fractured bones. Mixing with water turns it back into hard solid Gypsum.

$$\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O}$$

Concept Deep Dive

Diluting Acids: Water to Acid or Acid to Water?

A matter of safety

Core Concept

The process of dissolving an acid or a base in water is a highly exothermic reaction.

If you add water to a concentrated acid, the heat generated is so massive and sudden that the mixture can violently splash out and cause severe acid burns. The glass container may even shatter due to local heating.

The Rule: ALWAYS add the acid to the water, slowly, while stirring constantly. The large volume of water absorbs the generated heat safely.

Water of Crystallization

Why are crystals dry but have water formulas?

Crucial Understanding

Look at the formula for blue Copper Sulphate crystals: $\mathrm{CuSO_4 \cdot 5H_2O}$. Does this mean it’s wet? No!

Water of crystallization is the fixed number of water molecules chemically trapped inside the crystalline framework of a salt. It gives the crystal its shape and color. If you heat blue copper sulphate crystals strongly, they lose this water and turn into a white, powdery “anhydrous” salt. Add a few drops of water, and the blue color instantly returns!

Compare & Contrast

✗ Strong Acids/Bases

Ionize completely (100%) in aqueous solution.
Produce a very high concentration of $\mathrm{H^+}$ (or $\mathrm{OH^-}$) ions.
Excellent conductors of electricity.
Examples: Hydrochloric acid ($\mathrm{HCl}$), Sulphuric acid ($\mathrm{H_2SO_4}$), Sodium Hydroxide ($\mathrm{NaOH}$).

✓ Weak Acids/Bases

Ionize partially in aqueous solution.
Produce a lower concentration of $\mathrm{H^+}$ (or $\mathrm{OH^-}$) ions.
Poor conductors of electricity.
Examples: Acetic acid ($\mathrm{CH_3COOH}$ – Vinegar), Citric acid (Lemons), Ammonium Hydroxide ($\mathrm{NH_4OH}$).

Common Mistakes to Avoid

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Mistake 1

Confusing Basic and Acidic Salts: If you mix a Strong Acid and a Weak Base, the resulting salt is NOT neutral; it is acidic (pH < 7). If you mix a Weak Acid and a Strong Base, the salt is basic (pH > 7). Only a Strong Acid + Strong Base yields a perfectly neutral salt (pH = 7) like $\mathrm{NaCl}$.

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Mistake 2

Thinking “Alkali” and “Base” are identical terms: All alkalis are bases, but not all bases are alkalis. An alkali is specifically a base that dissolves in water. For example, Copper(II) Oxide ($\mathrm{CuO}$) is a base, but it is insoluble in water, so it is NOT an alkali. $\mathrm{NaOH}$ dissolves in water, so it is an alkali.

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Mistake 3

Misidentifying the Anode and Cathode products in Chlor-alkali: During the electrolysis of brine, remember the locations! Chlorine gas ($\mathrm{Cl_2}$) is given off at the Anode (+), Hydrogen gas ($\mathrm{H_2}$) is given off at the Cathode (-), and Sodium Hydroxide ($\mathrm{NaOH}$) forms near the Cathode.

Exam Tips

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Tip 1

The Pop Sound Test: Examiners frequently ask how to identify the gas produced when an acid reacts with a metal. The answer is always: “Bring a burning matchstick near the mouth of the test tube. If the gas burns with a ‘pop’ sound, it confirms the presence of Hydrogen gas.”

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Tip 2

The Milky Lime Water Test: When an acid reacts with a metal carbonate (like baking soda or eggshells), it produces Carbon Dioxide. To test for $\mathrm{CO_2}$, pass it through freshly prepared lime water ($\mathrm{Ca(OH)_2}$). The lime water will turn milky due to the formation of insoluble calcium carbonate ($\mathrm{CaCO_3}$).

Expected Exam Questions

Board Pattern Questions

Class 10 · Science · CBSE Exam

Class 10 · Chemistry

Tooth enamel is one of the hardest substances in our body, yet it gets damaged by eating chocolates and sweets. Why? What should we do to prevent it? [2 marks]

Answer Acid produced by bacteria drops pH below 5.5. Use basic toothpaste. 📝

Explanation

Bacteria present in the mouth degrade the sugar from chocolates and sweets to produce acids. When the pH of the mouth falls below $5.5$, tooth decay starts because the acid corrodes the calcium hydroxyapatite (enamel). To prevent this, we should clean our teeth after eating using toothpaste, which is generally basic in nature and neutralizes the excess acid.

A compound ‘X’ of sodium is commonly used in the kitchen for making crispy pakoras. It is also used for curing acidity in the stomach. Identify ‘X’. What is its chemical formula? State the reaction that takes place when it is heated during cooking. [3 marks]

Answer Baking Soda ($\mathrm{NaHCO_3}$) 📝

Explanation

The compound ‘X’ is Baking Soda.
Its chemical formula is $\mathrm{NaHCO_3}$ (Sodium hydrogen carbonate or Sodium bicarbonate).
When it is heated during cooking, it undergoes thermal decomposition to release carbon dioxide gas, which makes the food fluffy and crispy.
Reaction: $\mathrm{2NaHCO_3 \xrightarrow{Heat} Na_2CO_3 + H_2O + CO_2 \uparrow}$

Why should Plaster of Paris be stored in a moisture-proof container? Write the chemical equation for the reaction. [2 marks]

Answer Moisture converts it back into hard solid Gypsum, rendering it useless. 📝

Explanation

Plaster of Paris ($\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$) is a white powder. If it comes in contact with moisture (water), it absorbs water molecules and rapidly sets into a hard solid mass called Gypsum. To prevent this accidental hardening, it must be stored in a moisture-proof container.
Equation: $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O}$ (Gypsum)

Concept Map

Acids, Bases & Salts connects to →

Chemical Behavior

Metals & Non-Metals (Metallic vs Non-metallic oxides)

Life Processes (Digestion, Blood pH)

Environment (Acid rain effects)

Acids Bases and Salts – Concept Booster | Class 10 Science CBSE

Key Concepts

Acids, Bases and Salts

Concept Deep Dive