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Acids, Bases and Salts - Definitions and general properties

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Acids are substances that taste sour and turn blue litmus red. According to the Arrhenius theory, acids release hydrogen ions H+H^+ in aqueous solution, which exist as hydronium ions H3O+H_3O^+.

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Bases are substances that taste bitter, feel soapy to touch, and turn red litmus blue. They release hydroxide ions OH−OH^- in aqueous solution. Bases that are soluble in water are known as Alkalis (e.g., NaOHNaOH, KOHKOH).

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Indicators are substances used to test the acidic or basic nature of a solution. Natural indicators include Litmus and Turmeric. Synthetic indicators include Phenolphthalein (colorless in acid, pink in base) and Methyl Orange (red in acid, yellow in base).

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Acids react with active metals to produce a salt and evolve hydrogen gas H2H_2. This gas can be tested by the 'Pop Sound' test.

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Acids react with metal carbonates and metal hydrogencarbonates to produce a salt, water, and carbon dioxide gas CO2CO_2. The CO2CO_2 gas turns lime water milky due to the formation of CaCO3CaCO_3.

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Neutralization Reaction: When an acid reacts with a base, they neutralize each other's effects to form a salt and water. This is an exothermic reaction.

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The pH scale (ranging from 0 to 14) measures the strength of an acid or base based on the concentration of H+H^+ ions. A pH<7pH < 7 is acidic, pH=7pH = 7 is neutral, and pH>7pH > 7 is basic.

📐Formulae

H(aq)++H2O(l)→H3O(aq)+H^+_{(aq)} + H_2O_{(l)} \rightarrow H_3O^+_{(aq)}

Acid+Metal→Salt+H2(g)↑Acid + Metal \rightarrow Salt + H_2(g) \uparrow

Acid+Base→Salt+WaterAcid + Base \rightarrow Salt + Water

Metal Carbonate+Acid→Salt+CO2(g)+H2O(l)Metal\ Carbonate + Acid \rightarrow Salt + CO_2(g) + H_2O(l)

Metal Oxide+Acid→Salt+WaterMetal\ Oxide + Acid \rightarrow Salt + Water

Non-metallic Oxide+Base→Salt+WaterNon\text{-}metallic\ Oxide + Base \rightarrow Salt + Water

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

💡Examples

Problem 1:

Write the balanced chemical equation for the reaction of Zinc granules with dilute Sulfuric acid. How do you test for the gas evolved?

Solution:

Zn(s)+H2SO4(aq)→ZnSO4(aq)+H2(g)↑Zn(s) + H_2SO_4(aq) \rightarrow ZnSO_4(aq) + H_2(g) \uparrow

Explanation:

Zinc, being a reactive metal, displaces Hydrogen from the acid to form Zinc Sulfate (salt) and Hydrogen gas. To test for H2H_2 gas, bring a burning splinter near the mouth of the test tube; the gas burns with a characteristic 'pop' sound.

Problem 2:

What happens when Carbon dioxide gas is passed through Lime water? Provide the equations for both short and long durations.

Solution:

Short duration: Ca(OH)2(aq)+CO2(g)→CaCO3(s)↓+H2O(l)Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) \downarrow + H_2O(l). Long duration: CaCO3(s)+H2O(l)+CO2(g)→Ca(HCO3)2(aq)CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)

Explanation:

Initially, the lime water turns milky due to the formation of insoluble Calcium Carbonate (CaCO3CaCO_3). On passing excess CO2CO_2, the milkiness disappears because soluble Calcium Bicarbonate (Ca(HCO3)2Ca(HCO_3)_2) is formed.

Problem 3:

Identify the nature of the reaction: NaOH(aq)+HCl(aq)→NaCl(aq)+H2O(l)NaOH(aq) + HCl(aq) \rightarrow NaCl(aq) + H_2O(l).

Solution:

This is a Neutralization Reaction.

Explanation:

Sodium hydroxide (NaOHNaOH), a strong base, reacts with Hydrochloric acid (HClHCl), a strong acid, to form Sodium Chloride (NaClNaCl), a neutral salt, and water. The H+H^+ from the acid and OH−OH^- from the base combine to form H2OH_2O.