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The Amazing World of Solutes, Solvents, and Solutions - Acids, Bases, and Natural Indicators

Grade 7CBSE

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

🔑Concepts

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A solution is a homogeneous mixture of two or more substances. It consists of a solute (the substance being dissolved) and a solvent (the medium in which the solute dissolves).

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Acids are chemical substances that taste sour and turn blue litmus paper red. Examples include HClHCl (Hydrochloric acid) and H2SO4H_{2}SO_{4} (Sulphuric acid).

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Bases are substances that taste bitter, feel soapy to the touch, and turn red litmus paper blue. Common examples are NaOHNaOH (Sodium hydroxide) and Mg(OH)2Mg(OH)_{2} (Magnesium hydroxide).

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Indicators are special substances used to test whether a substance is acidic or basic by changing color. Natural indicators include Litmus (extracted from lichens), Turmeric, and China Rose petals.

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Neutralization is the reaction between an acid and a base which results in the formation of salt and water, accompanied by the release of heat.

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The pHpH scale measures how acidic or basic a substance is, ranging from 00 to 1414. A pHpH of 77 is neutral, less than 77 is acidic, and greater than 77 is basic.

📐Formulae

Acid+Base→Salt+Water+HeatAcid + Base \rightarrow Salt + Water + Heat

HCl+NaOH→NaCl+H2OHCl + NaOH \rightarrow NaCl + H_{2}O

H2SO4+Ca(OH)2→CaSO4+2H2OH_{2}SO_{4} + Ca(OH)_{2} \rightarrow CaSO_{4} + 2H_{2}O

2HCl+Mg(OH)2→MgCl2+2H2O2HCl + Mg(OH)_{2} \rightarrow MgCl_{2} + 2H_{2}O

💡Examples

Problem 1:

A student accidentally spills a drop of dilute HClHCl on a piece of blue litmus paper. What change will be observed?

Solution:

The blue litmus paper will turn red.

Explanation:

Acids like HClHCl have the property of changing the color of blue litmus indicator to red, indicating an acidic nature.

Problem 2:

When a turmeric stain on a white cloth is washed with soap, it turns reddish-brown. Why?

Solution:

The turmeric acts as a natural indicator and reacts with the basic nature of the soap.

Explanation:

Soap contains bases like NaOHNaOH. Turmeric remains yellow in acidic or neutral solutions but turns reddish-brown when it comes into contact with a base.

Problem 3:

Explain why an antacid tablet like Milk of Magnesia, Mg(OH)2Mg(OH)_{2}, is taken when one suffers from acidity.

Solution:

Mg(OH)2+2HCl→MgCl2+2H2OMg(OH)_{2} + 2HCl \rightarrow MgCl_{2} + 2H_{2}O

Explanation:

The stomach produces HClHCl for digestion. Excessive acid causes indigestion. Milk of Magnesia is a base (Mg(OH)2Mg(OH)_{2}) that neutralizes the excess acid to form salt and water, providing relief.

Acids, Bases, and Natural Indicators Revision - Class 7 Science CBSE