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Acids and Bases - Indicators

Grade 9IB

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

🔑Concepts

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Indicators are substances (usually weak acids or bases) that change color in response to changes in the concentration of hydrogen ions (H+H^+) or hydroxide ions (OH−OH^-) in a solution.

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The pHpH scale is a logarithmic scale ranging from 00 to 1414 used to specify the acidity or basicity of an aqueous solution.

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At 25∘C25^\circ C, a solution with pH<7pH < 7 is acidic, pH>7pH > 7 is basic (alkaline), and pH=7pH = 7 is neutral.

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Litmus paper is a common indicator: Red litmus turns blue in bases, while blue litmus turns red in acids.

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Phenolphthalein is an indicator that remains colorless in acidic solutions (pH<8.3pH < 8.3) and turns pink/magenta in basic solutions (pH>10.0pH > 10.0).

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Methyl orange appears red in acidic conditions (pH<3.1pH < 3.1) and yellow in basic conditions (pH>4.4pH > 4.4).

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Universal Indicator is a mixture of several indicators that exhibits a smooth color change over a wide pHpH range, typically from red (strong acid) to purple (strong base).

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Natural indicators, such as red cabbage juice, contain anthocyanins that change color based on the acidity of the environment.

📐Formulae

pH=−log⁡10[H+]pH = -\log_{10}[H^+]

[H+][OH−]=1.0×10−14 at 25∘C[H^+][OH^-] = 1.0 \times 10^{-14} \text{ at } 25^\circ C

HIn(aq)⇌H+(aq)+In−(aq)HIn(aq) \rightleftharpoons H^+(aq) + In^-(aq) (General indicator dissociation)

💡Examples

Problem 1:

A student tests a clear solution with a few drops of phenolphthalein and the solution turns a deep pink color. What can be inferred about the [H+][H^+] concentration and the nature of the solution?

Solution:

The solution is basic (pH>8.3pH > 8.3) and has a lower [H+][H^+] concentration compared to neutral water.

Explanation:

Phenolphthalein changes from colorless to pink in the presence of excess OH−OH^- ions. This indicates a basic solution where [OH−]>[H+][OH^-] > [H^+].

Problem 2:

Given a solution of HClHCl with a concentration of 0.01 M0.01\,M, calculate the pHpH and predict the color when Universal Indicator is added.

Solution:

pH=2pH = 2; The indicator will turn red.

Explanation:

Since HClHCl is a strong acid, [H+]=0.01 M=10−2 M[H^+] = 0.01\,M = 10^{-2}\,M. Using pH=−log⁡10(10−2)pH = -\log_{10}(10^{-2}), we get pH=2pH = 2. Universal Indicator shows red for strongly acidic solutions (pHpH 0−20-2).

Problem 3:

Explain the equilibrium shift in the indicator HIn⇌H++In−HIn \rightleftharpoons H^+ + In^- when NaOHNaOH is added.

Solution:

The equilibrium shifts to the right (towards In−In^-).

Explanation:

Adding NaOHNaOH provides OH−OH^- ions which react with H+H^+ to form H2OH_2O. This removal of H+H^+ (a product) causes the equilibrium to shift to the right according to Le Chatelier's Principle, increasing the concentration of the conjugate base form (In−In^-), which has a different color than HInHIn.