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Chemistry: Properties of Matter - Acids, Bases, and the pH Scale

Grade 8IB

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

🔑Concepts

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Acids are substances that release hydrogen ions (H+H^+) when dissolved in water. They typically have a sour taste and a pHpH value less than 77.

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Bases are substances that can neutralize acids by accepting H+H^+ ions or releasing hydroxide ions (OH−OH^-) in aqueous solutions (alkalis). They have a pHpH value greater than 77.

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The pHpH scale ranges from 00 to 1414, measuring the concentration of H+H^+ ions. A pHpH of 77 is considered neutral, such as pure water (H2OH_2O).

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Indicators are chemical substances (like Litmus, Phenolphthalein, or Universal Indicator) that change color based on the acidity or alkalinity of a solution.

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Neutralization is a chemical reaction where an acid and a base react to form a salt and water, effectively moving the pHpH closer to 77.

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Strong acids like hydrochloric acid (HClHCl) dissociate completely in water, while weak acids like ethanoic acid (CH3COOHCH_3COOH) only partially dissociate.

📐Formulae

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

HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)HCl(aq) + NaOH(aq) \rightarrow NaCl(aq) + H_2O(l) (Neutralization of Hydrochloric Acid and Sodium Hydroxide)

H+(aq)+OH−(aq)→H2O(l)H^+(aq) + OH^-(aq) \rightarrow H_2O(l) (Net ionic equation for neutralization)

2HCl+Mg→MgCl2+H22HCl + Mg \rightarrow MgCl_2 + H_2 (Reaction of acid with metal producing Hydrogen gas)

💡Examples

Problem 1:

Identify the salt produced when sulfuric acid (H2SO4H_2SO_4) reacts with potassium hydroxide (KOHKOH) and write the balanced equation.

Solution:

H2SO4+2KOH→K2SO4+2H2OH_2SO_4 + 2KOH \rightarrow K_2SO_4 + 2H_2O

Explanation:

In this neutralization reaction, the potassium ion (K+K^+) from the base combines with the sulfate ion (SO42−SO_4^{2-}) from the acid to form the salt Potassium Sulfate (K2SO4K_2SO_4).

Problem 2:

What color would Universal Indicator turn in a solution of sodium hydroxide (NaOHNaOH) with a pHpH of 1313?

Solution:

Purple or Dark Violet.

Explanation:

Universal Indicator follows the pHpH scale color spectrum (ROYGBIV). A pHpH of 1313 indicates a very strong base, which corresponds to the purple/violet end of the spectrum.

Problem 3:

If a solution has a hydrogen ion concentration that increases by a factor of 1010, how does the pHpH change?

Solution:

The pHpH decreases by 11 unit.

Explanation:

The pHpH scale is logarithmic. Because pH=−log⁡[H+]pH = -\log[H^+], every 1010-fold increase in H+H^+ concentration results in a decrease of 11 on the pHpH scale, making the solution more acidic.