Acids, Bases and Indicators

Introduction

In chemistry, we often encounter substances that are classified as acids or bases. Acids are substances that release hydrogen ions ($H^+$) when dissolved in water, while bases are substances that release hydroxide ions ($OH^-$) in water. Indicators are substances that change color in the presence of acids or bases, helping us to identify the nature of a solution.

Acids

Acids are substances that have a pH value less than 7. They are sour in taste and can conduct electricity when dissolved in water. Examples of common acids include hydrochloric acid ($HCl$) and sulfuric acid ($H2SO4$).

Key terms:

• Hydrogen ions ($H^+$): Positively charged ions released by acids in water.
• Concentration: The amount of acid in a given volume of solution.
• pH: A measure of the acidity or basicity of a solution.

Example: Calculate the concentration of a solution that contains 0.1 moles of hydrochloric acid in 250 cm$^3$ of solution.

Solution:
Given moles of acid = 0.1 moles
Volume of solution = 250 cm$^3$ = 0.25 L
Concentration = $\frac{moles}{volume}$
Concentration = $\frac{0.1}{0.25}$ = 0.4 mol/L

Bases

Bases are substances that have a pH value greater than 7. They are bitter in taste and feel slippery to the touch. Bases can also conduct electricity when dissolved in water. Common examples of bases are sodium hydroxide ($NaOH$) and ammonia ($NH3$).

Key terms:

• Hydroxide ions ($OH^-$): Negatively charged ions released by bases in water.
• Alkalinity: The ability of a base to neutralize an acid.
• Neutralization: The reaction between an acid and a base to form water and a salt.

Example: Determine the concentration of a solution that contains 0.2 moles of sodium hydroxide in 500 cm$^3$ of solution.

Solution:
Given moles of base = 0.2 moles
Volume of solution = 500 cm$^3$ = 0.5 L
Concentration = $\frac{moles}{volume}$
Concentration = $\frac{0.2}{0.5}$ = 0.4 mol/L

Indicators

Indicators are substances that change color in acidic or basic solutions. They help in determining the pH of a solution by the color change they exhibit. Examples of indicators include litmus paper, phenolphthalein, and universal indicator.

Key terms:

• pH range: The range of pH values over which an indicator changes color.
• Color change: The specific color change exhibited by an indicator in the presence of acids or bases.

Common Mistakes

1. Confusing acids with bases: Remember that acids release hydrogen ions while bases release hydroxide ions.
2. Incorrect dilution calculations: Pay careful attention to units when calculating concentrations of solutions.
3. Misinterpreting indicator colors: Make sure to match the color change of the indicator with the correct pH range.

Key Points

• Acids release hydrogen ions ($H^+$) and have a pH less than 7.
• Bases release hydroxide ions ($OH^-$) and have a pH greater than 7.
• Indicators change color in the presence of acids or bases, helping to determine the pH of a solution.

Practice Questions

1. Calculate the concentration of a solution containing 0.3 moles of sulfuric acid in 750 cm$^3$ of solution.

   Answer:
   Given moles of acid = 0.3 moles
   Volume of solution = 750 cm$^3$ = 0.75 L
   Concentration = $\frac{moles}{volume}$
   Concentration = $\frac{0.3}{0.75}$ = 0.4 mol/L

2. Identify the indicator that changes color in the pH range of 8-10.

   Answer:
   Phenolphthalein is an indicator that changes color in the pH range of 8-10.

3. Explain the difference between acids and bases in terms of the ions they release when dissolved in water.

   Answer:
   Acids release hydrogen ions ($H^+$) in water, while bases release hydroxide ions ($OH^-$).

4. What is the pH of a solution that turns red litmus paper blue?

   Answer:
   The solution is basic with a pH greater than 7.

5. Describe the process of neutralization between an acid and a base.

   Answer:
   Neutralization is the reaction between an acid and a base to form water and a salt.