Problem 116 - Chemistry and Chemical Reactivity

Question

You purchase a bottle of water. On checking its \(\mathrm{pH}\), you find that it is not neutral, as you might have expected. Instead, it is slightly acidic. Why?

Step-by-Step Solution

Answer

The water is slightly acidic due to dissolved carbon dioxide forming carbonic acid or minerals/salts.

Step 1: Understand the Concept of Neutral pH

The pH scale ranges from 0 to 14 where a pH of 7 is neutral. This means the concentration of hydrogen ions (H⁺) and hydroxide ions (OH⁻) are equal.

Step 2: Recognize the Nature of Slight Acidity

Since the pH is slightly less than 7, the water has a slightly higher concentration of hydrogen ions than hydroxide ions, making it slightly acidic.

Step 3: Identify Common Causes of Acidity in Bottled Water

Carbon dioxide from the air can dissolve in water, forming carbonic acid which lowers the pH slightly. Additionally, salts or minerals present naturally in water can contribute to slight acidity.

Key Concepts

Neutral pH

The pH scale is a numeric scale used to specify the acidity or basicity of an aqueous solution. It ranges from 0 to 14. A pH value of 7 is considered neutral. This is because at this point, the concentration of hydrogen ions (\(\text{H}^+\)) in the solution is equal to the concentration of hydroxide ions (\(\text{OH}^-\)). In pure water at 25°C, \(\text{H}^+\) and \(\text{OH}^-\) ions are present in equal concentrations, which is why water is ideally neutral.

The neutrality here means that the solution is neither acidic nor basic. It's a perfectly balanced state. However, once other substances are added, this balance can shift, causing the solution to become acidic or basic. In the real world, many factors can influence this balance, such as temperature changes or the addition of environmental CO2.

Acidic Water

Acidic water means the water has more hydrogen ions (\(\text{H}^+\)) than hydroxide ions (\(\text{OH}^-\)). When the pH is less than 7, it indicates acidity. The more it decreases below 7, the more acidic the solution is.

This lower pH can be caused by various factors. For example, when carbon dioxide (\(\text{CO}_2\)) from the air dissolves in water, it forms carbonic acid (\(\text{H}_2\text{CO}_3\)). This weak acid can make water slightly acidic. Additionally, other substances such as salts or minerals that are naturally present in water can also contribute to making it more acidic.

• CO2 exposure: Dissolved carbon dioxide forms carbonic acid.
• Mineral content: Certain salts and minerals can lower pH.

Understanding why water might be acidic helps in knowing how various environmental factors and contaminant levels affect water quality.

Hydrogen Ions

Hydrogen ions (\(\text{H}^+\)) are essential in determining the acidity of a solution. Their concentration is what actually dictates the pH level. When there's an increase in hydrogen ions, the pH decreases, signaling an acidic environment.

• In water, hydrogen ions and hydroxide ions are in equilibrium at a neutral pH.
• Any changes in their concentration disrupt the neutrality, leading to either an acidic or basic environment.

For example, carbon dioxide dissolving in water increases the \(\text{H}^+\) concentration, turning it acidic. Hence, monitoring hydrogen ions is crucial for maintaining water quality and understanding its chemical properties.

Such knowledge is especially valuable for processes involving water purification, chemical reactions, and biochemistry, where pH plays a critical role.