Question

What happens to the \(\mathrm{pH}\) of an acidic solution as pure water is added?

Short answer

pH increases as pure water is added, making the solution less acidic.

Step-by-step solution

Understand the Concept of pH

The pH scale measures how acidic or basic a solution is. The pH is derived from the hydrogen ion concentration \([H^+]\), where a low pH indicates high acidity and a high pH indicates low acidity. Pure water has a neutral pH of 7.

Consider the Effect of Dilution

When pure water is added to an acidic solution, the concentration of hydrogen ions \([H^+]\) decreases because pure water does not contribute any additional hydrogen ions. This dilution effect lowers the concentration of hydrogen ions in the solution.

Apply the Dilution to pH Calculation

The formula for calculating pH is \( ext{pH} = - ext{log}_{10}[H^+] \). As \([H^+]\) decreases due to the dilution with water, the logarithmic function means that the pH value will increase, making the solution less acidic.

Conclude with the Change in pH

Since adding water decreases the acidity level and raises the pH, the solution becomes closer to neutral. The pH moves towards 7 as more water is added.

Key concepts

pH scale

The pH scale is a numerical value used to specify the acidity or basicity of a solution. It ranges from 0 to 14, with 7 being neutral.
Solutions with a pH less than 7 are acidic, while those with a pH greater than 7 are basic (alkaline). This scale is logarithmic, calculated using the expression \( \text{pH} = -\log_{10}[H^+] \), where \([H^+]\) represents the hydrogen ion concentration.
In simpler terms, each unit change in pH corresponds to a tenfold change in the hydrogen ion concentration. For example, a solution with a pH of 5 is ten times more acidic than one with a pH of 6. Understanding this scale helps analyze the acidity or basicity of everyday solutions, like lemon juice or soap.

dilution effect

The dilution effect explains how the concentration of solutes in a solution reduces when more solvent is added. In the context of pH, when pure water is added to an acidic solution, it's a classic example of dilution.
Pure water doesn't add any hydrogen ions, so it decreases the overall concentration of hydrogen ions present. This effect leads to a reduction in the acidity of the solution. The formula for calculating the change in hydrogen ion concentration due to dilution is:

• Initial Concentration: \( [H^+]_0 \)
• Final Concentration after dilution: \( [H^+]_f \)
• Volume change is key: more volume usually means lower concentration.

The more water you add, the lower the concentration of hydrogen ions, and thus the less acidic the solution becomes. This in turn impacts the pH, pushing it towards neutral, around a pH of 7.

hydrogen ion concentration

Hydrogen ion concentration \([H^+]\) is a critical measure of a solution's acidity. It directly determines the pH of the solution.
The higher the hydrogen ion concentration, the lower the pH, making the solution more acidic. Conversely, a lower concentration of hydrogen ions equates to a higher pH, indicating a less acidic or more basic solution.
The relationship is expressed mathematically as \( \text{pH} = -\log_{10}[H^+] \). This means that slight changes in hydrogen ion concentration cause significant changes in pH because of the logarithmic nature of the pH scale.
Understanding how hydrogen ion concentration shifts are crucial in contexts like buffering solutions, where maintaining a stable pH is necessary for scientific experiments and biological applications.