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Chapter 1: Problem 34

A bottle of water is left outside early in the morning. The bottle warms gradually over the course of the day. What will happen to the pH of the water as the bottle warms? (A) Nothing; pure water always has a pH of 7.00. (B) Nothing; the volume would have to change in order for any ion concentration to change. (C) It will increase because the concentration of \(\left[\mathrm{H}^{+}\right]\) is increasing. (D) It will decrease because the auto-ionization of water is an endothermic process.

Short Answer

Expert verified
The pH of the water in the bottle will decrease as it warms because the auto-ionization of water is an endothermic process. Hence, the correct answer is (D)

Step by step solution

01

Understand the Concept of pH

pH is a measure of hydrogen ion concentration, \(\left[\mathrm{H}^{+}\right]\), a measure of the acidity or alkalinity of a solution. Water by nature undergoes auto ionization or self-ionization where one water molecule donates a proton (H⁺) to another one, becoming \(\mathrm{OH}^-\), and the other molecule becomes \(\mathrm{H}_3\mathrm{O}^+\), creating equal amount of these ions.
02

Identify the Impact of Temperature on pH

Increase in temperature can cause an increase in \(\left[\mathrm{H}^{+}\right]\) and \(\left[\mathrm{OH}^-\right]\) due to the increase in auto-ionization of water. Since pH is the negative logarithm of hydrogen ion concentration, the pH of pure water decreases as the temperature increases because the increase in \(\left[\mathrm{H}^{+}\right]\) concentration results in an increased acidity and therefore lower pH.
03

Identify the Correct Answer

Looking at the options, the one reflecting our understanding is (D) It will decrease because the auto-ionization of water is an endothermic process. Endothermic because the reaction absorbs heat energy during the process.

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