To determine the best description of the pH of pure water, we need to analyze the relationship between hydronium ions (H3O+) and hydroxide ions (OH-) in water.

Solution By Steps

Step 1: Understanding pH and Ion Concentrations

The pH of a solution is a measure of the concentration of hydronium ions (H3O+) in the solution. A pH of 7 is considered neutral, which means the concentration of hydronium ions equals the concentration of hydroxide ions.

Step 2: Analyzing the Ion Concentrations in Pure Water

In pure water, at 25°C, the concentration of hydronium ions (H3O+) and hydroxide ions (OH-) is each

1.0 imes 10^{-7} \mathrm{M}. This is due to the autoionization of water, where a small fraction of water molecules dissociate into H3O+ and OH-.

Step 3: Evaluating the Statements

The first statement correctly states that pure water is neutral because the concentration of hydronium ions equals that of hydroxide ions.

The second statement is incorrect because pure water does contain both hydronium and hydroxide ions, albeit in very low concentrations.

The third statement is incorrect because a hydronium ion concentration of

1.0 imes 10^{-7} \mathrm{M} indicates neutrality, not acidity.

The fourth statement is incorrect because “balsic” is not a recognized term in chemistry, and the hydroxide ion concentration of

1.0 imes 10^{-7} \mathrm{M} also indicates neutrality.

Final Answer

The best statement that describes the pH of pure water is: “It is neutral because the concentration of hydronium ions equals that of hydroxide ions.”