To determine which statement best describes the pH of pure water, let's analyze each of the statements provided.
1. It is neutral because the concentration of hydronium ions equals that of hydroxide ions.
- In pure water, both hydronium ions (H₃O⁺) and hydroxide ions (OH⁻) are present in equal concentrations of [tex]\(1.0 \times 10^{-7} \, \text{M}\)[/tex]. The pH of a solution is defined as the negative base-10 logarithm of the hydronium ion concentration:
[tex]\[
\text{pH} = -\log[H₃O⁺]
\][/tex]
For pure water:
[tex]\[
[H₃O⁺] = 1.0 \times 10^{-7} \, \text{M}
\][/tex]
Therefore:
[tex]\[
\text{pH} = -\log (1.0 \times 10^{-7}) = 7
\][/tex]
This indicates a neutral solution. Hence, the statement is correct as it correctly describes the neutrality and equal concentrations of hydronium and hydroxide ions.
2. It is neutral because the pure liquid contains neither hydronium ions nor hydroxide ions.
- This statement is incorrect. Pure water does contain hydronium ions and hydroxide ions, though in very low but equal concentrations of [tex]\(1.0 \times 10^{-7} \, \text{M}\)[/tex]. The neutrality of water is because of this equal concentration, not because of the absence of these ions.
3. It is acidic because it has a hydronium ion concentration of [tex]\(1.0 \times 10^{-7} \, \text{M}.\)[/tex]
- This statement is misleading. While pure water does have a hydronium ion concentration of [tex]\(1.0 \times 10^{-7} \, \text{M}\)[/tex], this concentration represents a neutral pH of 7. A solution is considered acidic if its pH is less than 7, which corresponds to a hydronium ion concentration greater than [tex]\(1.0 \times 10^{-7} \, \text{M}\)[/tex].
4. It is basic because it has a hydroxide ion concentration of [tex]\(1.0 \times 10^{-7} \, \text{M}.\)[/tex]
- This statement is also misleading. The basicity of a solution is determined when the pH is greater than 7, which means the hydronium ion concentration would be less than [tex]\(1.0 \times 10^{-7} \, \text{M}\)[/tex], and hydroxide ion concentration would be greater than [tex]\(1.0 \times 10^{-7} \, \text{M}\)[/tex]. Since both hydronium and hydroxide ion concentrations are equal in pure water, it remains neutral.
Thus, the best statement to describe the pH of pure water is:
It is neutral because the concentration of hydronium ions equals that of hydroxide ions.
