Understanding the pH of Multiple-Distilled Water

The question asks about the pH value of a sample of multiple-distilled water. To answer this, we need to understand what pH represents and what happens during water distillation.

What is pH?

pH is a measure of the acidity or basicity of an aqueous solution. It is defined as the negative logarithm (base 10) of the concentration of hydrogen ions (H⁺) in moles per litre.

\( \text{pH} = -\log_{10}[\text{H}^{+}] \)

In pure water, there is a natural process called autoionization where water molecules dissociate into hydrogen ions (H⁺) and hydroxide ions (OH⁻):

\( \text{H}_2\text{O} \rightleftharpoons \text{H}^{+} + \text{OH}^{-} \)

At 25°C, the concentration of H⁺ ions and OH⁻ ions in pure water is equal, typically \( 1.0 \times 10^{-7} \) M (moles per litre). Since the concentrations are equal, pure water is considered neutral.

Using the pH formula for pure water at 25°C:

\( \text{pH} = -\log_{10}[1.0 \times 10^{-7}] \)

\( \text{pH} = -(-7) \)

\( \text{pH} = 7 \)

So, the pH of pure water at 25°C is 7.

What is Multiple-Distilled Water?

Distillation is a process used to purify water by separating it from dissolved salts and other impurities. Water is heated to boiling, the steam is collected and then condensed back into liquid water. Multiple distillation means this process is repeated several times. This rigorous purification process aims to remove almost all dissolved substances, including minerals, salts, organic matter, and dissolved gases like carbon dioxide, which can affect the water's pH.

Highly purified water, like multiple-distilled water, is very close to theoretical pure \( \text{H}_2\text{O} \). Therefore, its pH value should be determined by the autoionization of water itself.

pH of Highly Purified Water

Since multiple-distilled water is intended to be extremely pure, it should ideally behave like theoretical pure water. In perfectly pure water, the concentrations of H⁺ and OH⁻ ions from autoionization are equal, resulting in a neutral pH of 7 (at 25°C).

While it's difficult to achieve absolutely perfect purity and slight variations can occur due to factors like temperature or minimal absorption of gases from the atmosphere (though multiple distillation minimises this), the pH of multiple-distilled water will be very close to 7. It will not be acidic (pH < 7) or basic (pH > 7) unless contaminated.

Analyzing the Options

Let's look at the given options:

• Zero: A pH of zero represents a very strong acidic solution (\([\text{H}^{+}] = 1\) M). This is not possible for water.
• 14: A pH of 14 represents a very strong basic solution (\([\text{OH}^{-}] = 1\) M, which means \([\text{H}^{+}] = 10^{-14}\) M). This is not possible for water.
• Very near to zero: This suggests a very strong acid, which is incorrect for purified water.
• Very near to seven: This aligns with the properties of pure or highly purified water, where the concentration of H⁺ and OH⁻ ions are virtually equal, leading to a neutral pH.

Therefore, the pH value of a sample of multiple-distilled water is expected to be very close to seven.

Revision Table: pH and Water Purity

Additional Information: Factors Affecting Water pH

While multiple-distilled water is very pure, even small amounts of dissolved substances can influence pH. For example:

• Dissolved Gases: Carbon dioxide from the air can dissolve in water to form carbonic acid (\(\text{H}_2\text{CO}_3\)), which is acidic. \( \text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \). This can slightly lower the pH below 7. Multiple distillation removes dissolved gases, helping maintain a pH closer to 7.
• Dissolved Minerals/Salts: Impurities like dissolved salts can affect the balance of H⁺ and OH⁻ ions, changing the pH. Distillation removes these.
• Temperature: The autoionization of water is temperature-dependent. At temperatures other than 25°C, the concentration of H⁺ and OH⁻ in neutral water changes, and thus the neutral pH value shifts slightly (e.g., neutral pH is lower than 7 at higher temperatures, higher than 7 at lower temperatures). However, for standard reference, pH 7 is considered neutral.

For highly purified water like multiple-distilled water, these factors are minimized, making its pH value very close to the theoretical value for pure water at a standard temperature, which is 7.