Understanding pH and Substance Properties

The pH scale is a measure used to specify the acidity or basicity of an aqueous solution. It ranges typically from 0 to 14. Solutions with a pH less than 7 are considered acidic, a pH of 7 is neutral (like pure water), and solutions with a pH greater than 7 are considered basic or alkaline. The pH value is inversely related to the concentration of hydrogen ions ($H^+$) in the solution.

A pH value of about 14 indicates a very strong basic or alkaline solution, meaning it has a very low concentration of $H^+$ ions and a very high concentration of hydroxide ions ($OH^-$).

Analyzing the Given Substances and Their Typical pH Values

Let's examine the typical pH range for each substance provided in the options:

• Blood: Human blood has a tightly regulated pH, typically ranging from 7.35 to 7.45. This is slightly alkaline.
• Sodium hydroxide: Sodium hydroxide ($NaOH$) is a very strong base. Solutions of sodium hydroxide, especially at higher concentrations, can have a very high pH, approaching 14. For example, a 1 M solution of NaOH has a pH of 14.
• Milk of magnesia: Milk of magnesia is a suspension of magnesium hydroxide ($Mg(OH)_2$). Magnesium hydroxide is a weak base. Its pH is typically around 10 to 11, making it alkaline but not as strongly basic as concentrated sodium hydroxide.
• Lemon Juice: Lemon juice is highly acidic due to the presence of citric acid. Its pH is typically around 2 to 3.

Comparing pH Values

We can summarize the approximate pH ranges of the substances:

Determining the Substance with pH About 14

Based on the typical pH values, sodium hydroxide is the substance among the options that can have a pH value of about 14, especially when in a concentrated solution. Blood is slightly alkaline, milk of magnesia is a weaker base with a lower alkaline pH, and lemon juice is acidic.

Revision Table: pH Scale and Substances

Additional Information on Strong Bases

Strong bases like sodium hydroxide ($NaOH$) completely dissociate in water, releasing a high concentration of hydroxide ions ($OH^-$). This high concentration of $OH^-$ ions leads to a very low concentration of $H^+$ ions, resulting in a high pH value, potentially reaching 14 for a 1 molar solution.

Other examples of strong bases include potassium hydroxide ($KOH$) and barium hydroxide ($Ba(OH)_2$).

Weak bases, like magnesium hydroxide ($Mg(OH)_2$) in milk of magnesia, do not completely dissociate in water, resulting in a lower concentration of $OH^-$ ions compared to strong bases of similar concentration, and thus a lower pH.