Which one of the following gives the highest amount of hydrogen ions (

Which one of the following gives the highest amount of hydrogen ions (H⁺) ?

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Sodium hydroxide solution

Milk of magnesia

Lemon juice

Gastric juice

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Answer is Wrong!

This question was previously asked in

UPSC NDA-1 – 2018

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The correct answer is D) Gastric juice.

The amount of hydrogen ions (H⁺) in a solution determines its acidity, measured by pH. A lower pH indicates a higher concentration of H⁺ ions and thus higher acidity.
A) Sodium hydroxide solution (NaOH) is a strong base, having a very high pH and thus a very low concentration of H⁺ ions.
B) Milk of magnesia (Mg(OH)₂) is a weak base, having a high pH but lower than NaOH, still a very low concentration of H⁺ ions compared to acids.
C) Lemon juice contains citric acid and has a pH typically between 2.0 and 2.5, indicating a significant concentration of H⁺ ions.
D) Gastric juice in the stomach contains hydrochloric acid (HCl) and has a very low pH, typically ranging from 1.5 to 3.5. This extremely low pH indicates a very high concentration of H⁺ ions, necessary for digesting proteins and killing ingested microorganisms. Comparing the typical pH values, gastric juice is the most acidic among the options, meaning it has the highest concentration of hydrogen ions.

pH is defined as the negative logarithm (base 10) of the hydrogen ion concentration: pH = -log₁₀[H⁺]. Therefore, a decrease of 1 unit in pH corresponds to a tenfold increase in H⁺ concentration. Gastric juice’s low pH (e.g., 2) means a much higher H⁺ concentration than lemon juice (e.g., 2.5).

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