The pH of pure water at room temperature is nearly 7. For this pH value to increase above 7, which of the following substances must be added to the water?

Introduction / Context:
The pH scale is used to describe how acidic or basic an aqueous solution is. Pure water at room temperature has a pH close to 7 and is considered neutral. Adding acids lowers the pH, while adding bases raises it. Some dissolved substances, however, are neutral and do not significantly change pH. This question asks which listed substance, when added to pure water, will increase the pH above 7, meaning the solution becomes basic or alkaline.

Given Data / Assumptions:

• Pure water has pH approximately equal to 7 at room temperature.
• We are adding small amounts of various substances and observing the effect on pH.
• The options include ammonia, cane sugar, common salt, hydrochloric acid and glucose.
• We assume dilute aqueous solutions, not extreme concentrations or temperatures.

Concept / Approach:
A base in water produces hydroxide ions or accepts protons, leading to an increase in pH above 7. Ammonia in water acts as a weak base, reacting with water to form ammonium ions and hydroxide ions. Neutral substances such as cane sugar and glucose dissolve without ionising significantly and therefore do not change pH much. Common salt, sodium chloride, is the salt of a strong acid and strong base and produces a nearly neutral solution. Hydrochloric acid is a strong acid and lowers pH when added to water. The correct approach is to identify which substance behaves as a base in aqueous solution and therefore raises the pH.

Step-by-Step Solution:
Step 1: Recognise that ammonia (NH3) in water acts as a weak base, reacting according to NH3 + H2O ⇌ NH4⁺ + OH⁻, which increases hydroxide ion concentration. Step 2: Increased hydroxide ion concentration means a basic solution, so the pH rises above 7. Step 3: Cane sugar is a neutral carbohydrate that dissolves in water but does not release acids or bases; its solution remains close to pH 7. Step 4: Common salt, NaCl, is formed from strong acid HCl and strong base NaOH; its aqueous solution is also nearly neutral, with pH close to 7. Step 5: Hydrochloric acid is a strong acid and will increase the hydrogen ion concentration in water, lowering pH below 7. Step 6: Therefore, only ammonia clearly raises the pH of water above 7 by acting as a base.

Verification / Alternative check:
Using a pH meter or universal indicator in simple laboratory experiments confirms these effects. A solution of aqueous ammonia gives a pH between 9 and 11 depending on concentration, indicating basic character. Solutions of cane sugar or glucose show pH values close to 7, indicating neutrality. A sodium chloride solution also measures close to pH 7 because the ions do not hydrolyse significantly. A dilute hydrochloric acid solution gives a pH well below 7, often around 1 to 3. These measurements support the conclusion that ammonia is the only option that increases the pH above 7.

Why Other Options Are Wrong:
Option B, cane sugar, is neutral and does not appreciably change pH. Option C, common salt, is the salt of a strong acid and strong base and gives a nearly neutral solution, not a basic one. Option D, hydrochloric acid, is a strong acid that decreases pH rather than increasing it. Option E, glucose, behaves similarly to cane sugar; it dissolves but does not ionise to produce hydroxide ions and does not increase pH. Only option A, aqueous ammonia, acts as a weak base and raises the pH of pure water above 7.

Common Pitfalls:
A common confusion is to assume that any dissolved substance, especially salts, must change pH significantly. In reality, only substances that produce or consume hydrogen or hydroxide ions will strongly affect pH. Another pitfall is to forget that weak bases like ammonia can still raise pH even though they do not dissociate completely. Remember that salts of strong acids and strong bases tend to be neutral, while salts involving weak acids or weak bases can produce acidic or basic solutions. For this question, focusing on the behaviour of ammonia in water quickly leads to the correct answer.

Final Answer:
The pH of pure water increases above 7 when Aqueous ammonia solution, which acts as a weak base is added, because it produces hydroxide ions in solution.