In acid–base chemistry, which of the following substances has a pH value of 7 and is considered neutral under standard conditions?

Introduction / Context:
The pH scale is a fundamental concept in chemistry used to describe how acidic or basic a solution is. A pH value of 7 at 25 degrees Celsius is defined as neutral, values less than 7 are acidic, and values greater than 7 are basic or alkaline. Many exam questions test whether students can quickly identify common substances as acidic, basic, or neutral. This question focuses on which of the listed substances typically has a pH of 7 under standard conditions.

Given Data / Assumptions:

• We are using the standard pH scale at around 25 degrees Celsius.
• A neutral solution has pH 7, acidic solutions have pH below 7, and basic solutions have pH above 7.
• The substances listed include pure water, sulfuric acid, sodium carbonate solution, and hydrochloric acid solution.
• We assume typical laboratory concentrations where sulfuric acid and hydrochloric acid are clearly acidic and sodium carbonate is basic.

Concept / Approach:
In pure water at 25 degrees Celsius, the concentrations of hydrogen ions and hydroxide ions are equal, each being 1.0 * 10^-7 mol per litre. This balance defines a neutral solution with pH 7. Strong acids such as sulfuric acid and hydrochloric acid, when dissolved, produce high concentrations of hydrogen ions and thus have pH values well below 7. Sodium carbonate is a basic salt that hydrolyses in water to produce carbonate ions, which can accept hydrogen ions and increase hydroxide ion concentration, making the solution basic with pH above 7. Therefore, pure water is the only substance listed that is neutral with pH 7 under standard conditions.

Step-by-Step Solution:
Step 1: Recall that a pH of 7 at 25 degrees Celsius corresponds to a neutral solution where hydrogen and hydroxide ions are balanced. Step 2: Identify pure water as the standard reference for a neutral solution with equal concentrations of hydrogen and hydroxide ions. Step 3: Recognise that sulfuric acid (H2SO4) is a strong acid and, in aqueous solution, produces a large excess of hydrogen ions, lowering the pH well below 7. Step 4: Note that hydrochloric acid (HCl) is also a strong acid and similarly generates high hydrogen ion concentration, resulting in pH values less than 7. Step 5: Remember that sodium carbonate (Na2CO3) forms basic solutions because carbonate ions react with water to form bicarbonate and hydroxide ions, raising the pH above 7. Step 6: Conclude that only pure water at 25 degrees Celsius has a pH of 7 and is neutral, so it is the correct answer.

Verification / Alternative check:
Standard chemistry references define the pH of pure water at 25 degrees Celsius as exactly 7 based on the ionic product of water. They also classify sulfuric acid and hydrochloric acid as strong acids that fully ionise in water, giving solutions with pH well below 7, even at modest concentrations. Sodium carbonate is grouped with basic salts because it raises the pH of water due to hydrolysis. Laboratory experiments with pH indicators or pH meters confirm that pure water does not turn indicators either strongly red or blue, indicating neutrality, whereas acid and base solutions do. These observations support pure water as the neutral example.

Why Other Options Are Wrong:
Sulfuric acid solution is strongly acidic and therefore cannot have a neutral pH of 7. Hydrochloric acid solution is another strong acid and also produces pH values below 7. Sodium carbonate solution is basic, not neutral, because its carbonate ions generate hydroxide ions in water, leading to pH values above 7. These substances do not match the definition of a neutral solution and therefore cannot be correct answers to the question as stated.

Common Pitfalls:
Students sometimes confuse pure water with natural water from taps, rivers, or lakes, which may contain dissolved salts and gases that slightly shift the pH. Another pitfall is assuming that all clear, colourless liquids are neutral, which is not true for strong acids and bases. To avoid these mistakes, remember that in theory and in carefully prepared laboratory conditions, pure water at 25 degrees Celsius is the standard neutral reference with pH 7. Recognising this helps quickly identify neutral solutions in exam questions.

Final Answer:
The substance that has a pH value of 7 and is neutral under standard conditions is pure water at 25 degrees Celsius.