An aqueous solution of hydrochloric acid has a pH of $2$. If this solution is diluted by adding a large amount of distilled water, will the pH value increase, decrease, or remain the same? What maximum limit will the pH approach upon extreme dilution at $25^{\circ }\text{C}$? Explain why concentrated acid should always be added to water slowly with constant stirring, rather than water to acid.

Step-by-step solution

1. Step 1: Address the effect of dilution on pH. Diluting an acid decreases the concentration of ${\text{H}}^{+}$ ions per unit volume. Since $\text{pH}=-\log [{\text{H}}^{+}]$, a lower ${\text{H}}^{+}$ concentration results in an increased pH value.
2. Step 2: State the limiting value of pH. As dilution approaches infinity, the concentration of hydrogen ions approaches that of pure neutral water ($10^{-7}\text{M}$). Thus, the pH increases but will never exceed $7$; it approaches $7$ as its upper limit.
3. Step 3: Explain the safety precaution for diluting concentrated acid. The mixing of a concentrated acid with water is a highly exothermic process.
4. Step 4: Contrast the two methods. If water is added to concentrated acid, a large amount of heat is generated suddenly in a localized area, which may cause the acidic mixture to splash out and cause chemical burns, or break the glass container. Adding acid slowly to a large volume of water with continuous stirring allows the heat evolved to be absorbed safely by the water.

Concept tested: Dilution of acids and pH scale limits

Published 24 August 2026 · How this solution was prepared and checked

Related questions

• 1 Rahul added dilute hydrochloric acid to solid sodium hydrogen carbonate taken in a test tube. A gas was evolved which turned lime water milky. When the gas was passed through lime water in excess, the milkiness disappeared. Explain the chemical reactions involved by writing balanced chemical equations for both stages.
• 2 Calculate the pH of a 0.10 M solution of sodium hydroxide (NaOH). Give your answer correct to two decimal places.
• 3 A 25.0 mL sample of 0.10 M acetic acid (CH_3COOH) is titrated with 0.10 M NaOH. Calculate the pH of the solution at the half‑equivalence point. (Acetic acid K_a = 1.8×10^-5.) Give your answer correct to two decimal places.
• 4 Plaster of Paris (CaSO_4 · 1/2H_2O) sets into a hard solid mass on mixing with water. Write the balanced chemical equation for this setting reaction. Also, calculate the percentage of water of crystallisation by mass present in hydrated copper sulphate (CuSO_4 · 5H_2O). (Given atomic masses: Cu = 63.5 u, S = 32 u, O = 16 u, H = 1 u)
• 5 Which of the following indicators turns from yellow to red when added to an acidic solution?
• 6 A student tests three oxides X, Y, and Z with litmus paper and records the following observations: (i) Oxide X dissolves in water to turn blue litmus red. (ii) Oxide Y dissolves in water to turn red litmus blue. (iii) Oxide Z reacts with both dilute hydrochloric acid and sodium hydroxide solution to form salt and water. Identify the general nature (acidic, basic, or amphoteric) of oxides X, Y, and Z. Give one example of a chemical formula for each type of oxide.