The compound commonly used in prickly-heat powders to reduce irritation and help prevent excessive sweating is which of the following acids?

Introduction / Context:
Prickly heat powders are applied to the skin in hot and humid climates to relieve heat rash, reduce irritation, and help control excessive sweating. These powders usually contain a base like talc along with mild antiseptic or astringent compounds. Understanding which chemical ingredients are used in common medicinal and cosmetic products is useful in applied chemistry and general awareness. This question asks you to identify which acid is commonly used in such powders to help prevent excessive sweating and irritation.

Given Data / Assumptions:

• The product mentioned is prickly-heat powder.
• The role of the compound is to prevent excessive sweating and soothe the skin.
• The options given are boric acid, phosphoric acid, carbonic acid, and sulphonic acid.
• We assume typical formulations in which boric acid is used as a mild antiseptic and astringent.

Concept / Approach:
Boric acid (H3BO3) is a weak acid used in many topical preparations as a mild antiseptic and astringent. In prickly-heat powders, it helps reduce bacterial growth and has a soothing effect on irritated skin. Its mild astringent properties can also assist in reducing excessive sweating in affected areas. Phosphoric acid is stronger and mainly used in fertilisers and soft drinks; carbonic acid exists only weakly in carbonated beverages; sulphonic acids are strong organic acids used in detergents and industrial applications. These other acids are not typically associated with cosmetic or medicated powders meant for direct application on sensitive skin.

Step-by-Step Solution:
Step 1: Consider the requirements of a prickly-heat powder. It must be safe for direct application to skin, help reduce irritation, and mildly control sweating. Step 2: Evaluate boric acid. Boric acid is widely used as a mild antiseptic and astringent in topical preparations and powders. Step 3: Evaluate the other acids. Phosphoric acid is too strong and corrosive for direct skin application at effective concentrations. Carbonic acid is weak and unstable, mainly relevant in carbonated drinks, not powders. Sulphonic acids are strong organic acids used primarily in detergents; they are not suitable as soothing skin ingredients. Step 4: Conclude that boric acid is the compound used in prickly-heat powder.

Verification / Alternative check:
Labels and ingredient lists of traditional prickly-heat powders frequently show boric acid among the active or supporting ingredients, alongside talc, zinc oxide, and sometimes salicylic acid. Pharmacy textbooks describe boric acid as a mild antiseptic and astringent used in dusting powders and eye washes at low concentrations. In contrast, phosphoric and sulphonic acids are usually discussed in industrial or food chemistry contexts, not dermatology. This consistent use of boric acid in skin powders confirms it as the correct choice for this question.

Why Other Options Are Wrong:
Option B (Phosphoric acid): Primarily used in fertilisers and cola beverages; it is not a standard ingredient in prickly-heat powders. Option C (Carbonic acid): Exists transiently in carbonated water; it is not used in dry powders for skin application. Option D (Sulphonic acid): Strong organic acids derived from sulphuric acid, commonly used in detergents and surfactants, not as soothing skin agents.

Common Pitfalls:
Some students may be tempted to choose phosphoric acid because it sounds familiar or “pharma-like”, or they may confuse boric acid with borax or other boron compounds. To avoid errors, remember that boric acid is the one traditionally associated with mild antiseptic and astringent skin preparations, especially in dusting powders and eye washes. Its gentle nature at low concentrations makes it suitable for sensitive skin, unlike the stronger mineral or organic acids listed in the other options.

Final Answer:
The compound used in prickly-heat powders to help prevent excessive sweating is Boric acid.