Bar soap does not melt in the conventional sense, like ice. Instead, when exposed to sun and heat, soap undergoes physical and chemical changes that alter its texture, appearance, and quality. These changes are distinct from a true phase transition but can lead to a softened, degraded product.
Understanding Soap’s Composition
Soap is a salt of a fatty acid, created through saponification. This chemical reaction combines fats or oils, like triglycerides, with a strong alkali such as sodium hydroxide for hard bar soaps or potassium hydroxide for softer soaps. The reaction breaks down fats and oils into soap molecules and glycerol. The resulting soap is solid at room temperature due to its molecular structure, where long-chain fatty acid salts associate. This composition gives bar soap a relatively high melting point, much higher than typical ambient or hot outdoor temperatures.
The Effects of Sun and Heat on Soap
Direct sunlight and elevated temperatures cause several alterations to bar soap. One common effect is softening, as heat weakens the interactions between long-chain fatty acids. Soaps with higher plant-based oil content can become more fluid under heat. Prolonged heat exposure can also lead to the soap drying out and cracking as moisture evaporates.
Beyond physical changes, sunlight’s ultraviolet (UV) radiation degrades the soap’s fragrance and color. UV light breaks down volatile compounds responsible for scent and causes dyes and natural colorants to fade over time. This can result in discoloration, such as white soaps turning yellowish. Additionally, some soaps may develop a sticky or slimy residue due to moisture accumulation or chemical structure changes, exacerbated by humidity and heat.
Proper Soap Storage
Proper storage preserves the quality and extends the life of bar soap. Keeping soap in a cool, dry place away from direct sunlight minimizes softening and degradation. High temperatures and sun exposure accelerate moisture evaporation and ingredient breakdown. Using a soap dish with proper drainage prevents the soap from sitting in water, which can make it mushy and dissolve faster.
Ensuring good air circulation allows the soap to dry completely between uses, maintaining firmness. For unused bars, storing them in an airtight container or original packaging protects them from humidity and environmental factors that cause scent and color loss. These measures maintain the soap’s integrity and effectiveness.