A. First, what is the rate of evaporation? It is “the speed at which a scent spreads through a space.” The lower the boiling point, the greater the volatility and the faster the evaporation rate.
What, then, does it mean for a scent to spread through a space? A scent is an invisible collection of molecules. Hundreds of different types of molecules make up the smell of a rose; when they stimulate our sense of smell, we recognize the scent as rose. In other words, “a scent spreading through a space means scent molecules dispersing through that space.” Scent molecules move and disperse more readily as temperature rises. Fragrance materials have different boiling points depending on the molecules they contain; think of this as the speed at which their engines start.
Applying perfume to the skin means warming its materials with body heat. Thus, the materials whose engines start quickly swiftly disappear, while those whose engines are slow to start keep lingering.
Molecules are actually in constant motion, flying about. Molecular motion stops completely at or below absolute zero (−273.15°C). In other words, the molecules of all the fragrance materials begin escaping from the skin simultaneously the moment you apply them. However, because their boiling points differ, some scents are clearly perceptible while others are difficult to recognize at first.
It is therefore more accurate to say, “They give off scent from the beginning, but become easier to recognize as time passes,” rather than, “The slower-evaporating ones emerge later.”
For example, in a perfume composed of grapefruit, rose (in Japanese), and sandalwood (in Japanese), these scents do not wait their turn, with grapefruit smelling first, followed by rose and then sandalwood. All the scents are present from the beginning, but grapefruit, which evaporates quickly, smells strongest at first. Picture its molecules moving quickly, with many escaping from the skin. After a certain amount of time, the grapefruit scent molecules disappear from the skin. Rose, with the next-fastest evaporation rate, slower than grapefruit, then becomes easier to perceive strongly. Sandalwood, which evaporates relatively slowly, remains on the skin until the end.
The fragrance pyramid (in Japanese) and the terms top, middle, and last notes make this easier to understand.
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