When coconut oil (and many other oils) is hot it is transparent but when it is cold, solid it is opaque and white why?



Gr. Noida, Uttar Pradesh

Any oil looking transparent or translucent (semi-transparent) facilitating see through condition under normal conditions in summer or warm climate transforms to white solid or gel like form in winter, especially in very cool climate/regions. This situation is similar to transparent water under normal temperature transforming to white or bluish white ice block upon freezing with the difference of oils having much higher freezing temperature than does water at 0°C.

Oils, basically made of triglycerides comprising several fatty acids, have much higher freezing/melting temperature with the extent of saturation determining the melting-freezing point. In the case of coconut oil freezing starts around 25°C which is why we see the oil getting stuck to the container during chilly winter season.

Temperature is an important attribute determining the physical state of any material. So oil or water upon cooling down to near freezing conditions starts solidifying, through condensation and by virtue of their inter atomic distances becoming shorter also start crystallizing. Depending on the purity or impurity or chemical content, the body color of the solid varies usually remain near white condition with exceptions.

When light (usually white light from the sun comprising all colors of the visible spectrum viz VIBGYOR) falls on a material, either it is transmitted through ( so looking transparent) or partly absorbed and reflected back depending on the chemical composition and constituents, state of matter of the material. Of the three processes, transmission, reflection and absorption, depending on which dominates, the material turns-out to be looking transparent or white if it fully reflects the light or colored if partly reflects the light or black if there is complete absorption of light. In any crystalline solid having regular arrangement of ions, the process of light scattering, reflected at various angles dominates over transmission.

So we can no longer see the frozen oil in solid form transparent and hence appears as white solid. meaning almost all the light falling on it is reflected. Once this frozen oil block is melted, it turns back to liquid form it starts transmitting visible light with the reflection component absent which is a temperature dependent reversible process.


Luminescence Group

CECRI, Karaikudi, Tamil Nadu

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Printable version | Sep 21, 2017 8:31:38 PM |