What is a real life example of sublimation?
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As you know due to the presence of moisture in the food products they are at a risk to be spoiled by microorganisms such as bacteria, yeast, and mold. So, many of the food products need to be dried by removing the moisture from them. There are also naturally occurring substances, such as enzymes, which cause foods to ripen. And so drying is also needed to slow down the action of enzymes.
There is no direct method to remove the moisture from the food products. When we freeze the food products (by putting it in a freezer) it slows down their decomposition by turning the residual moisture into ice, thus stopping the growth of most bacterial species.
But sometimes we need to preserve food for a long period of time, say more than a month, and in that case, freezing is not enough. We then need to remove the moisture from the food products rather than just turning the moisture into ice > done by freezing (as mentioned above). This is where comes the role of sublimation.
Usually, solid ice starts to melt when placed somewhere at room temperature. Suppose we don't want it to melt and instead we want it to evaporate by converting the ice to vapours directly. This is where sublimation comes in when we directly want the solid (ice) to be converted into gas (vapours) and not allowing the ice to melt first.
We need the process of sublimation in the food products so that after freezing the food the ice in them can be sublimated and converted to gas (vapours) and thus making the food products completely dry and thus preventing their decay by microorganisms such as bacteria, yeast, and mold. There is a special process carried out to accomplish this goal and it is called freeze-drying or vacuum drying. Also, freeze-drying significantly reduces the total weight of the food product for easy transportation. You can see a picture on this webpage where it shows the three versions of a fruit > before freezing, after freezing and finally dried.
Freeze-drying is actually used in many industries. Since many pharmaceuticals degrade pretty quickly when exposed to air and water, freeze-drying is also used in the pharmaceutical industry and also used by research centers to preserve biological samples.
Freeze-drying is carried out in a vacuum chamber. In a vacuum chamber, the pressure can be extremely low. So low, in fact, that water can actually boil at room temperature. We all know that if you heat water up, it will boil. What you may not know is that the temperature at which water (or any liquid) will boil depends on the atmospheric pressure. Water actually boils at a lower temperature if the pressure around it is lowered. That is the reason at high altitude places water boils at a temperature below 100 degrees.
During vacuum drying, high-energy water molecules diffuse to the surface and evaporate due to low pressure. Due to the absence of air, vacuum drying prevents oxidation and maintains the color, texture, and flavor of the dried products.
Actually, any pure substance can exist in three phases > it can be a solid or a liquid or a gas (vapor). Now imagine the same substance existing as a solid, a liquid or a gas all at the same time. This is called the triple point where all the three phases of the substance exist at the same time (co-exist) but only at a particular temperature and a particular pressure.
For example, pure water (h2o) has a triple point temperature of 0.01-degree Celcius and a triple point pressure of 611.5 Pa. It means if we somehow bring the temperature and pressure values down to the values as mentioned above for the normal pure water (currently existing in the room temperature) then at that particular temperature and pressure all the three phases (solid, liquid and gas) of water will co-exist. You can watch this youtube video to watch this happening live.
For most substances, the triple point pressure is lower than the atmospheric pressure. These substances can exist at atmospheric pressure as liquids. If the triple point pressure is higher than the atmospheric pressure, the liquid phase does not exist at atmospheric pressure and the solid substance directly evaporates (sublimates).
Written by: Rajesh Bihani who is the webmaster of this website. Know more about Rajesh Bihani)