Most of the things that we observe around us generally exist in three primary forms or phases: solid, liquid, and gas. The phase of the matter is temperature-dependent, i.e., with the change in temperature, every material changes its phase from one state to another. Generally, this change in phase of matter is gradual, i.e., from solid to liquid, and eventually, from liquid to gas; however, under certain specific conditions of temperature and pressure, a solid may directly change into a gas. This process is known as Sublimation. The variation of matter with respect to changes in pressure and temperature is given by something called a “phase diagram.”
Comparison of phase diagrams of carbon dioxide (red) and water (blue) showing the carbon dioxide sublimation point (middle-left) at 1 atmosphere. As dry ice is heated, it crosses this point along the bold horizontal line from the solid phase directly into the gaseous phase. Water, on the other hand, passes through a liquid phase at 1 atmosphere
In a phase diagram, “triple point” is a unique configuration of temperature and pressure where all three phases of matter can coexist together in thermal equilibrium. In other words, it corresponds to the lowest pressure at which the substance can exist as a liquid (intermediate state) for a given temperature. Sublimation takes place when the pressure drops below the value of triple point, without any change in the temperature. One way to think about this is that if you want to observe sublimation, you need to get the substance beneath the triple point by lowering the pressure. Consequently, if the triple point of a substance is at high pressure, then sublimation takes place, bypassing the process of melting into a liquid phase. However, one should not get confused sublimation with the transformation of a solid into a gas in a chemical reaction. For example, the dissociation on heating of solid ammonium chloride into hydrogen chloride and ammonia is not sublimation but a chemical reaction. Similarly, the combustion of candles containing paraffin wax into carbon dioxide and water vapor is also not sublimation, but a chemical reaction with oxygen. In general, sublimation only attributes to the change of the physical state of the matter. Let’s discuss a few examples of sublimation in daily life:
1. Dry Ice
Carbon Dioxide usually exists as a gas in our environment. However, it can also exist in solid form as “Dry Ice.” Unlike its name, dry ice has nothing to do with frozen water (ice). It is named so because it resembles normal ice and does not melt but evaporates, hence the term “dry ice.” It goes directly from a solid to gas by the sublimation process. At pressures below 5.13 atm and temperatures below −56.4 °C (216.8 K; −69.5 °F) (the triple point), CO2 changes from a solid to a gas with no intervening liquid form. It is used primarily as a cooling agent but is also used in fog machines at theatres for dramatic effects. Interestingly, it provides a lower temperature than that of water ice and not leaving any residue (other than incidental frost from moisture in the atmosphere). It is useful for preserving frozen foods where mechanical cooling is unavailable. However, it is advised to not deal with solid carbon dioxide with bare hands as it can leave severe frostbites on the skin.
2. Water Cycle
In the water cycle, sublimation is most commonly used to explain the process of snow and ice turning directly into water vapors without going into the liquid phase. The opposite of sublimation is “deposition,” where water vapors change directly into ice, such as snowflakes and frost. When weather conditions such as low relative humidity and dry winds are present, sublimation happens more readily. It also happens at higher altitudes where the air pressure is low as compared to lower altitudes. Energy is also needed, such as harsh or intense sunlight, to maintain the temperature at the triple point. If one had to choose a location on Earth where sublimation happens a lot, Mt. Everest’s south face would be a perfect choice. Low temperatures, strong winds, bright sunshine, very low air pressure are the necessary condition for sublimation to take place.
3. Mothballs
Every year, when the summer arrives, people put their warm out-of-season clothes in storage until next winter or autumn. Every time they do so, they also put tiny but notably pungent mothballs along with the clothes to protect them from moth. During the fall, when we take those clothes out again, we find that balls are significantly shrunk in size, but there is no residue on the clothes. This occurs because, in general, the mothballs we use are made of naphthalene, which can sublimate from solid to gas under normal room temperature conditions during the summer. The strong order of naphthalene prevents insects from approaching the clothes and causing any harm. However, since naphthalene can catch fire easily, modern mothballs use 1,4-dichlorobenzene instead. Moreover, 1,4-dichlorobenzene mothballs can go under sublimation even at a lower temperature of 21-24 °C.
4. Dye-Sublimation Printing
Sublimation printing is becoming more and more popular nowadays. The best designed graphic apparel, home décor, and advertising displays use the dye sublimation process to achieve vibrant and everlasting graphics. The process uses the science of sublimation, not in an exact manner but in the opposite sense. The heat and pressure are applied to a solid color, turning it into a gas through an endothermic reaction without passing through the liquid phase. This process is known as “Deposition” or “Desublimation.” In sublimation printing, unique sublimation dyes are transferred to sheets of “transfer” paper via liquid gel ink through a piezoelectric print head. The ink is deposited on these high-release inkjet papers, which are used for the next step of the sublimation printing process. After the digital design is printed onto sublimation transfer sheets, it is placed on a heat press along with the substrate to be sublimated. In order to transfer the image from the paper to the substrate, it requires a heat press process that is a combination of temperature and pressure. The heat press applies this special combination, which can change depending on the substrate, to “transfer” the sublimation dyes at the molecular level into the substrate. The most common dyes used for sublimation activate at 350 ºF. However, a range of 380 to 420 ºF is normally recommended for optimal color. The inks then penetrate the fibers of the material and are permanently bonded to it, resulting in a high-definition print with an extremely intricate level of detail.
5. Forensics
Solving a crime is never a cakewalk. Thanks to the scientific methods that provide concrete evidence to make sure that justice is being served. The criminal investigation includes means for developing and analyzing physical evidence from crime scenes. Modern-day criminal investigations commonly employ many modern scientific techniques, collectively known as forensic science. DNA is the gold standard for criminal evidence. But often, there is no DNA at the scene of a crime. Then, an old-fashioned fingerprint analysis can be used. A skilled analyst can identify an individual from a single good print because no two people have identical fingerprints. When people touch objects with their fingers, they may leave behind traces of skin oils. This oil outline the pattern of whorls on their fingers. One method for bringing latent prints into view is called iodine fuming. Under normal temperature and pressure conditions, iodine is a solid crystalline substance with a metallic luster. However, it is volatile, meaning that it sublimes easily. Therefore, when its fumes are blown over a surface containing fingerprints, iodine can react with the fatty acid secreted by fingertips, producing dark color prints. Another sublimation process that helps in a criminal investigation is dye sublimation. It is used to create digital watermarks on documents. It enables a forensic examiner to differentiate an authentic document from a forgery.
6. Perfume Tablets
We are all familiar with scented perfume tablets used for rendering a pleasant fragrance in the bathroom or wardrobe. They give your home a subtle and personalized scent. They are made of 100% organic wax permeated with pure essential oils that enhance their effectiveness and make them long-lasting. Depending on the type of usage, a perfume tablet can last for 3-6 months. Most of these tablets include camphor as a major ingredient for scent. Camphor can go under sublimation at room temperature. Therefore, these aromatic tablets appear to decrease in size over time.
7. Accretion of Matter in Space
Once in a while, most of us wonder how this solar system, planets, comets, moons, and other celestial objects came into existence. In astrophysics, the aggregation of particles into a large object by gravitationally attracting more matter, normally gaseous matter, to the accretion disk is known as the accretion of matter. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes. Planets form in protoplanetary discs of gas and dust that surround young stars. It has been acknowledged that drifting pebbles play an important role in the core accretion scenario by triggering streaming instabilities or by aiding the growth of planetary cores. Moreover, ice lines of volatile species such as water seem to be promising sites for this process. At the water ice line, the higher surface energy of ice promotes coagulation, and the sublimated vapor can diffuse outward in the disk and deposit onto pebbles, allowing fast growth.