Freezing is a phase transition phenomenon, in which a liquid state of a given substance is changed to a solid-state. It is also known as “solidification”. The most common example of freezing, which is observed every day, is the formation of ice cubes in ice-tray when water is kept in the freezer for some time.
The freezing point is defined as a temperature at which this phenomenon of phase transfer takes place. Solid and liquid phases of a substance exist in equilibrium at the freezing point. Therefore, for most substances, the melting and freezing temperatures are the same.
How does freezing occur?
As we know that, when liquids are cooled, the freezing of a substance takes place. This is because when the temperature is decreased, it means that we are extracting heat from the particles of a substance. As the heat energy is removed from particles, their kinetic energy decreases. This decrease in kinetic energy, in turn, decreases the speed of particles. When the speed of the particles decreases, they come closer to each other. As the inter-particle space decreases, the intermolecular forces of attraction between the particles increases, which tends to bring the particles very close to each other, and therefore, the liquid form of substance changes to the solid form. This is how the freezing of a substance takes place.
Factors affecting Freezing Point
The nature of the intermolecular forces of a liquid directly affects the freezing point of a substance. For instance, when the intermolecular forces of attraction between the particles of a liquid are strong, then its freezing point will be high, and if the intermolecular forces are weak, then the freezing point becomes low. The change in the pressure conditions can also alter the freezing point of a substance.
Super-Cooled Liquids
The theory states that the melting point and the freezing point of a substance have to be the same. This applies to the majority of substances, but few exceptions are there, i.e., few substances have a slight difference between their freezing point and melting points. These substances can be cooled beyond their freezing point while retaining their liquid form. Hence, super-cooling is a state where liquids do not solidify even below their normal freezing point. This phenomenon can be observed every day in meteorology; at high altitudes, clouds are an accumulation of super-cooled droplets of water below their freezing point.
Examples
1. Snowfall
Snow falling is one of the most attractive sights, which fascinate everyone. But have you ever wondered how does snow formation take place, and what is the science behind it? Well, the answer to this question is based on the freezing concept. When the atmospheric temperature is at or below the freezing point of water, i.e., 0°C, and there is the minimum amount of moisture present in the air, at such conditions, the formation of snow in the atmosphere takes place. Once the formation of snow crystals takes place in the atmosphere, they mature by absorbing water droplets from the surrounding. After a snow-fall, snow present on the ground may melt or evaporate, or it may persist for long periods. Snow can even form at incredibly low temperatures as long as there is moisture present in the air and some way to lift or cool the air. The heaviest snow-falling takes place when there is relatively warm air near the ground because warmer air has the capacity to hold more water vapors. As snow formation requires moisture and cold temperature, dry areas may rarely receive snow. For example, Antarctica’s dry valleys are quite cold with low humidity, and strong winds, which removes any moisture from the air. Therefore, this extremely cold region receives a little snow.
Snowflakes
The snowflakes are the accumulation of many snow crystals. Mostly, their sizes are less than 1.3cm. The near-freezing temperatures, light winds, and unstable atmospheric conditions can form much larger and irregular snowflakes, which may be near 5 cm across, however, their exact size is not known.
2. Sea Ice
When the ocean water begins to freeze, the formation of small needle-like ice crystals, which are known as frazil form, takes place. Their size varies from 3 to 4 millimeters in diameters. The salt present in the ocean does not freeze, therefore, the crystals expel salt into the water, and the frazil crystals consist of nearly pure freshwater.
3. Frozen Food
Freezing is a very safe method for preserving food. There will be no growth in food poisoning or food spoilage due to microorganisms when the food is stored below -12°C. During the initial freezing operation, the temperature of food is reduced to below -12°C in a reasonable time, i.e., for few hours rather than days, and it is not allowed to rise above 7°C during thawing (it means the process of warming the food that has been frozen so that it can be eaten). However, once thawed, any microorganism present can again become active, and under certain conditions, it can multiply to levels that can lead to foodborne illness. Therefore, it is important to ensure that processing before freezing results in safe food when frozen so that the thawing process does not result in the growth of microorganisms. Generally, frozen food can retain its vitamins and minerals, and there is no change to the fat, carbohydrate, or protein content. It can be a convenient way to incorporate healthy food from every food item, including whole-grains, fruits, vegetables, protein, and dairy products.
4. Lava Hardening into Solid Rock
Magma is a molten form of rock mixture, which is found under the earth’s surface. When it is ejected by a volcano or other event, the material that is extracted is known as lava. Magma is extremely hot, having a temperature range of 700-1300°C, and this extremely hot temperature is responsible for its fluid state. This liquid form of magma solidifies when it is cooled down. That solidified form of magma acquires the form of rocks, which is known as igneous rock.
5. Solidification of Melted Candle Wax
It is a well-known fact that wax contracts on solidification. It means that if the molten wax is taken in large vessels, and it is allowed to cool slowly, then it will solidify. This solidified wax can be converted into different shapes.
6. Anti-freezer
Anti-freezer, such substances that lower the freezing point of water, which protects a system from the harmful effects of ice formation. Anti-freezers, such as ethylene glycol or propylene glycol, which is commonly added in automobile cooling system in order to prevent damage to radiators caused due to freezing of water.
7. Embryo Freezing
Embryo freezing is a medical procedure that allows people to store embryos, which can be used later for different purposes. It is also possible to freeze eggs, which are not fertilized. The frozen eggs are sometimes, used to become pregnant, and doctors may recommend, in vitro fertilization (IVF) process, in which the doctor exposes the egg to sperm and leaves them in the lab for fertilization, and that fertilized egg is termed as an embryo. According to medical theory, a correctly frozen embryo can remain viable for any length of time. These can be stored in sealed containers at a temperature of -321°F. Embryo freezing is beneficial for the following type of people:
- People with some genetic disorders, which may affect the reproduction process.
- Such people who will soon undergo chemotherapy.
- For same-sex couples, who wish to have children.
Although embryo freezing treatment is expensive, it can be considered as a safe and successful option for pregnancy.
7. Cryonics (Freezing bodies): Life after death?
At a very low temperature with certain conditions, it is possible to preserve tissues for centuries, possibly including the neurological basis of the human mind. The brain tissues can be cooled to cryogenic temperatures, in such a way that the formation of ice does not take place inside the cells. This is done through a process, called vitrification. Cryonics is a medical practice, whose aim is of preserving humans and animals at cryogenic temperatures with the expectation that future science can restore them to healthy living conditions. At present, cryonics can only be performed after the declaration of the legal death of the cryonics subject. The scientific justification for the practice of cryonics is based on the following concepts:
- Sufficiently low temperatures can virtually stop chemical changes in the bodies for centuries.
- Vitrification mixtures can be used to reduce or even eliminate the ice formation in tissues.
- Death is a process, not an event, and this is a long process.
- The damage, which is associated with low-temperature preservation, and clinical death that is not reversible in nature today, theoretically will be reversible in the future.
Cryonics is not yet a proven, and recognized method for a medical procedure. The processes involved in cryopreservation are as follows:
- In the first stage, the circulation and respiration of the cryonics subject are restored mechanically, and it is rapidly cooled to a temperature between 10°C to 0°C.
- Then, the subject’s blood is washed out and a significant amount of body water is replaced with a cryoprotectant mixture, which prevents the ice formation in tissues.
- Then cooled to a temperature below -120°C and held in cryostasis.
- It is done with the hope that in the future, some medicine or technology will be established that has the capacity to rewarm the subject, the cryo-protectant will be removed, tissues will be repaired, the disease will be cured, and the subject will be rejuvenated.
8. Congealing of Bacon Grease
Bacon is a fatty portion of meat, which is taken from pork belly. When it is cooked properly, i.e., at low temperatures, lots of its fat is rendered off, and it becomes a liquid. The benefit of this slow and low bacon cooking is that we are left over with grease. The rendered fat is crystal clear, and on cooling, it takes creamy white color. Bacon fat can be treated as a substitute for butter in our diet.
9. Freeze-Drying
Freeze-drying is a process, which is used in the food processing industry with the aim to remove water from food-stuff. The main purpose of freeze-drying is to increase the shelf life of food-stuffs. The various steps in this process are as follows:
- The temperature of the product is lowered, i.e., around -40°C, which freezes the free water in foodstuff.
- Then pressure is lowered, and thus sublimation of frozen water takes place. This comes under primary drying.
- Finally, the targeted value of residual moisture is achieved by increasing and decreasing the temperature of the product. This comes under secondary drying.
10. Superfluid Helium (Overflowing of Soft-drinks)
You might have observed while drinking soft-drink, there is a spontaneous overflow of its rim, and shooting up of the straw from which you are drinking. This is nothing but superfluid helium. When liquid helium is cooled just below its boiling point, i.e., -269°C, it shows exceptional behavior of climbing up and over the sides of a dish. Here, it is not merely a liquid, it becomes superfluid now. The term superfluid means a liquid that flows without friction. When most of the liquids are cooled, their particles are settled into a regular manner, i.e., it becomes a solid. But liquid helium shows an exceptional behavior of its liquidity even at low temperatures, and never settled into a solid-state.