Mixing different substances together is something we do every day. Most of the experiments that we conduct in the chemistry lab often require the mixing of two liquids. Any two liquids are said to be miscible if they mix in all proportions, at any concentration, to form a homogeneous solution. In simple terms, miscibility is a property of two liquids that can completely dissolve in each other in all proportions. By contrast, any two liquids are said to be immiscible if there are certain proportions in which the mixture doesn’t form a solution. Liquids tend to be immiscible when the force of attraction between the molecules of the same liquid is greater than the force of attraction between the two different liquids. Such a system consists of 2 phases, though it is usually referred to as a mixture. Since by definition, immiscible liquids do not interact with each other. When put in the same vessel, they will evaporate completely independent of each other. In other words, each liquid will contribute its own characteristic equilibrium vapor pressure regardless of the presence of the other liquid. Thus, the total pressure exerted by the mixture is the sum of the equilibrium vapor pressures. If you have two immiscible liquids in a closed flask and keep everything still, the vapor pressure you measure will simply be the vapor pressure of the one which is floating on top. The energy required to break down the existing bonds between the liquid molecules and form new ones is known as enthalpy. Polarity in liquids commonly used as solvents is one property that can prevent mixing due to enthalpy. Polar liquids have molecules with positive and negative ends that attract to form strong intermolecular bonds. Non-polar liquids are held together by weaker van der Waals forces. Liquids with similar polarities can mix, but low-polarity molecules cannot overcome the strong bonds in a higher polarity liquid, so they remain separate. Although most of the liquids can get easily mixed, there are a few examples of immiscible liquids in our daily life.
1. Oil and Water
“Oil and Water” is perhaps the most common example of two immiscible liquids. No matter how much you mix oil and water, they do not mix. The reason this happens is because of the chemical nature of oil and water molecules. There is a popular saying in chemistry that “like dissolves like.” This means that polar liquids (like water) dissolve in other polar liquids, while nonpolar liquids (usually organic molecules) mix well with each other. Each water molecule is polar because it has a bent shape in which the negatively charged oxygen atom is attached to positively charged hydrogen atoms on each side of the molecule. Water forms hydrogen bonds between oxygen and hydrogen atoms of different water molecules. When water encounters nonpolar oil molecules, it sticks to itself rather than mingles with the organic molecules. That is why we need soap or liquid soap solution to clean the dishes that we use for cooking. Soap cleans oil and grease because one end of the soap molecule is polar and so is soluble in water, and the other end is non-polar and so similar to oil and grease. The soap molecules surround the grease leaving the water-soluble parts on the outside so the water can help wash the grease away. Thus, the soap molecule provides a link between two substances that would otherwise be immiscible. It is important to note that immiscibility does not account for why oil floats over water. Oil floats on water because it is less dense or has lower specific gravity. The immiscibility of oil and water, however, is not related to the difference in density.
2. Kerosene and Water
Kerosene, also known as paraffin, is a combustible hydrocarbon liquid that is derived from petroleum. It is widely used as a fuel in aviation as well as in households. It is a low viscosity clear liquid that is made of 10m to 16 carbons chained hydrocarbons obtained from the fractional distillation of petroleum at 150 to 250 C. Like many other hydrocarbons, kerosene is also immiscible with water. Most of the hydrocarbons are non-polar compounds. Such compounds cannot attract water molecules out of their hydrogen-bonded network. Kerosene and water will remain as two separate layers when put in the same vessel. Moreover, the weight or density of kerosene is lighter than water and has buoyancy. If you take water and put some drops of kerosene, then the kerosene oil will float. The immiscibility of kerosene and water plays an important role in chemistry. In X-Ray crystallography, kerosene can be used to store crystals. When a hydrated crystal is left in the air, dehydration may occur slowly. This makes the color of the crystal dull. Kerosene can keep the form of the crystal intact.
3. Gasoline (Petrol) and Water
During the rainy season, one of the major concerns of the people who own a vehicle is that if water sweeps into the fuel tank, it may cause harm to the engine. Modern cars have good sealing against water. However, there would be a chance in which water does come into your car’s fuel tank. So what if water gets in the fuel tank? Gasoline or petrol is a clear petroleum-derived flammable liquid that is used primarily as a fuel in most spark-ignited internal combustion engines. It consists mostly of organic compounds obtained by the fractional distillation of petroleum, enhanced with a variety of additives. Like many other hydrocarbons, petrol (gasoline) and water don’t mix because they have two different densities. Petrol is also very hydrophobic and doesn’t like water much. One can try and mix them, but they eventually separate again. Petrol is less dense than water, so water settles to the bottom and the petrol floats on top of the water. The advantage of this is that if waters accidentally get inside the fuel tank, it will settle down mostly in the reserve section, saving the engine from any problems.
4. Corn Syrup and Vegetable Oil
Another most common example that one can find in their kitchen for two immiscible liquids is corn syrup and vegetable oil. Corn syrup is a sweet syrup produced by milling corn into cornstarch and then putting the cornstarch through the process of acid hydrolysis. The density of corn syrup is approximately 1.4 g/mL. Corn syrup is almost entirely made of dextrose, a sugar. Density is an important concept in chemistry that is defined as the mass of an object per unit volume. Density is a physical property of matter that describes how closely packed together the atoms or molecules of a substance are. The formula used to calculate density is d = m/v, where d is the density, m is the mass of the object, and v is the volume of the object. Scientists use density in different ways. They use it to identify unknown substances and to separate different liquids. If a solution with a lower density is added to a solution with a greater density, the less dense solution will rest on top of the denser solution. On the contrary, if a solution with a greater density is added to a solution with a lower density, the higher density solution will naturally fall to the bottom. The density of vegetable oil is approximately 0.8 g/mL. Therefore, when mixed together, the corn syrup will settle down and the vegetable oil will float over it.
5. Wax and Water
When you go to a home decor store, you come across an interesting kind of lamp known as the Lava lamp. They are also an interesting example of immiscible liquids. The lamp consists of a bolus of a special colored wax mixture inside a glass vessel, the remainder of which contains clear or translucent liquid. The working of a lava lap depends primarily on three things: density, immiscibility, and convection. In any mixture of immiscible liquids, each immiscible liquid is called a phase. A mixture with two immiscible liquids is called a biphasic mixture. A mixture with more than two immiscible liquids is called a multiphasic mixture. When you watch the globs float around in a lava lamp, you’re looking at a biphasic mixture. At room temperature, the globs are a bit denser than the surrounding liquid. That’s why they sit at the bottom of the lamp. But when you turn the lamp on, the globs heat up. The molecules move faster. The globs become less dense than the surrounding liquid. They rise and start to float around. Lava lamps are designed so that the temperature at the top is a bit cooler than at the bottom. When the molecules reach the top, they cool down and lose energy, and move closer together. So when a glob reaches the top of the lava lamp, it contracts. It becomes denser than the surrounding liquid and begins to sink. When it reaches the bottom, the whole cycle repeats. A lava lamp is an example of a convection current. Convection currents cause liquids and gases to rise and fall because of changes in their density. There are convection currents all around you, even in the Earth’s crust.