Why Do Oil And Water Not Mix? Amazing Science Facts For You

Adekunle Olanrewaju Jason
Jan 24, 2024 By Adekunle Olanrewaju Jason
Originally Published on Nov 16, 2021
Water and oil in glass
Age: 3-18
Read time: 5.2 Min

Oil and water are both liquids, but why does their chemical composition not allow them to mix?

Water is the universal solvent; both solids and liquids can be dissolved in water. One of the exceptions to water's dissolving power is oil, oil and water do not form a homogenous mixture.

Most substances like sugar, salt, paint, or dyes mix with water and form a homogenous mixture. A mixture is formed by two substances or more when they mix together in equal ratios throughout the volume and one component of the mixture can not be differentiated from the others. This uniform mixing is possible because of the polarity of water and the components mixed with it. A single water molecule consists of two positive hydrogen atoms and one negative oxygen atom, which leads to an uneven charge distribution across the molecule, causing its polarity. The water molecule is partially positively charged due to the hydrogen atom and negatively charged due to the oxygen atom. The positively charged and negatively charged ends on the water molecule aid it in breaking the chemical structure of the substance being mixed with water. The reason why oil and water molecules don't mix; oil is a non-polar molecule. Therefore, water cannot attract oil molecules, rather oil molecules repel water molecules. This water repelling property makes the oil molecule hydrophobic or water-fearing. As the density of oil is lower than water when the two are mixed together, they form separate layers. The oil will float on top of the water, the latter of which will sink.

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Why don't oil and water mix?

Due to the polarity difference between the water and oil molecules, oil and water do not mix. Water is considered a polar molecule that has a partial positive charge on one end and a negative charge on one end. The water molecules will attract each other due to the hydrogen bonds formed between them. The hydrogen atoms of one molecule bond with the oxygen atom of another molecule and vice versa. It can dissolve both liquid and solid substances due to this property. Due to the lack of polarity in the oil molecule, it can not dissolve in water as polar molecules can. Oil has non-polar hydrophobic (water-repelling) molecular chemistry; when the molecules interact with water, they don't let the polar bonds of the water molecule break the chemical bonds in the oil molecule. Oil is less dense compared to water and other polar liquid like food coloring, dye, paint, etc. As it is less dense, when oil and water mix, they form layers; the oil floats above the water, and the water will sink to the bottom. The type of oil used in the mixture is not related to this phenomenon; all types of oils and fats like vegetable oil or petroleum oil float on top of the water.

Can oil and water molecules be forced to dissolve?

Yes, using some special techniques and chemical resources, water and oil can be forced to mix together. We can mix oil and water molecules with the help of surfactants and emulsifiers. Surfactants reduce the surface tension of a liquid. A popular surfactant is detergent. It weakens the bonds between the surface molecules of a liquid, effectively breaking it into tiny pieces. They have a polar end and non-polar end; the non-polar end latches on to the oil molecules and the polar end to the water molecules, breaks the oil molecule and suspends it the water medium. This chemistry of surfactants makes them a popular choice for removing oil and grease from clothes and utensils. Emulsifiers work with the mechanism in chemistry. Their non-polar end bonds with the non-polar molecules in the mixture and the polar end with the polar molecules. When two immiscible liquids mix together they form an emulsion in colloidal form. This mixture will soon separate into layers if an emulsifier is not added to stabilize the emulsion. For example, if oil and water are placed in the same container, agitated by shaking the container, and then left alone, the components will quickly separate. If the same experiment is conducted with the addition of an emulsifier, the solution formed will be colloidal and will stay that way for longer. The simple explanation for what happens in this example is that, in the oil and water mix, the emulsifier will stick with both water and oil, and thus does not let them separate. In food, oil and water interact temporarily with the help of heat, but when the food is left standing for a while, the oil will separate and float to the top.

A school kid mixing water and oil.

How can oil and water be separated?

Oil and water don't mix and therefore can be separated very easily since they form distinct layers when placed in the same container. Oil and water can be separated using decantation, which uses a separator funnel. This funnel has an inverted teardrop shape with a stoppered opening on the wide end and a screw nozzle on the narrow end. The denser liquid can be removed from the nozzle at the bottom into one vessel and then the other liquid can be removed into a different one. In industrial applications, a belt skimmer is used the skim oil off the top of the water. To separate oil and water from a stable emulsion, demulsifying agents are used which absorb water and break the break oil film.

Where can we see examples of water and oil not mixing?

You can see plenty of examples in our daily life where oil and water don't mix. Some examples are; the shining rainbow patches in puddles after the rain, oil spills from ships and boats in the ocean which cause water pollution, when we try to was oily hands with water, and when the oil separates from cooked food and floats to the top.

Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestion for why do oil and water not mix article, then why not take a look at why do boats float, or why do leaves fall.

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Written by Adekunle Olanrewaju Jason

Bachelor of Science specializing in Mass Communication.

Adekunle Olanrewaju Jason picture

Adekunle Olanrewaju JasonBachelor of Science specializing in Mass Communication.

With over 3+ years of professional experience, Olanrewaju is a certified SEO Specialist and Content Writer. He holds a BSc in Mass Communication from the University of Lagos. Throughout his dynamic career, Olanrewaju has successfully taken on various roles with startups and established organizations. He has served as a Technical Writer, Blogger, SEO Specialist, Social Media Manager, and Digital Marketing Manager. Known for his hardworking nature and insightful approach, Olanrewaju is dedicated to continuous learning and improvement.
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