Azeotrope

Sometimes, liquids mix together in a special way. This is called an azeotrope.

When you boil these liquids, they do not change. The parts stay in the same amount. This is hard to fix. You cannot separate them by boiling.

Some mixtures boil at a low heat. These are called positive types.

Other mixtures boil at a high heat. These are called negative types.

Scientists study these to understand how liquids work. It helps them separate other things.

Sometimes, liquids mix in a special way. This is called an azeotrope.

When you boil these liquids, they do not change. The parts stay in the same amount. This happens because the vapor has the same mix as the liquid. Usually, boiling helps separate liquids. This is called distillation. But you cannot use simple distillation to separate an azeotrope.

There are two main types. A positive azeotrope boils at a low temperature. It boils even lower than the liquids it is made of.

One example uses ethanol and water. Ethanol boils at 78.4 °C. Water boils at 100 °C. But the mixture boils at 78.2 °C. This is lower than both!

A negative azeotrope boils at a high temperature. It boils higher than the liquids it is made of.

Hydrochloric acid and water form a negative azeotrope. This mix boils at 110 °C. This is higher than water or the acid alone.

Some mixtures are even more complex. A double azeotrope has two different boiling points.

A saddle azeotrope uses three or more liquids.

Sometimes, liquids mix in a very special way. This is called an azeotrope.

Usually, we can separate liquids by boiling them. This process is called distillation. When you boil a mixture, the vapor often has a different mix than the liquid. This allows us to collect one liquid separately. However, an azeotrope is a mixture that stays the same. When you boil it, the vapor has the exact same proportions as the unboiled liquid. Because of this, simple distillation cannot change the mix.

There are two main types of these mixtures. A positive azeotrope boils at a lower temperature than its parts. We call these minimum boiling mixtures. For example, ethanol boils at 78.4 °C and water boils at 100 °C. But a mix of 95.63% ethanol and 4.37% water boils at only 78.2 °C. This is the lowest temperature possible for that mix at normal pressure.

On the other hand, a negative azeotrope boils at a higher temperature. These are called maximum boiling mixtures. A mix of 20.2% hydrochloric acid and 79.8% water boils at 110 °C. This is much higher than water or the acid alone.

Scientists have studied these mixtures for a long time. The word azeotrope comes from Greek words. It means "no change on boiling." This term was created in 1911. It was made by English chemists John Wade and Richard William Merriman. They wanted a shorter way to describe mixtures with a constant boiling point. They wanted to avoid using very long, clunky phrases. Using this single word made science much easier to discuss.

Some mixtures are even more complex than just two liquids. A double azeotrope has two different boiling points. One example is a mix of benzene and hexafluorobenzene. There are also saddle azeotropes. These only happen when you have three or more liquids. A known example uses acetone, chloroform, and methanol. This specific mix boils at 57.5 °C.

You can also find heterogeneous azeotropes. These are mixtures that form two separate liquid layers. For example, chloroform and water can form these layers. When boiled, they create a vapor with a fixed ratio of 97.0% chloroform and 3.0% water.

Understanding azeotropes is very useful for real work. Engineers use this knowledge to separate liquids that are hard to part. For instance, it is hard to get pure acetic acid from water. It is not very practical to keep distilling it. Instead, workers can add ethyl acetate to the mix. This creates an azeotrope with water that boils at 70.4 °C. This allows the water to be boiled away easily. This leaves nearly pure acetic acid behind in the container.

An azeotrope is a special type of liquid mixture. It consists of two or more liquids that cannot be separated by simple distillation. In a normal mixture, boiling the liquid creates a vapor with a different composition than the liquid itself. This difference allows scientists to collect different substances through repeated boiling and cooling. However, an azeotrope behaves differently. When an azeotrope boils, the resulting vapor has the exact same proportions of ingredients as the unboiled liquid. Because the composition remains constant during this process, these mixtures are often called constant boiling point mixtures.

The mechanism of an azeotrope is best understood through vapor-liquid equilibrium. This describes the balance between the liquid phase and the vapor phase at a specific temperature. In a phase diagram, we can track how temperature and composition change during boiling. For a positive azeotrope, the boiling point curve reaches a minimum point. At this specific composition, the liquid curve and the vapor curve touch. If you attempt to distill a mixture that is not yet at the azeotropic composition, the vapor will become richer in one component. However, as you repeat the process, you will only ever move closer to the azeotropic point, never past it.

Scientists classify azeotropes into two primary types based on their boiling points. A positive azeotrope, also known as a minimum boiling mixture, boils at a temperature lower than any of its individual parts. A well-known example is a mixture of ethanol and water. Pure ethanol boils at 78.4 °C, and pure water boils at 100 °C. Yet, a mixture containing 95.63% ethanol and 4.37% water boils at only 78.2 °C.

Conversely, a negative azeotrope, or maximum boiling mixture, boils at a temperature higher than any of its individual constituents. For instance, a mixture of 20.2% hydrochloric acid and 79.8% water boils at 110 °C. This is significantly higher than the boiling point of water at 100 °C or hydrogen chloride at −85 °C.

Beyond these two types, more complex systems exist. A double azeotrope is a system that contains both a minimum and a maximum boiling point. One example of this is a mixture of benzene and hexafluorobenzene.

There are also saddle azeotropes, which only occur in systems with three or more liquids. These mixtures do not fit into the positive or negative categories because their boiling point falls between the boiling points of the individual components. A known example is a ternary azeotrope made of 30% acetone, 47% chloroform, and 23% methanol, which boils at 57.5 °C.

The term "azeotrope" has a specific history in chemistry. It was coined in 1911 by English chemists John Wade and Richard William Merriman. The name is derived from Greek roots. The prefix "a-" means "no," "zein" means "to boil," and "tropos" means "turning." Together, the word literally means "no change on boiling." The chemists created this term to replace much longer, more cumbersome descriptions like "mixtures having a minimum or maximum boiling point."

Some azeotropes are categorized by how well the liquids mix together. If the liquids are completely miscible, meaning they mix perfectly in all proportions, it is a homogeneous azeotrope. However, if the components do not mix completely, they form a heterogeneous azeotrope, or heteroazeotrope. In these cases, the mixture will separate into two distinct liquid layers. For example, chloroform and water can form two layers when shaken and left to stand. If you boil this heteroazeotrope, the vapor will always have a fixed ratio of 97.0% chloroform and 3.0% water.

Understanding these properties is vital for industrial and technical applications. Engineers must know the pressure, temperature, and composition behavior of mixtures to manage chemical processes. Azeotropes are particularly useful when trying to separate "zeotropic" mixtures, which are substances that do not form azeotropes. For example, separating pure acetic acid from water is very difficult using only distillation. To solve this, engineers can add ethyl acetate as an "entrainer." The ethyl acetate forms an azeotrope with water that boils at 70.4 °C. By distilling this new mixture away, the worker can leave nearly pure acetic acid behind in the container.