Difference Between Volatile and Nonvolatile Substances

Main Difference – Volatile vs Nonvolatile Substances

Substances can be classified into two categories based on volatility: volatile and nonvolatile substances. The volatility of a substance refers to its ability to transfer into the vapour phase from the liquid phase. A substance that may transform into gaseous phase directly from solid phase through sublimation is also considered volatile. The main difference between volatile and nonvolatile substances is that volatile substances easily transfer into gaseous phase whereas nonvolatile substances don’t easily transfer into gaseous phase.

This article looks at, 

1. What is Volatility
2. What are Volatile Substances
      – Definition, Properties, Characteristics, Examples
3. What are Nonvolatile Substances
      – Definition, Properties, Characteristics, Examples
4. What is the difference between Volatile and Nonvolatile Substances

What is Volatility  

Volatility is directly associated with vapour pressure of a substance. Vapour pressure is the pressure of the substance after transferring to the gaseous phase. Volatility is also closely associated with boiling point. A substance with a lower boiling point has higher volatility and vapour pressure.

The volatility of a substance is affected by the strength of intermolecular forces. For example, water is not readily volatile at room temperature and needs to be heated in order to evaporate. This is because of the hydrogen bonding between the molecules. As hydrogen bonds are much stronger, water has a higher boiling point and comparatively lesser volatility. In contrast, non-polar organic solvents such as hexane are readily volatile since they have weak Van Der Waals forces. Therefore, they also have low boiling points.

Molecular weight too plays a role in volatility. Substances of higher molecular weight have less tendency to be vaporised whereas, lower molecular weight compounds can be easily vaporised. 

What are Volatile Substances

Volatile substances are the substances which have a higher capability to transfer into the vapour phase. They have much weaker intermolecular attractions, hence can be easily transformed into the vapour phase. They also have higher vapour pressures and lower boiling points. Most organic compounds are volatile. They can be easily separated using distillation or rotary evaporators by providing only a small amount of heat. Most of them evaporate at room temperature when exposed to air. This is because of the weak intermolecular forces.

Let’s take acetone as an example. Acetone (CH3COCH3) is a highly volatile compound, which readily evaporates off when exposed to air. When a little amount of acetone is poured into a watch glass and kept for some time, the acetone molecules at the top most layer easily get released from other molecules and transform into the vapour phase. This exposes the next layers, and eventually, all the remaining acetone molecules transform into the vapour phase.

Most of the products we use on a daily basis contain volatile substances. Some examples include fossil fuel, paints, coatings, perfumes, aerosols and etc. These are somewhat harmful to health. Organic volatile compounds can retain in the atmosphere and enter our systems through inhalation. These compounds can cause harmful effects on chronic exposure. Furthermore, these cause harmful environmental conditions such as global warming and ozone layer depletion.

Figure 1: Perfume, an example of a volatile substance

What are Non-Volatile Substances

Compounds which do not readily turn into vapour are called non-volatile compounds. This is mainly due to their stronger intermolecular forces. The common features of such compounds are lower vapour pressure and high boiling points. The presence of a solute in a solvent lowers the ability of that particular solvent to evaporate. However, after evaporation, the non-volatile solute will not appear in the vapour phase of the volatile solvent.

There are several non-volatile liquids. Water having a boiling point of 100  ̊C, is a fine example of a non-volatile liquid. As discussed earlier, this is due to the presence of strong hydrogen bonds between water molecules. Mercury is also a non-volatile liquid. Mercury is the only metal which is a liquid at room temperature. Since it contains metallic bonds, metal mercury ions embedded in a sea of electrons, cannot be easily evaporated and has a very high boiling point and a low vapour pressure.

Figure 2: Mercury, an example of nonvolatile substance

Difference Between Volatile and Nonvolatile Substances 

Definition

Volatile Substance: Volatile substances readily transfer into the gaseous phase.

Nonvolatile Substances: Nonvolatile substances do not readily transfer into the gaseous phase. 

Vapour Pressure

Volatile Substance: Volatile substances have a comparatively high vapour pressure. 

Nonvolatile Substances: Nonvolatile substances have a comparatively low vapour pressure.

Boiling Point

Volatile Substance: The boiling point of volatile substances is comparatively low.

Nonvolatile Substances: The boiling point of non-volatile substances is comparatively high.

Intermolecular Attractions

Volatile Substance: These have weaker intermolecular attractions. 

Nonvolatile Substances: These have strong intermolecular attractions. 

Conclusion

Volatile compounds can be readily sent into the vapour phase. Usually, volatile substances have boiling points that are lower than 100  ̊C. In contrast, non-volatile compounds are difficult to be transferred to the gaseous phase, and they have much higher boiling points. Also, volatile compounds have higher vapour pressure compared to non-volatile compounds.

Volatile compounds also have weaker intermolecular forces such as Van Der Waals forces. Most volatile compounds are non-polar organic compounds. Therefore, they do not have stronger intermolecular attractions. Non-volatile compounds are mostly polar, and they have stronger interactions between molecules. This is the difference between volatile and nonvolatile substances. 

Reference:
1. “Helmenstine, Anne Marie. “Here’s What Volatile Means in Chemistry.” About.com Education. N.p., 17 Feb. 2017. Web. 21 Feb. 2017.
2. “Vapor Pressure.” Department of Chemistry. Purdue University, n.d. Web. 21 Feb. 2017.
3. “Volatile Organic Compounds (VOCs).” Enviropedia. N.p., n.d. Web. 21 Feb. 2017.
4. “Helmenstine, Anne Marie. “Understand What Nonvolatile Means in Chemistry.” About.com Education. N.p., 14 Oct. 2016. Web. 21 Feb. 2017.

Image Courtesy:
1. “Vintage Atomizer Perfume Bottle”By Angela Andriot – Vetiver Aromatics. (CC BY-SA 3.0) via Commons Wikimedia
2. “Hydrargyrum”By Hi-Res Images of Chemical Elements (CC BY 3.0) via Commons Wikimedia

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