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In chemistry, pure chemicals refer to substances that only contain one type of fundamental unit, like an atom or molecule. However, in some disciplines, the definition of a pure substance extends to homogeneous mixtures. Therefore, in order to define what pure chemicals are, we must first specify which perspective we’re looking at them from. 

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3d illustration of molecule model

What Is A Chemically Pure Substance?

A chemically pure substance is defined by the absence of contaminants or undesired chemicals within a mixture or solution. Chemical purity basically means that only one type of atom, molecule, or compound is present in a sample. In the case of purified water, for example, only H2O molecules are present due to the purification methods that it has been treated with. 

However, it’s important to understand that no chemically pure substance is actually 100% pure. Purification methods, including things like distillation and extraction, are limited in terms of how accurately they can measure or detect contaminants in a compound. Even if 100% purity was achieved, it then becomes an issue of how long the compound is able to stay pure for. We’ll use purified water again to give you an example:

  • As soon as purified water is exposed to air, its purity level starts to decrease
  • This is because gases in the air, like carbon dioxide, are able to dissolve in the water
  • Containers are also a source of impurity because, unless hermetically sealed, air can easily get in
  • If left in direct sunlight, the molecules in the container could also begin leaching into the water

All of these circumstances can introduce contaminants into a mixture, whether it’s purified water or a chemical compound. Therefore, in reality, it’s virtually impossible to have a 100% chemically pure substance that maintains its purity for an extended period of time. 

That said, we can get substances extremely close to 100% purity. Reagent grade isopropyl alcohol (IPA), for example, has a purity level of 99.8%, and this still makes it suitable for use in many analytical laboratory experiments, such as when synthesising chemicals. 

Illustration of distillation process
Distillation is one of the most popular purification methods used to make a chemically pure substance

What Is An Example Of A Chemically Pure Substance?

We touched on it earlier, but one of the most common examples of a chemically pure substance is purified water. Whether it’s been distilled or deionised, water is purified for many reasons, but mainly so that it can be used in laboratory testing as a reliable cleaning agent, buffer, or even a reagent. 

Most chemically pure substances are produced in factories, where they’re able to get as close to 100% purity as possible. Some other examples of chemically pure substances are:

  • Alcohols: From ethanol and methylated spirits to IPA, many types of alcohols can be purified through distillation techniques. Even some commercially available liquors have high purity, like the Polish vodka Spirytus, which is 96% alcohol.
  • Sulphuric acid: One of the most ubiquitous acids in industry, sulphuric acid is necessary for manufacturing  and processing  a wide range of products that include fertilisers, explosives, detergents, gasoline, kerosene, plastics, and steel. Because of these roles, sulphuric acid used in industries needs a purity range of between 95% and 98%.
  • Gold: In nature, this metal is almost always found in pure elemental form because it doesn’t easily react with other chemicals. Even in ore forms, gold can be manually extracted from rocks. However, the purity of gold in jewelry making is never 24-karat, i.e. 100% pure, because in this purest form, it’s too soft and malleable, making it incredibly difficult to fashion into jewelry.
  • Water: Pure water makes another appearance in this article because it’s important to note that it doesn’t occur naturally. Pure water undergoes several stages of filtration and distillation in order to be classed as purified. This means that bottled drinking water that’s marketed as pure simply isn’t: to be potable, water must contain certain minerals and vitamins, which are classed at contaminants. Only laboratory grade and reagent grade water products are close to being absolutely pure.
  • Sugars: Sugar crystals can be purified to near absolute purity levels, depending on the intended use. For example, common table sugar is actually almost 100% sucrose. This is because fibres, minerals, and other biochemicals are removed from sugar cane extracts in order to produce refined white sugar. 

What Does Chemically Pure Grade Mean?

Various countries and international organisations have their own grading systems for pure grade  chemicals. These systems are based on the quality, purity, and how chemicals have contaminants removed. Therefore, a chemically pure grade may have several meanings depending on where you live. Here in the UK, the most common grades for general chemical products are:

  • Technical grade: Chemicals under this category are intended for industrial applications. This is also the lowest grade of purified chemicals.
  • GRG: The General Reagent Grade means that a chemical is suitable for laboratory use and has a purity level of 99.9%.
  • BP grade: This stands for the British Pharmacopoeia grade. Chemicals that pass this standard are commonly used in cosmetics because they’re biologically safe and non-hazardous. 
  • Analytical research grade: This is the highest standard, wherein the chemicals classified under it are suitable for use in precise testing.
Pure substances and mixtures diagram
Pure substances are only composed of one type of fundamental unit, while impure substances are composed of many other types of compounds

What Is A Pure Chemical Compound?

A pure chemical compound is a chemical that contains only one type of molecule, which is the molecule of a compound. This includes things like tin, sulphur, and diamonds. 

Homogenous mixtures, like gasoline, air, or salt water, aren’t considered pure chemical compounds because they have different compounds in them. However, these can be separated through various purification methods in order to produce a chemically pure sample. 

What Are the Differences Between Pure And Impure Substances?

Pure substances may either refer to chemically pure substances or homogeneously pure substances:

  • Pure substances contain only one type of fundamental unit, like atoms or molecules
  • Homogeneously pure substances contain several compounds or elements, but maintain a consistent composition throughout the mixture

Impure substances, on the other hand, refer to compounds that are either chemically impure or homogeneously impure. As such, impure substances usually contain undesirable compounds, like dissolved gases, minerals, vitamins, pollutants, or any other kind of unwanted compound.

About the author

Jessica Clifton

Director

Jessica is a Director at ReAgent and leads a variety of growth projects. She has an extensive background in marketing, and has worked in the chemical manufacturing industry since 2019. When she’s not writing articles for ReAgent, Jessica can be found on a run, in her campervan, building LEGO, or watching Star Wars.
View all posts by Jessica Clifton

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