What is Chemical Composition in Chemistry?

Kate Onissiphorou

by Kate Onissiphorou

31st March 2022

Scientist examining a spectroscopy picture in a lab

Chemical composition refers to the precise number of elements per molecule in a substance. The chemical composition of a substance determines its physical and chemical properties under certain conditions. 

Substances can exist in many states of matter, including solid, liquid, gas, plasma, and Bose-Einstein condensate. They can also exist in different combinations, such as diatomic elements, compounds, solutions, suspensions, alloys, and colloids. A sufficiently stable substance under standard conditions is defined by its elements or compounds. 

In chemistry, mixtures can either be homogeneous or heterogeneous.

  • In a homogeneous mixture, the constituent substances are indistinguishable, uniform throughout and can’t be separated easily.
  • The constituent substances in a heterogeneous mixture are distinct, not uniform and can be easily separated. 

Air is an example of a homogeneous mixture, while colloids such as milk, fog, and butter are types of heterogeneous mixtures. Colloidal mixtures are defined by the size of the particles and the way they disperse light. Colloids typically have a particle size range of between 2 nm and 500 nm and they scatter visible light.

How to Determine the Chemical Composition of a Substance

Substances are fundamentally composed of elements or compounds (two or more chemical elements bonded together). You can therefore determine the chemical composition of a substance by looking at which elements are chemically bonded together.

Most substances do not exist in pure forms in nature as some impurities will always be present. Purification processes, such as macroscopic filtration, reverse osmosis, centrifugal separation, and distillation are therefore used to produce purer forms of substances without destroying their chemical composition. These processes are often used during the initial stage of determining the chemical composition of substances.

There are various techniques you can employ to identify a substance’s chemical composition. These fall into two categories – qualitative methods and quantitative methods.

Qualitative methods

Qualitative methods are mainly descriptive techniques used to identify the composition of an unknown analyte. They do not involve precise measurements or calculations and may be performed as a preliminary step in ruling out the presence of certain substances. 

Some examples of qualitative methods include:

  • Iodine test – commonly used to detect the presence of starch
  • Flame test – used to detect certain metals or ions based on the colours they produce when subjected to a flame
  • Precipitate tests – various chemical reagents can be used to determine the absence or presence of certain compounds or ions based on the colour and appearance of precipitates
  • Spectroscopy – elements can be identified based on the colour bands or black lines they produce on the visible spectrum when analysed using a spectroscope.

    Scientist examining a spectroscopy picture in a lab

    Spectroscopy is an example of a qualitative method of finding a chemical composition.

Quantitative methods 

Unlike qualitative techniques, quantitative methods use precise measurements and calculations to determine the proportions and structure of a substance. They do not simply detect the presence of a substance or rule out certain elements or functional groups. 

Some examples of quantitative methods include:

  • Titration (volumetric analysis) – this is typically used in neutralisation reactions between acids and bases. The neutralisation threshold volume is used to determine the molar concentration of an analyte. Titration is mainly used for neutralisation reactions
  • Gravimetric analysis – in this technique, a purified substance of unknown composition is weighed before reagents are applied to produce well-known reactions. The weight of the isolated substance is then compared to the weight of the original sample substance.
  • X-ray fluorescence spectroscopy – in this method, high-energy x-rays or gamma rays are directed at an unknown substance. The radiation emitted by the substance is then analysed for the chemical signatures of elements. X-ray fluorescence spectroscopy can be used to identify metals, glass, ceramics and building materials. It’s also useful in forensic work, for example, in determining the authenticity of a painting.

How to Calculate the Chemical Composition of a Mixture

As we’ve already explained, there are various qualitative and quantitative methods you can use to analyse the composition of an unknown mixture. When it comes to calculating the exact chemical composition of a mixture, you’ll need to look at measurements like molar mass and density.

Molar mass

One way to calculate a mixture’s chemical composition is by examining its molar mass. You may simply write down the chemical formulas and then multiply the number of atoms of an element per molecule by the atomic weight. This will give you the molar mass, which you can then correlate with the mass of the sample. However, it’s important to make sure you have a highly pure sample of the substance.

For example, you can calculate the molar mass of water as follows:

Water = H2O

  • Oxygen = 1 atom x 16 u or 16 grams per litre to get the molarity equivalent
  • Hydrogen = 2 atoms x 1 u or 2 grams per litre to get the molarity equivalent
  • TOTAL: 18 grams

As one gram of water is equal to 1 mL at standard conditions, one mole of water is therefore 18 mL. 

Density

Another way to calculate the composition of a substance is through density. If you know the mass and volume of a substance, you simply need to divide the mass by the volume. The density equation

This formula is useful if you want to determine the purity of a substance of known density.

For example, gold has a density of 19.32 grams per cubic centimetre. This means a metal with a density that’s lower than this is not pure gold. To claim a gold sample has high purity, its density must be near to this value. The Archimedes principle uses density to test the purity of a substance.

How do you Write Chemical Composition?

The chemical composition of any substance is written as a formula for either a compound or an element, or sometimes a combination of the two.

If the substance is a mixture, you need to write the chemical names and/or the respective chemical formulas and the percentage of all the substances that are present.

For example, kerosene is a complex mixture of various hydrocarbons that generally contains:

  • Paraffins (55.2%)
  • Naphthenes (40.9%)
  • Aromatic hydrocarbons (3.9%).

You can further subdivide these chemical categories into specific compounds but the list would be very long.

Chemical Compositions Examples

Chemical compositions are typically written as chemical formulas, which are element symbols with subscripts. These represent a molecule of a particular substance. Chemical compositions may also be written as structural formulas or skeletal structures if the molecule is large and very complex, such as organic compounds.

In some instances, chemical compositions can be represented by generalised formulas containing functional groups. Here are some examples:

Chemical formula

  • Table sugar or sucrose: C12H22O11
  • Limestone or calcium carbonate: CaCO3

Structural formula

  • Butane: The structural formula of butane
  • Ethylene: The structural formula of ethylene

Skeletal formula

  • Ibuprofen: The skeletal formula of ibuprofen
  • Escitalopram antidepressant: The skeletal formula of escitalopram antidepressant

What is the Chemical Composition of Air?

Air is a homogeneous mixture that’s chemically composed of elements and compounds, the largest proportion of which is diatomic nitrogen at 78.08%. This is followed by diatomic oxygen at 20.95%. Carbon dioxide and other gaseous compounds are also found in air, but only in trace amounts.

What is the Chemical Composition of Rust?

Rust is generally understood as the oxidation of iron commonly observed in building structures, bridges and automobiles. Rust is essentially hydrated iron oxide or Fe2O3nH2O. It may also contain iron hydroxide (FeO(OH)).

What is the Chemical Composition of Alcohol? Various glasses of different types of alcohol

Alcohol generally has a hydroxyl or ―OH functional group attached to an alkyl group, otherwise known as an alkane substituent. There are various types of alcohols, one of which is ethanol, or: 

C2H5OH

Ethanol is a non-lethal alcohol found in alcoholic beverages such as wine, gin, and beer. The ethanol percentage varies depending on the type of alcoholic beverage. 

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