What is Chemical Composition in Chemistry?

Introduction to Chemical Composition 

Understanding chemical composition is fundamental to many fields of science and engineering, as knowing the chemical composition allows us to predict how substances will interact with each other, which is vital in creating new materials, medicines, and countless other applications.

But what is it? Well, chemical composition is the arrangement, ratio and type of atoms in the molecules of that substance. This can vary with adding or removing chemicals from the substance, by changing the ratio of substances within it, or when other chemical changes occur.

With this in mind, how do we approach this topic when learning about the ins and outs of chemistry? As an essential part of what we do, it’s crucial to understand these markers. That’s why we’re ready to lead through the different elements, give some examples, and define solid methods used to make use of all the elements that go into chemical composition.

Key Takeaways

• Chemical composition refers to the types, arrangement, and ratio of atoms within a substance, which determines its properties and behaviour

• Knowing chemical composition is vital for predicting interactions between substances in various scientific fields.

• Two key methods, quantitative and qualitative, are used to help identify this composition

• Molar mass and density can be used to help with calculating a chemical composition’s formula

• When documenting these compositions, chemical and structural formulas are used

How to Determine the Chemical Composition of a Substance

Substances are fundamentally composed of elements or compounds (two or more chemical elements bonded together). This means you can 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 used to produce purer forms of substances without destroying their chemical composition. These processes usually mark the initial stage of determining the chemical composition of substances, and help form the basis of what goes into chemical solutions.

There are various techniques you can employ to identify a substance’s chemical composition. These fall into two categories – qualitative methods and quantitative methods. These approaches can be employed individually or in combination to provide a complete picture of a substance’s makeup.

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

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.
• 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, and in vivo analysis of plants.

How to Calculate the Chemical Composition of a 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. This can be found by writing down the chemical formulas and then multiplying 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 pure sample of the substance.

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.

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.

Writing 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. This is especially important in manufacturing, such as with chemical blending, to ensure that you have all necessary components available.

For instance, 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, creating an endless list of the elements in each of these. For simplicity, most will stick to the more common subgroups of chemical compositions within a mixture.

Examples of Chemical Composition in Everyday Substances

Now we’ve explored the different terms and methods used to document chemical composition, let’s go over some key examples you may encounter out in the world. These have been split into the two methods of documentation you’ll see: Chemical formulas and structural formulas.

Chemical compositions are typically written as chemical formulas, which are element symbols with subscripts. These represent a molecule of a particular substance. They 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:
• Ethylene:

Skeletal formula

• Ibuprofen:
• 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)), but its exact makeup can vary depending on environmental conditions.

What is the Chemical Composition 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.

This isn’t its only use, however, as it is also used as a solvent, antiseptic, and even as a fuel source in some biofuels.

What is the Chemical Composition of Water?

Water, a simple molecule with the formula H₂O, is formed of one oxygen atom joined with two hydrogen atoms.. This composition gives water its unique properties, such as high surface tension and the ability to dissolve many other substances.

Conclusion

As we continue to innovate, insights from chemical composition will remain a cornerstone of scientific progress. From basic analyses that identify a substance’s components to precise calculations that determine exact ratios, chemical composition is essential for applications across science, industry, and everyday life.  

Of course, once you have your composition, you can explore ways of using it. Make sure to use the proper measurements, and keep to mind the different types of chemical bonds, and you’ll be making sense of these formulas in no time.