O Level Chemistry & IP Chemistry Notes by 10 Year Series Author – Chemistry Specialist

O-Level & IP Pure Chemistry: Examination Tips on Elements, Compounds & Mixtures

Matters has been classified as 3 states of matter, namely Solid, Liquid & Gas.

Matters can also be classified as Elements, Compounds & Mixtures.

So, how do we tell if a substance is an element, a compound or a mixture?

You need to know their definitions and properties very well in order to do so.

Let’s get started ………

A) Elements

It is a pure substance that cannot be split into two or more simpler substances by chemical means.

For example, carbon, hydrogen and oxygen are known as elements since they cannot be broken down further into simpler substances.

Elements are represented by chemical names & chemical symbols. The names and chemical symbols of some common elements are listed below:

Very often, you can guess the chemical symbol from the first two letters of the element’s chemical name. You should refer to the Periodic Table for the full list of elements.

Elements can be classified in different ways:

(i)    By State – Solids, Liquids & Gases

(ii)   As Metals, Non-Metals & Metalloids

Metalloids have properties of both metals and non-metals. e.g. Si and Ge

Elements can exist as atoms or molecules.

An atom is the smallest particle of an element that has the chemical properties of an element.

Each element contains only one type of atom.

Group 0 Noble Gases such as helium, neon, argon, kryton and xenon are known as monoatomic elements i.e. they exists as individual atoms.

A molecule is a group of two or more atoms that are chemically combined together.

Diatomic molecules refers those that are formed by the combination of two atoms. Elements which exists as diatomic elements are hydrogen (H2), oxygen (O2), nitrogen (N2), chlorine (Cl2), etc.

Polyatomic molecules are those that contain three or more atoms. Common example will be ozone (O3) which is triatomic.

B) Compounds

A pure substance that contains two or more elements chemically combined together in a fixed ratio.

A compound can be decomposed by chemical processes such as thermal decomposition and electrolysis (which is the process of using electricity to break down a compound).

A compound may be made up of molecules (covalent compound) or ions (ionic compound). For example, carbon dioxide is a compound made up of molecules while sodium chloride is a compound made up of positive sodium ions and negative chloride ions.

Compounds have a chemical name which indicate the types of elements present.

CompoundElements present
sodium chloride (common salt)sodium and chlorine
carbon dioxidecarbon and oxygen
copper (II) sulfatecopper, sulfur and oxygen
magnesium oxidemagnesium and oxygen

Compounds can also be represented by chemical formulae.

The chemical formula of a compound can be determined by putting the formula of the elements that made up the compound.

The chemical formula shows us the

Tips on writing chemical formula of compound:

RulesExamples
For compounds that contain both
Metallic & Non-Metallic elements,
Symbol of Metallic Element is written 1st
Sodium chloride (NaCl)
No of atoms of elements is written as subscriptsWater (H2O and not H2O)
Not necessary to write subscript “1”Water (H2O and not H2O1)
Oxygen atom is usually written at the endCarbon dioxide (CO2 and not O2C)
Reduce the “subscript” to the lowest termMgO and not Mg2O2

C) Mixtures

A mixture is made up of two or more substances that are not chemically combined.

Mixtures do not have a chemical formula to represent it. This is different from elements and compounds.

Mixtures can be made up of elements or compounds.

The composition of a mixture are not fixed i.e. they can be present in any ratio.

We can have a mixture of:

a) Two elements e.g. helium (He) and oxygen (O2)

b) Two compounds e.g. water (H2O) and carbon dioxide (CO2)

c) Element and compound e.g. oxygen (O2) and carbon dioxide (CO2)

The components of a mixture can be separated by physical separation techniques such as filtration, simple distillation and sublimation, etc.

An example of a widely used mixture is alloys which is defined as a mixture of a metal with other element(s) (can be metals or non-metals). e.g. Steel is made up iron and carbon.

It is important for students to know the four main differences between a mixture and a compound, in terms of:

In order to remember the four main differences easily, we told our Sec 3 and 4 GCE O-Level and IP Pure Chemistry Tuition Classes to remember the mnemonics S.P.E.C.

Difference Between Compounds and Mixtures:

 CompoundsMixtures
SeparationCan only be broken down into its elements or into simpler compounds by chemical means i.e. thermal decomposition or electrolysisComponents can be separated by physical processes i.e. filtration, simple distillation and sublimation
PropertiesPhysical and chemical properties of a compound are different from that of its constituent elementsChemical properties of a mixture are the same as that of its components
Energy changeA chemical reaction tales place when a compound is formed i.e. energy change which involves heat/light is involvedNo chemical reaction takes place when a mixture is formed i.e. no energy change is involved
CompositionElements in a compound are always combined in a fixed ratioComponents of a mixture can be mixed in any proportion

Let us take a look at an example on how we can apply this table of differences between compounds and mixtures.

Example: Water (H2O) is formed when hydrogen (H2) and oxygen gas (O2) are chemically combined.

 Water (a compound)Mixture of hydrogen and oxygen
SeparationCan be broken down into hydrogen and oxygen by electrolysis (you will learn this in more detail under the topic of Electrolysis in Sec 4).Can be separated by physical process such as fractional distillation (you will learn this in more detail under the topic of Separation Techniques in Sec 3).
PropertiesWater is a colourless liquid while hydrogen and oxygen are colourless gases at room temperature and pressure.
Unlike oxygen which supports combustion, water does not.
Uniform mixture of colourless gases at room temperature.
Mixture of hydrogen and oxygen support combustion, just like the mixture of hydrogen and oxygen.
Energy changeAn energy change is observed when water is formed.No energy change when the mixture is formed.
CompositionRatio of hydrogen atom : oxygen atom is 2:1.Ratio of hydrogen gas to oxygen gas can vary.

Let me give you an exam-based multiple choice question (MCQ) on this topic of Elements, Compounds & Mixtures to test your understanding of the key concepts.

Question:

X and Y combine to form XY2. Which of the following statements is true?

A) Ratio of X:Y is 2:1

B) An energy change occurs when XY2 is formed

C) XY2 will have a density in between those of X and Y

D) XY2 will have the properties of both X and Y

Do leave your answers in the comments section below. You can also state your reasons for choosing that particular option (A, B, C or D).

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.

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