Chemistry (0620) | Elements, compounds and mixtures

Chemistry IGCSE (0620) Study Notes: Elements, Compounds, and Mixtures (Topic 2.1)

Welcome to one of the most fundamental chapters in chemistry! Don't worry, understanding the difference between elements, compounds, and mixtures is like learning the basic building blocks of all matter. Once you grasp these definitions, the rest of chemistry becomes much clearer!

In this topic, we will learn how to classify everything around us into one of these three categories.

1. Elements: The Simplest Building Blocks (Core Content)

Imagine LEGO bricks. If you only use one type of brick—say, only the red rectangular bricks—that’s an element!

Definition of an Element

An element is a pure substance that cannot be broken down into simpler substances by chemical methods. This is the simplest form of matter.

• Every particle in an element is identical.
• All atoms in an element have the same number of protons (this is the proton number or atomic number).

Structure of Elements

Elements exist as single atoms (like Helium, He) or as molecules containing two or more identical atoms joined together (like Oxygen, O₂, or Nitrogen, N₂). These are still considered elements because the atoms are all the same type.

Key Examples:

• Gold (Au)
• Oxygen (O₂)
• Carbon (C)

Quick Takeaway: Elements are pure substances made up of only one type of atom. You can’t break them down chemically!

2. Compounds: Chemically Joined (Core Content)

Now, take your red LEGO bricks and chemically weld them to blue rectangular bricks in a perfect, permanent pattern. You have created a compound!

Definition of a Compound

A compound is a pure substance formed when two or more different elements are chemically bonded together.

• The joining process is a chemical reaction, often involving a transfer or sharing of electrons.
• Compounds must be in a fixed ratio (or fixed proportion) by mass. For example, water always has two hydrogen atoms for every one oxygen atom (H₂O).

Properties of Compounds

The most important characteristic of a compound is that its properties are completely different from the properties of the elements that formed it.

• Example: Sodium (Na) is a reactive metal that explodes in water. Chlorine (Cl₂) is a poisonous, green gas. When they combine chemically, they form Sodium Chloride (NaCl) – common table salt – which you eat every day!

Separation of Compounds

Because the elements in a compound are chemically bonded, they can only be separated by another chemical reaction, which usually requires a lot of energy (like electrolysis or heating strongly).

Quick Takeaway: Compounds are pure substances made up of different types of atoms bonded in a fixed ratio. They have new properties.

3. Mixtures: Physically Blended (Core Content)

Take your red LEGO bricks, your blue LEGO bricks, and some green ones, and just toss them all into a bucket together. They aren't stuck together; they are just next to each other. That’s a mixture!

Definition of a Mixture

A mixture consists of two or more substances (elements or compounds) that are not chemically bonded together.

• The substances are only physically combined.
• Mixtures have a variable composition (you can change the amounts of each substance present).

Properties of Mixtures

Each substance in a mixture retains its individual chemical properties.

• Example: Salt water is a mixture. The salt still tastes salty, and the water is still wet.
• Example: Air is a mixture of gases (N₂, O₂, CO₂, etc.). The oxygen still supports combustion, and the nitrogen is still mostly unreactive.

Did you know? Most of the materials you see every day, like rocks, wood, milk, and blood, are mixtures!

Separation of Mixtures

Because there are no chemical bonds holding them together, mixtures can be separated relatively easily using physical methods (methods that don't involve a chemical reaction), such as:

• Filtration
• Distillation
• Crystallisation
• Chromatography

Quick Takeaway: Mixtures are impure substances made up of substances not bonded together. They have a variable ratio and can be separated easily by physical methods.

4. Comparing Elements, Compounds, and Mixtures

This is the core concept you must master. It's often helpful to compare their key features side-by-side.

Understanding Purity:
Elements and compounds are both pure substances. A pure substance consists of only one type of material (one type of atom or one type of molecule). Mixtures are impure because they contain two or more types of materials that aren't fixed.

The Three Key Differences

1. Composition (What are they made of?):

• Element: Contains only one type of atom.
• Compound: Contains two or more different elements, chemically bonded.
• Mixture: Contains two or more substances (elements or compounds), physically combined.

2. Ratio/Proportion:

• Element: Not applicable (it is one substance).
• Compound: Components must be present in a fixed, definite ratio.
• Mixture: Components can be mixed in any variable ratio.

3. Separation and Properties:

• Compound: Can only be separated by chemical reactions. Properties are new and distinct from the starting elements.
• Mixture: Can be separated by physical methods (like filtering or evaporating). Components keep their original properties.

Memory Trick: C vs M

Don't worry if this seems tricky at first. Remember this simple contrast:
The 'C' in Compound stands for Chemical bond and Chemical separation.
The 'M' in Mixture stands for Mingled (mixed) and can be separated by Mechanical (physical) means.