2.2 Elements, Compounds and Mixtures

This topic explains how matter is classified. All matter around us can be sorted into a few basic categories based on what it is made of.

Definition

Element

An element consists of atoms that have the same atomic number (also known as the proton number). This is the simplest form of matter which cannot be broken down through chemical means.

Simple explanation: An element is made of only one type of atom.

Example: Hydrogen (H), Oxygen (O), Sodium (Na), Carbon (C).

Compound

A compound is a substance formed when two or more different atoms chemically combine.

Simple explanation: A compound is formed when different elements react together chemically to make a completely new substance.

Example: Water (H₂O), Carbon dioxide (CO₂), Sodium chloride (NaCl).

Mixture

A mixture is a substance formed when two or more substances physically combine.

Simple explanation: In a mixture, substances are mixed together but not chemically bonded.

Example: Tea (milk + water + tea leaves + sugar), Air (nitrogen + oxygen + other gases).

Main Idea

Matter can be described with both physical properties and chemical properties. Matter can be classified as:

• a) Element
• b) Compound

• a) Homogeneous
• b) Heterogeneous mixtures:
• i) Colloid
• ii) Suspension

Simple explanation: Matter is either a pure substance (only one type of particle) or a mixture (more than one type of particle).

Core Concept

Physical Change vs Chemical Change

The book reminds us:

• A physical change alters the physical properties of a substance.
• A chemical change forms a new substance completely.

Simple explanation: In a physical change, the substance stays the same (like ice melting). In a chemical change, a new substance forms (like iron rusting).

Atom and Element

Earlier, we talked about the atom and how atoms make up all of matter. The same types of atoms are called elements.

• Atom = smallest particle of matter
• Element = made up of the same type of atoms

Compound Formation

When two or more elements chemically combine — meaning undergo a chemical reaction to form a new substance — this is called a compound.

As this is a completely new substance, it is completely different from the elements that were used to make it.

Simple explanation: Sodium (a soft, dangerous metal) + Chlorine (a poisonous gas) = Sodium chloride (common table salt). The compound is completely different from its elements.

Mixture Formation

Mixtures are the physical combinations of substances. A mixture does not contain the same types of particles. If you were to examine the chemical composition of the particles in a mixture, the particles would be chemically different from each other.

Example: Tea is made up of milk, water, tea leaves, and sugar — all of which have different chemical compositions.

Intermediate Level

Comparison Table

Why Compounds Are Different from Mixtures

• Compound: The properties of a compound are completely different from the elements that made it.
• Example: Na (soft metal) + Cl (poisonous gas) → NaCl (edible salt)

• Mixture: The properties of a mixture are a combination of the properties of its components.
• Example: Air shows properties of nitrogen, oxygen, and other gases mixed together.

Types of Mixtures

The book mentions homogeneous and heterogeneous mixtures:

• Homogeneous mixture: Same composition throughout (e.g., salt solution, air).
• Heterogeneous mixture: Different composition in different parts (e.g., sand in water).

• Colloid (particles 1–1000 nm; e.g., milk, blood)
• Suspension (particles > 1000 nm; e.g., chalk in water, muddy water)

Note: These are covered in detail in Section 2.5 (Colloids & Suspensions).

Important Points

• - Element: simplest form of matter; made of same type of atoms; cannot be broken down chemically.
• - Compound: formed when two or more different atoms chemically combine; new substance with new properties.
• - Mixture: formed when two or more substances physically combine; components retain their properties.
• - Physical change: alters physical properties; no new substance.
• - Chemical change: forms a new substance completely.
• - Classification of matter:
• - Pure substance → Element, Compound
• - Mixture → Homogeneous, Heterogeneous (Colloid, Suspension)

Remember

• - Element = same atoms.
• - Compound = different atoms chemically bonded.
• - Mixture = substances physically mixed.
• - Compound properties ≠ element properties.
• - Mixture properties = combination of component properties.
• - Physical change → no new substance.
• - Chemical change → new substance.

Common Mistakes

• 1. Thinking a compound is just a mixture. A compound is chemically bonded; a mixture is not.
• 2. Thinking elements can be broken down. Elements cannot be broken down by chemical means.
• 3. Confusing "different atoms" with "different substances." A compound needs different atoms; a mixture has different substances.
• 4. Thinking a mixture has fixed composition. Mixtures can have any ratio of components; compounds have a fixed ratio.
• 5. Confusing physical and chemical changes. Physical change → no new substance; chemical change → new substance.

Quick Revision

• Matter is classified as pure substance (element, compound) or mixture (homogeneous, heterogeneous).
• Element: same atoms; simplest form; cannot be broken down chemically.
• Compound: different atoms chemically combined; new substance; fixed ratio.
• Mixture: substances physically combined; variable ratio; retains properties.
• Physical change: alters physical properties; no new substance.
• Chemical change: forms new substance.
• Heterogeneous mixtures: colloid and suspension.

60-Second Revision

• Element = same type of atoms.
• Compound = different atoms chemically bonded.
• Mixture = substances physically mixed.
• Compound has new properties; mixture retains component properties.
• Physical change → no new substance.
• Chemical change → new substance.
• Warning: a compound is not just a mixture — it is chemically bonded.