1. Overview
Everything in the universe is made of matter, and all matter is composed of atoms. This topic explains how we classify substances based on whether they contain only one type of atom (elements), different atoms bonded together (compounds), or different substances simply tangled together (mixtures). Understanding these differences is fundamental to predicting how substances will behave and how they can be separated.
Key Definitions
- Atom: The smallest particle of an element that can take part in a chemical reaction.
- Element: A pure substance made of only one type of atom. It cannot be broken down into simpler substances by chemical means.
- Compound: A substance made of two or more different elements chemically joined (bonded) together in a fixed ratio.
- Mixture: A combination of two or more substances (elements or compounds) that are physically mixed but not chemically joined.
- Molecule: Two or more atoms chemically bonded together (can be the same element, e.g., $O_2$, or different elements, e.g., $H_2O$).
Core Content
Characteristics of Elements
- Elements are the basic building blocks of all matter.
- They are found on the Periodic Table (e.g., Carbon (C), Oxygen (O), Iron (Fe)).
- A box filled with identical single circles representing atoms of a monatomic element like Neon, and a box with pairs of identical joined circles representing a diatomic element like Oxygen ($O_2$)
Characteristics of Compounds
- Fixed Composition: Elements in a compound are always present in a fixed ratio (e.g., $H_2O$ always has two Hydrogen atoms for every one Oxygen atom).
- Chemical Bonding: Elements are joined by strong chemical bonds; energy (usually heat) is often required to form or break them.
- New Properties: A compound has completely different physical and chemical properties from the elements it is made of.
- Example: Sodium ($Na$) is a reactive metal and Chlorine ($Cl_2$) is a poisonous gas. Together they form Sodium Chloride ($NaCl$), which is harmless table salt.
- Separation: Can only be separated into elements by chemical reactions or electrolysis.
Characteristics of Mixtures
- Variable Composition: Substances can be present in any ratio (e.g., a cup of tea can have one or three spoons of sugar).
- No Chemical Bonding: Substances are not joined together.
- Retained Properties: Each substance in a mixture keeps its own original properties.
- Separation: Can be easily separated by physical methods (e.g., filtration, distillation, chromatography, or using a magnet).
- A box showing a variety of different shapes and colors—some single atoms, some bonded molecules—not organized in any specific pattern.
Summary Comparison Table
| Feature | Element | Compound | Mixture |
|---|---|---|---|
| Composition | Only one type of atom | Two or more types of atoms chemically bonded | Two or more substances physically combined |
| Properties | Unique to that element | Different from its component elements | Same as its components |
| Separation | Cannot be simplified | Chemical reactions or electrolysis | Physical methods (e.g. filtration) |
| Melting/Boiling Point | Fixed | Fixed | Over a range of temperatures |
Extended Content (Extended Only)
There are no specific "Supplement" learning objectives listed for sub-topic 2.1 in the current IGCSE Chemistry syllabus. All content in this section is Core.
Key Equations
While this topic is conceptual, equations are used to demonstrate the formation of compounds from elements.
1. Formation of Iron(II) Sulfide
- Word Equation: Iron + Sulfur $\rightarrow$ Iron(II) sulfide
- Symbol Equation: $Fe(s) + S(s) \rightarrow FeS(s)$
- Note: The mixture of Fe and S can be separated by a magnet. Once heated and reacted into $FeS$, it is no longer magnetic.
2. Formation of Water
- Word Equation: Hydrogen + Oxygen $\rightarrow$ Water
- Symbol Equation: $2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$
- Note: Hydrogen and Oxygen are explosive gases; water is a stable liquid used to put out fires.
Common Mistakes to Avoid
- ❌ Wrong: Thinking that $O_2$ or $N_2$ are compounds because they have two atoms.
- âś“ Right: $O_2$ is a molecule, but it is still an element because the atoms are the same type.
- ❌ Wrong: Saying that mixtures have a fixed melting point.
- âś“ Right: Only pure elements and compounds have sharp, fixed melting points. Mixtures melt over a range of temperatures.
- ❌ Wrong: Using the word "mixture" to describe a compound like $CO_2$.
- âś“ Right: $CO_2$ is a compound because the Carbon and Oxygen are chemically bonded.
Exam Tips
- Command Words: If a question asks you to "Distinguish between a compound and a mixture," always provide a comparison for both (e.g., "A compound has a fixed composition, whereas a mixture has a variable composition").
- Identifying Diagrams: In the exam, you will often see boxes with circles.
- Identical circles = Element.
- Two different circles joined together = Compound.
- Different circles/groups not joined together = Mixture.
- Real-world Context: Air is a common exam example. Remember: Air is a mixture of gases (Nitrogen, Oxygen, Argon, Carbon Dioxide, etc.) because they are not bonded to each other and can be separated by physical means (fractional distillation).
- Fixed Points: If a graph shows a substance melting at exactly $0^\circ C$, it is a pure substance (element or compound). If it melts from $-2^\circ C$ to $0^\circ C$, it is a mixture.