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Elements, Compounds and Mixtures
ICSE Class 8 Chemistry • Chapter 3 (Detailed Master Notes)
Chapter Overview
If matter is the building material of the universe, then elements are the fundamental Lego bricks. By combining these pure bricks in different ways, we can create complex compounds or simple mixtures. In this chapter, we classify all matter into pure and impure substances based on their chemical composition.
3.1 Pure Substances: Elements
A pure substance is made of only one kind of particle (atoms or molecules). It has a fixed, definite composition and constant properties.
Element: The simplest form of a pure substance which cannot be broken down into anything simpler by any ordinary physical or chemical methods.
An element is made up of identically alike atoms. Example: A piece of pure Gold ($Au$) contains only gold atoms. Oxygen gas ($O_2$) contains only oxygen atoms.
Classification of Elements:
- Metals: Usually solid, hard, lustrous, malleable, ductile, and good conductors of electricity (e.g., Iron, Copper, Gold).
- Non-metals: Soft solids or gases, non-lustrous, brittle, and poor conductors of heat and electricity (e.g., Carbon, Oxygen, Sulphur).
- Metalloids: Elements showing properties of both metals and non-metals (e.g., Silicon, Germanium).
- Noble Gases: Highly unreactive gases found in traces in the air (e.g., Helium, Neon, Argon).
3.2 Pure Substances: Compounds
What happens when you chemically force two different elements together? You get a compound.
Compound: A pure substance formed by the chemical combination of two or more radically different elements in a fixed proportion by mass.
Key Characteristics of Compounds:
- Fixed Proportion: Water ($H_2O$) is always strictly formed by $2$ atoms of Hydrogen and $1$ atom of Oxygen.
- Completely New Properties: Hydrogen is highly explosive. Oxygen fiercely supports burning. But when they combine chemically, they form Water ($H_2O$), which violently extinguishes fire! The compound loses the properties of its constituent elements.
- Chemical Separation: A compound cannot be separated by physical means like filtration. You need chemical methods (like electrolysis) to break it apart.
3.3 Impure Substances: Mixtures
If you just toss two things together without chemically reacting them, you get a mixture.
Mixture: An impure substance formed by physically mixing two or more substances (elements or compounds) in any random proportion, where no chemical reaction takes place.
Types of Mixtures:
- Homogeneous Mixture: The components are uniformly mixed and cannot be visibly distinguished. (e.g., Salt completely dissolved in water, clean Air, Alloys like Brass).
- Heterogeneous Mixture: The components are unevenly mixed and can be seen separately. (e.g., Sand and water, raw salad, Iron filings mixed with Sulphur powder).
AI Image Prompt: A 3-part graphic showing beakers. Beaker 1 (Element): Filled strictly with identical red spheres. Beaker 2 (Compound): Filled with paired molecules, each pair mathematically consisting of one red sphere attached to two white spheres (water-like). Beaker 3 (Mixture): A chaotic, random jumble of loose red spheres, loose green cubes, and blue triangles scattered freely.
3.4 Differences: Compound vs Mixture
| Parameter | Compound | Mixture |
|---|---|---|
| Combination | Constituents are chemically combined. | Constituents are only physically mixed. |
| Proportion | Always in a fixed, definite ratio by mass. | Can be mixed in any random ratio. |
| Properties | Properties are entirely new and unique. | Retains the properties of individual constituents. |
| Separation | Can only be separated by strong chemical methods. | Can be easily separated by simple physical methods. |
Q1. Air is considered a mixture, while water is considered a compound. Justify.
Answer: Air is a physical blend of various gases (nitrogen, oxygen, etc.) in variable proportions. The gases retain their individual properties and can be separated by physical methods like fractional distillation. Water, however, is a compound formed by chemically combining hydrogen and oxygen in a fixed fixed mathematically set $1:8$ ratio by mass. Water has properties entirely different from its constituent gases and cannot be separated physically.
Q2. How can you prove that iron sulfide ($FeS$) is a compound, not a mixture of iron and sulfur?
Answer: First, if you bring a magnet near iron sulfide, it will not attract the iron, showing iron has lost its magnetic property. Second, if you add dilute acid to it, it produces a foul-smelling gas (hydrogen sulfide) rather than hydrogen gas (which pure iron would produce). Finally, its components are chemically bonded and cannot be separated physically.