Is Matter Around Us Pure
Most of the matter around us is not pure but a mixture of substances. This Class 9 chapter explains the difference between pure substances (elements and compounds) and mixtures, classifies solutions, suspensions and colloids, introduces the Tyndall effect, and surveys the methods used to separate the components of a mixture.
Learning objectives
- Distinguish pure substances from mixtures.
- Differentiate elements and compounds.
- Classify mixtures as homogeneous or heterogeneous.
- Describe solutions, suspensions and colloids and the Tyndall effect.
- Choose suitable methods to separate mixtures.
Key concepts
Pure substances: elements and compounds
A pure substance has a fixed composition. An element is made of one kind of atom and cannot be broken down chemically (e.g. iron, oxygen). A compound is formed when elements combine chemically in a fixed ratio (e.g. water), and its properties differ from those of its elements.
Mixtures
A mixture contains two or more substances in any ratio, not chemically combined. Homogeneous mixtures have a uniform composition (e.g. salt solution); heterogeneous mixtures do not (e.g. sand in water).
Solutions, suspensions and colloids
A solution is a homogeneous mixture of a solute in a solvent with very small particles. A suspension has large, visible particles that settle. A colloid has medium-sized particles that do not settle and scatter light (the Tyndall effect), e.g. milk.
Separation techniques
Components of mixtures are separated by methods such as filtration, evaporation, distillation, centrifugation, chromatography, sublimation and using a separating funnel — the choice depends on the properties that differ between the components.
Important formulas
Concentration (mass %)
= (mass of solute ÷ mass of solution) × 100
Key definitions
- Element
- A pure substance made of only one kind of atom.
- Compound
- A substance formed by the chemical combination of elements in a fixed ratio.
- Solution
- A homogeneous mixture of a solute dissolved in a solvent.
- Colloid
- A mixture with medium-sized particles that do not settle and scatter light (Tyndall effect).
Solved examples
Q1. How is a compound different from a mixture?
Solution: In a compound, elements combine chemically in a fixed ratio and the properties change; in a mixture, substances are simply mixed in any ratio, keep their own properties, and can be separated physically.
Q2. How would you separate salt from a salt solution?
Solution: By evaporation — heat the solution so the water evaporates, leaving the salt behind as a solid.
Q3. Why does the path of a torch beam become visible through milk?
Solution: Milk is a colloid; its medium-sized particles scatter the light, making the beam's path visible — this is the Tyndall effect.
Common mistakes to avoid
- Calling air or salt solution a compound — they are mixtures.
- Thinking the particles of a solution can be seen or filtered (they cannot).
- Confusing a colloid with a true solution — colloids show the Tyndall effect.
- Believing a compound's properties are the same as its elements'.
Is Matter Around Us Pure — MCQ Quiz
12 questions with instant feedback. Use number keys 1–4 to answer.
A substance made of only one type of atom is a/an:
Practice questions
Short answer
Give one example each of an element and a compound.
Element: oxygen; Compound: water (H₂O).
What is the Tyndall effect?
The scattering of a beam of light by the particles of a colloid, making its path visible.
Name a method to separate iron filings from sand.
Magnetic separation (using a magnet).
Long answer
Compare the properties of solutions, suspensions and colloids.
Solutions are homogeneous with very small particles that do not settle, pass through filter paper, and do not scatter light. Suspensions are heterogeneous with large particles that settle, can be filtered, and are visible. Colloids are heterogeneous-looking but stable, with medium particles that do not settle, pass through filter paper, and scatter light (Tyndall effect).
Describe how a mixture of salt, sand and iron filings can be separated.
First use a magnet to remove the iron filings. Then add water to dissolve the salt and filter; the sand stays on the filter paper. Finally, evaporate the filtrate to recover the salt. Each step exploits a different property — magnetism, solubility and volatility.
HOTS (Higher Order Thinking)
Why can the components of a mixture be separated by physical methods but those of a compound cannot?
In a mixture the substances are not chemically bonded, so physical means suffice; in a compound the elements are chemically combined, so only a chemical reaction can break them apart.
Air is considered a mixture, not a compound. Give one reason.
Its components (nitrogen, oxygen, etc.) are present in variable proportions, are not chemically combined, and each keeps its own properties — all features of a mixture.
Quick revision
Revision notes
- Pure substance = element or compound (fixed composition).
- Mixture = any-ratio combination; homogeneous or heterogeneous.
- Solution (no scatter) vs colloid (Tyndall) vs suspension (settles).
- Separation: filtration, evaporation, distillation, chromatography, etc.
Key takeaways
- Compounds have fixed ratios and new properties; mixtures do not.
- The Tyndall effect distinguishes colloids from true solutions.
- Match the separation method to the differing property.
Frequently asked questions
What is the difference between an element and a compound?
An element has only one kind of atom; a compound is formed by chemically combining elements in a fixed ratio.
What is a colloid?
A mixture with medium-sized particles that do not settle and scatter light, showing the Tyndall effect — like milk.
How is salt obtained from sea water?
Usually by evaporation of the water, leaving the salt behind.