Pure Substances and Mixtures
NCERT Class 9 • Science • Chapter "Is Matter Around Us Pure"
Pure Substances and Mixtures
A pure substance is made of only one kind of particle and has a fixed composition, while a mixture contains two or more substances physically mixed in any proportion. Almost everything we use daily is a mixture, which is why separating and identifying its parts is a basic skill in chemistry.
Elements, Compounds and Mixtures
- An element is a pure substance made of only one kind of atom and cannot be broken into simpler substances by chemical means.
- Elements are classified as metals, non metals and metalloids.
- A compound is formed when two or more elements combine chemically in a fixed ratio by mass.
- In a compound the properties are entirely different from those of its elements, as water differs from hydrogen and oxygen.
- A mixture keeps the properties of its components and can be separated by physical methods.
- Homogeneous mixtures have a uniform composition throughout, such as salt dissolved in water.
- Heterogeneous mixtures have visibly different parts, such as sand in water or oil in water.
Solutions, Colloids and Suspensions
- A solution is a homogeneous mixture of a solute dissolved in a solvent, with particle size so small that it cannot be seen.
- A solution does not scatter light, particles never settle, and it cannot be separated by filtration.
- Concentration is the amount of solute present in a given amount of solution.
- A suspension is heterogeneous with visible particles that settle on standing and can be filtered out.
- A colloid lies between the two: it looks homogeneous but its particles scatter a beam of light, which is the Tyndall effect.
- Milk, fog, smoke, ink and butter are common colloids.
- Colloids are named by the state of the dispersed phase and the medium, as in foam, emulsion, sol and aerosol.
- A saturated solution holds as much solute as it can at that temperature; beyond it the extra solute settles.
| Property | Solution | Colloid | Suspension |
|---|---|---|---|
| Particle size | Smallest | Intermediate | Largest |
| Tyndall effect | Not shown | Shown | Shown |
| Settling | Never settles | Does not settle | Settles on standing |
| Filtration | Cannot be filtered | Ordinary filter fails | Can be filtered |
Tyndall effect — the scattering of a beam of light by the particles of a colloid, which makes the path of the beam visible.
Exam me kaise aata hai
- Scattering of light by colloid particles is called — Tyndall effect
- Milk is an example of a — colloid
- The substance that dissolves is called the — solute
- Sand in water is an example of a — suspension
UPSC/State PSC ke liye
- A compound has a fixed ratio by mass while a mixture has none, which is the sharpest test between the two.
- The Tyndall effect explains why a beam of sunlight is visible in a dusty room and why headlights are visible in fog.
Yahan confuse hote hain
✗ Air is a compound | Air is a homogeneous mixture of gases | ✓
✗ A colloid settles on standing | A colloid does not settle; a suspension does | ✓
✗ Compounds keep the properties of their elements | Compounds have entirely different properties | ✓
Ek nazar me
- Pure substances are elements or compounds; mixtures are physical combinations.
- Compounds have a fixed ratio and new properties; mixtures do not.
- Solution is homogeneous with the smallest particles and shows no Tyndall effect.
- Colloid shows the Tyndall effect; suspension settles and can be filtered.