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CBSE Class 9 : Science : Chapter-2 : "IS MATTER AROUND US PURE ?"
Pure Substance: A substance made of only one kind of particle; it has a uniform composition and properties. Examples include elements (like iron, oxygen) and compounds (like water, carbon dioxide).
Mixture: Consists of two or more different types of particles. Mixtures can be homogeneous or heterogeneous.
2. Types of Mixtures
- Homogeneous Mixtures: Uniform composition throughout. Particles are mixed so well that they can't be distinguished. Example: Saltwater, where salt is evenly dissolved in water.
- Heterogeneous Mixtures: Non-uniform composition; particles are visible and unevenly distributed. Example: Sand in water, where sand particles are visible and not fully mixed with water.
3. Separation of Mixtures
Methods of Separation: Techniques depend on the physical properties of components in the mixture:
- Filtration: Separates insoluble solids from liquids (e.g., sand from water).
- Evaporation: Removes a liquid from a solution to leave the solute behind (e.g., salt from saltwater).
- Distillation: Separates substances based on different boiling points (e.g., water and alcohol).
- Centrifugation: Separates particles in a liquid based on density (e.g., separating cream from milk).
- Sublimation: Separates substances that sublimate on heating (e.g., separating iodine from a mixture).
4. Types of Pure Substances
Elements: Consist of only one type of atom. Cannot be broken down into simpler substances. Divided into metals (like iron), non-metals (like sulfur), and metalloids (like silicon).
Compounds: Formed by chemically combining two or more elements in a fixed ratio. Properties of a compound differ from those of the individual elements. Example: Water (H₂O), where hydrogen and oxygen chemically combine to form water.
5. Solutions, Suspensions, and Colloids
Solution: Homogeneous mixture with a solute dissolved in a solvent. Particles are very small and cannot be seen (e.g., sugar in water).
Suspension: Heterogeneous mixture with large, visible particles that settle over time (e.g., sand in water).
Colloid: Appears homogeneous but is actually heterogeneous with intermediate-sized particles that do not settle (e.g., milk).
6. Properties of Colloidal Solutions
Tyndall Effect: Colloidal particles scatter light, making the path of light visible (e.g., light beam visible in fog).
Brownian Motion: The random, constant movement of colloidal particles in a fluid due to collisions.
7. Concentration of a Solution
Concentration refers to the amount of solute in a given amount of solution. Methods to Express Concentration:
- Mass by mass percentage.
- Mass by volume percentage.
- Volume by volume percentage.
8. Alloys and Their Importance
Alloy: A homogeneous mixture of two or more metals, or metals and non-metals. Alloys have improved properties (e.g., strength, resistance to corrosion).
Example: Steel (an alloy of iron and carbon).
9. Physical and Chemical Changes
Physical Change: No new substance forms; changes in state or appearance (e.g., melting, dissolving).
Chemical Change: A new substance forms with different properties (e.g., burning, rusting).
Key Points to Remember
- Mixtures can be separated by physical methods; pure substances cannot.
- Solutions are homogeneous, suspensions are heterogeneous, and colloids have unique properties (like the Tyndall Effect).
- The concentration of solutions can be measured in different ways, important in chemistry applications.
- Alloys are practical applications of mixtures for industrial uses.
A. Multiple Choice Questions (MCQs)
1. Which of the following is a homogeneous mixture?
a) Sand and salt
b) Sugar and water
c) Oil and water
d) Sand in water
Answer: (b) Sugar and water
2. Which method can be used to separate a mixture of salt and water?
a) Filtration
b) Evaporation
c) Sublimation
d) Distillation
Answer: (b) Evaporation
3. Tyndall effect is shown by which of the following?
a) Solution
b) Colloid
c) Suspension
d) Compound
Answer: (b) Colloid
4. An example of a chemical change is:
a) Melting of ice
b) Dissolving sugar in water
c) Rusting of iron
d) Boiling of water
Answer: (c) Rusting of iron
B. Fill in the Blanks
1. A substance made of only one kind of particle is called a ______.
Answer: Pure substance
2. ______ is a method used to separate cream from milk.
Answer: Centrifugation
3. The smallest particles of a solution are not visible under a microscope and do not show ______ effect.
Answer: Tyndall
4. ______ is the process used to separate volatile and non-volatile substances from a mixture.
Answer: Distillation
5. The concentration of a solution is measured in terms of ______.
Answer: mass by mass, mass by volume, or volume by volume percentage.
C. True or False
1. A compound has properties that are different from its constituent elements.
Answer: True
2. Colloids are homogeneous mixtures.
Answer: False (they are heterogeneous but appear homogeneous)
3. Milk is an example of a suspension.
Answer: False (milk is a colloid)
4. Mixtures can only be separated by chemical methods.
Answer: False (mixtures can often be separated by physical methods)
5. In a solution, solute particles are visible to the naked eye.
Answer: False (solute particles in a solution are too small to be seen.
D. Very Short Answer Questions
1. What is a pure substance?
Answer: A pure substance consists of only one type of particle with uniform composition and properties throughout, like elements or compounds.
2. Name two types of mixtures.
Answer: Homogeneous and heterogeneous mixtures.
3. What is a colloid? Give one example.
Answer: A colloid is a heterogeneous mixture where particles are not visible and do not settle. Example: Milk.
4. Define the Tyndall effect.
Answer: The Tyndall effect is the scattering of light by colloidal particles, making the light beam visible.
E. Short Answer Questions
1. Differentiate between a compound and a mixture.
Answer: A compound is a pure substance with two or more elements chemically combined in a fixed ratio, having unique properties different from its elements (e.g., water). A mixture consists of two or more substances physically combined, where each retains its properties and can vary in composition (e.g., salt and sand).
2. Explain the method of separation used for separating a mixture of sand and water.
Answer: Filtration is used to separate sand (solid, insoluble) from water. The mixture is passed through filter paper, where sand remains as residue, and water passes as filtrate.
3. What is meant by ‘concentration of a solution’? How is it expressed?
Answer: Concentration refers to the amount of solute dissolved in a given quantity of solvent or solution. It is commonly expressed as a percentage, like mass by mass %, mass by volume %, or volume by volume %.
F. Long Answer Questions
1. Describe various methods for separating mixtures and give examples of each.
Answer:
- Filtration: Separates insoluble solids from liquids, e.g., sand from water.
- Evaporation: Used to separate soluble solids from liquids by heating, e.g., salt from saltwater.
- Distillation: Separates mixtures based on different boiling points, e.g., separating alcohol from water.
- Centrifugation: Separates components based on density by rapid spinning, e.g., cream from milk.
- Sublimation: Separates substances that can directly change from solid to gas, e.g., iodine from sand.
2. Explain the characteristics of a suspension, colloid, and solution with examples.
Answer:
- Suspension: A heterogeneous mixture with large particles that settle over time, e.g., sand in water. Particles can be filtered.
- Colloid: Appears homogeneous, particles do not settle or filter, and show the Tyndall effect, e.g., milk.
- Solution: A homogeneous mixture with solute completely dissolved in the solvent, e.g., saltwater. Particles are too small to be filtered or seen.
