Chapter 2: Is Matter Around Us Pure?
This comprehensive study material is fully aligned with the KVS Class IX Science Student Support Material (2025-26). It details all foundational concepts of pure substances, mixtures, solutions, colloids, and suspensions, with complete solved questions from the official bank.
1. What is a Substance?
A pure substance is a single kind of matter which cannot be separated into other kinds of matter by any physical method.
All substances around us have different shapes, sizes, and textures. Everything in the universe is made up of matter. Pure substances can be classified into elements and compounds.
- Elements: Sodium ($\mathrm{Na}$), Hydrogen ($\mathrm{H}$), Magnesium ($\mathrm{Mg}$), Gold ($\mathrm{Au}$).
- Compounds: Sodium chloride ($\mathrm{NaCl}$), Silver nitrate ($\mathrm{AgNO_3}$), Potassium sulphate ($\mathrm{K_2SO_4}$).
2. Elements
The term "element" was first used by Robert Boyle (1661). Antoine L. Lavoisier established the first useful scientific definition of an element: The basic form of a substance which cannot be broken down into simpler substances by chemical reactions.
- An element is made up of one kind of atom only.
- Elements are arranged systematically in the Modern Periodic Table. There are currently 118 elements.
Classification of Elements based on Physical State (at Room Temperature)
- Solid Elements: Gold ($\mathrm{Au}$), Silver ($\mathrm{Ag}$), Copper ($\mathrm{Cu}$), Platinum ($\mathrm{Pt}$), Lead ($\mathrm{Pb}$), Sodium ($\mathrm{Na}$), Iodine ($\mathrm{I}$), etc.
- Liquid Elements: Only two elements are liquid at room temperature: Mercury ($\mathrm{Hg}$) and Bromine ($\mathrm{Br}$). However, Gallium ($\mathrm{Ga}$) and Caesium ($\mathrm{Cs}$) also become liquid at temperatures slightly above room temperature ($>30^{\circ}\mathrm{C}$) and can melt on our palm.
- Gaseous Elements: There are 11 elements found in the gaseous state at room temperature. These are: Hydrogen ($\mathrm{H}$), Oxygen ($\mathrm{O}$), Nitrogen ($\mathrm{N}$), Fluorine ($\mathrm{F}$), Chlorine ($\mathrm{Cl}$), and the 6 noble gases—Helium ($\mathrm{He}$), Neon ($\mathrm{Ne}$), Argon ($\mathrm{Ar}$), Krypton ($\mathrm{Kr}$), Xenon ($\mathrm{Xe}$), and Radon ($\mathrm{Rn}$).
Classification of Elements based on Characteristics
i) Metals
Elements that can lose electrons easily and carry a positive charge are called metals. Properties include:
- Lustre: Possess a shiny surface.
- Malleable: Can be beaten into thin sheets (e.g., Gold is highly malleable).
- Ductile: Can be drawn into thin wires.
- Sonorous: Generate a ringing sound when hit.
- Conduction: Good conductors of heat and electricity.
ii) Non-metals
Elements that can gain electrons easily and carry a negative charge are called non-metals. Properties include:
- Non-lustrous (dull appearance).
- Non-malleable and non-ductile.
- Non-sonorous.
- Bad conductors of heat and electricity.
iii) Metalloids
Elements which possess characteristics of both metals and non-metals are called metalloids (or semi-solids). They are located along a zig-zag line separating metals and non-metals in the periodic table. Examples: Boron ($\mathrm{B}$), Silicon ($\mathrm{Si}$), Germanium ($\mathrm{Ge}$), etc.
3. Compounds vs. Mixtures
| Property | Compound | Mixture |
|---|---|---|
| Definition | Pure substance formed by two or more elements chemically combined in a fixed proportion by mass. | Combination of two or more substances physically mixed but not chemically combined. |
| Composition | Always fixed proportion by mass. | Variable composition. |
| Formula | Has a definite chemical formula (e.g., $\mathrm{NaCl}$). | Does not have any chemical formula. |
| Separation | Can only be broken down into constituent elements by chemical or electrochemical reactions. | Can be separated into constituents by physical methods. |
| Properties | Properties are entirely different from those of its constituent elements. | Shows the properties of its constituent substances. |
| Examples | Water ($\mathrm{H_2O}$), Carbon dioxide ($\mathrm{CO_2}$), Sodium chloride ($\mathrm{NaCl}$). | Fruit juice, sugar solution, soil, air, copper sulphate solution. |
4. Types of Mixtures
i) Homogeneous Mixture
A mixture in which the constituent substances are uniformly mixed throughout, leaving no visible boundaries of separation. Solutions are homogeneous mixtures.
Examples: Salt dissolved in water, sugar dissolved in water, air.
ii) Heterogeneous Mixture
A mixture that contains physically distinct parts and has a non-uniform composition, leaving visible boundaries of separation between constituents.
Examples: Sand and common salt, oil and water, muddy water. Heterogeneous mixtures can be divided into suspensions and colloids.
5. Concentration of a Solution
Solutions are homogeneous mixtures consisting of a solute (the substance dissolved, present in lesser quantity) and a solvent (the dissolving medium, present in greater quantity).
- Dilute Solution: Contains a relatively small amount of solute in a large volume of solvent.
- Concentrated Solution: Contains a relatively large amount of solute.
- Saturated Solution: A solution that has dissolved the maximum amount of solute possible at a particular temperature.
- Unsaturated Solution: A solution containing less than the maximum saturation amount of solute at that temperature.
- Solubility: The maximum amount of solute present in a saturated solution at a given temperature is called its solubility.
Formulas for Expressing Concentration
a) Mass by Mass percentage:
Note: $\text{Mass of solution} = \text{Mass of solute} + \text{Mass of solvent}$.
b) Mass by Volume percentage:
c) Volume by Volume percentage:
6. Suspensions and Colloids
Suspension
A heterogeneous mixture in which solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particle size is greater than $100\mathrm{~nm}$ ($10^{-7}\mathrm{~m}$). Examples: Chalk powder in water, muddy water, milk of magnesia, paints.
Colloid (Colloidal Solution)
A heterogeneous mixture in which particles are uniformly spread throughout but are too small to be seen by the naked eye. Particle size ranges between $1\mathrm{~nm}$ and $100\mathrm{~nm}$. Examples: Milk, blood, smoke, face cream.
The phenomenon of scattering of a beam of light as it passes through a colloid is called the Tyndall effect. It was discovered by John Tyndall in 1859. This effect is also observed when a fine beam of light enters a dark room through a small hole, owing to scattering by dust and smoke particles in the air.
Classification of Colloids
Colloids are classified based on the state of the dispersed phase (solute-like component) and the dispersing medium (solvent-like component):
| Dispersed Phase | Dispersing Medium | Type of Colloid | Real-Life Examples |
|---|---|---|---|
| Solid | Solid | Solid Sol | Milky glass, coloured gemstones |
| Solid | Liquid | Sol | Milk of magnesia, mud, starch solution, gold sol |
| Solid | Gas | Aerosol | Smoke, dust storm, automobile exhaust |
| Liquid | Solid | Gel | Jelly, cheese, butter, curd, boot polish |
| Liquid | Liquid | Emulsion | Milk, face cream, liquid tonics |
| Liquid | Gas | Aerosol | Fog, clouds, mist, insecticide sprays |
| Gas | Solid | Foam | Froth, rubber, sponge, bread |
| Gas | Liquid | Foam | Soap bubbles, shaving cream, soda water |
Note: Gas-gas mixtures are not possible as colloids because gas-gas mixtures are homogeneous (true gaseous solutions).
Comparison: True Solution, Colloid, and Suspension
| Property | True Solution | Colloid | Suspension |
|---|---|---|---|
| Nature | Homogeneous | Heterogeneous | Heterogeneous |
| Particle Size | Less than $1\mathrm{~nm}$ ($10^{-9}\mathrm{~m}$) | Between $1\mathrm{~nm}$ and $100\mathrm{~nm}$ | Greater than $100\mathrm{~nm}$ |
| Visibility | Invisible to naked eyes and microscope | Invisible to naked eyes | Visible to the naked eye |
| Filterability | Passes through filter paper | Passes through filter paper | Cannot pass through filter paper |
| Stability | Highly stable (does not settle) | Stable (does not settle) | Unstable (settles down) |
| Tyndall Effect | Does not show | Shows | Shows |
| Diffusion | Diffuses rapidly | Diffuses slowly | Does not diffuse |
7. Physical vs. Chemical Changes
| Physical Change | Chemical Change |
|---|---|
| Only physical properties (color, state, density, volume) change; chemical identity is unchanged. | Chemical composition and molecular properties undergo a complete change. |
| No new substance is formed. | A completely new substance is formed with unique properties. |
| Very little or no energy (heat, light, sound) is absorbed or given out. | Accompanied by a distinct absorption or release of energy. |
| Temporary change and easily reversible. | Permanent change and generally irreversible. |
| Original substance can be regained by simple physical methods. | Original substance cannot be regained by simple physical methods. |
8. Chapterwise Question Bank
Multiple Choice Questions (1 Mark Each)
MCQ 1 Which of the following best describes a pure substance?
(a) A substance composed of two or more elements physically mixed
(b) A substance composed of only one type of atom or molecule
(c) A substance that can be separated into components by physical means
(d) A substance that has variable composition
MCQ 2 Which of the following is an example of a homogeneous mixture?
(a) Sand and water (b) Oil and water (c) Sugar dissolved in water (d) Salt and pepper
MCQ 3 Which of the following is a method used to separate a mixture of ammonium chloride and sodium chloride?
(a) Filtration (b) Sublimation (c) Centrifugation (d) Distillation
MCQ 4 Which of the following substances, on sufficient cooling, produces a solid which undergoes sublimation under normal conditions?
(a) Water vapour (b) Ammonia (c) Carbon dioxide (d) Chlorine
MCQ 5 For which of the following mixtures, the same method of separation can be followed as a mixture of naphthalene and sand?
(a) Iron filings and camphor (b) Iodine in carbon tetrachloride (c) Sulphur in carbon disulphide (d) Camphor and ammonium chloride
MCQ 6 A student crushed a piece of chalk and mixed the chalk powder in 100 mL water. The water appeared white and cloudy. After some time the particles settled at the bottom of the container. She claims that the mixture is a suspension. What justifies her claim?
(a) The particles mix completely with water.
(b) The particles of chalk form a separate layer.
(c) The particles of chalk are visible through the naked eye.
(d) The particles of chalk are uniformly distributed in water.
Assertion-Reason Based Questions (1 Mark Each)
Directions: Choose (a) if both A and R are true and R is correct explanation of A; (b) if both A and R are true but R is not correct explanation; (c) if A is true but R is false; (d) if A is false but R is true.
A/R 7
Assertion: When a beam of light is passed through a colloidal solution placed in a dark place, the path of the beam becomes visible.
Reason: Light gets scattered by the colloidal particles.
A/R 8
Assertion: A mixture of sugar and benzoic acid can be separated by shaking with ether.
Reason: Sugar is insoluble in water.
A/R 9
Assertion: A mixture of sand and water can be separated by filtration.
Reason: Sand particles are larger than water molecules.
A/R 10
Assertion: Pure water is a compound.
Reason: Pure water has a fixed composition and properties.
Very Short Answer Questions (2 Marks Each)
Q11 Sucrose (sugar) crystals obtained from sugarcane and beetroot are mixed together. Will it be a pure substance or a mixture? Give reasons for the same.
Q12 Identify the physical changes from the examples given below: Melting of iron metal, rusting of iron, bending of iron rod, drawing a wire of iron metal.
Q13 Mohnish has a saturated solution in a beaker and he wants to convert it into an unsaturated solution. Suggest a method for it.
Q14 A solution of alcohol in water has been prepared by mixing $150\mathrm{~mL}$ of alcohol with $600\mathrm{~mL}$ of water. Calculate the volume percentage of the solution.
Q15 Copper sulphate solution in water does not show the Tyndall effect, but a mixture of water and milk shows it. Why?
Short Answer Questions (3 Marks Each)
Q16 (a) Give any one point of difference between true solution, colloidal solution, and suspension.
(b) $20\mathrm{~g}$ of sodium chloride is dissolved in $100\mathrm{~mL}$ of water. How will you test whether the given solution is saturated or unsaturated at the given temperature?
(c) Suggest any one method by which we can increase the solubility of saturated solutions.
Q17 (a) List any three characteristics of a colloid.
(b) Name the two components of a colloid.
(c) Identify the colloids from the following mixtures: Muddy water, sugar in water, ink, blood, soda water, foam.
Long Answer Questions (5 Marks Each)
Q18 Three students A, B and C prepared mixtures using chalk powder, common salt and milk respectively in water. Whose mixture:
(i) would not leave residue on filter paper after filtration?
(ii) would show Tyndall effect?
(iii) would give transparent/clear solution?
(iv) would settle down at the bottom when left undisturbed?
(v) could be filtered by filter paper?
Q19 During an experiment, students were asked to prepare a $10\mathrm{\%}$ (Mass/Mass) solution of sugar in water. Mohnish dissolved $10\mathrm{~g}$ of sugar in $100\mathrm{~g}$ of water, while Harshita prepared it by dissolving $10\mathrm{~g}$ of sugar in water to make $100\mathrm{~g}$ of solution.
(a) Are the two solutions of the same concentration?
(b) Compare the mass % of the two solutions.
Case Study Based Questions (4 Marks Each)
Case Study 20 A group of students took an old shoe box and covered it with a black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a tumbler in the box as shown in the figure. They were amazed to see that milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it.
Questions:
1. Explain why the milk sample was illuminated. Name the phenomenon involved.
2. Same results were not observed with a salt solution. Explain.
3. Can you suggest two more solutions which would show the same effect as shown by the milk solution?
4. Give one example of the above phenomenon observed in our surroundings.
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