Is Matter Around Us Pure?
Welcome to HSLC Guru. This article presents complete English-medium notes, textbook question-answers, and additional practice questions for Class 9 Science Chapter 2 — Is Matter Around Us Pure? as per the ASSEB (Assam State School Education Board) syllabus. The chapter explores the difference between pure substances and mixtures, types of solutions and colloids, methods of separation, and the classification of pure substances into elements and compounds.
Chapter Summary
In everyday language we use the word “pure” loosely, but in science a pure substance means a single kind of particle. Milk, soft drinks, sea water, and even the air we breathe are not pure — they are mixtures of two or more substances. A mixture in which the components are completely uniform throughout is called a homogeneous mixture (for example, salt dissolved in water, brass, air). When the components are not uniform and we can see different parts, the mixture is heterogeneous (for example, sand and water, oil and water, soil).
A solution is a homogeneous mixture of two or more substances. The component present in larger amount is the solvent and the one present in smaller amount is the solute. The amount of solute present in a given amount of solvent or solution is called concentration. It is commonly expressed as mass percentage (mass of solute / mass of solution × 100) or volume percentage (volume of solute / volume of solution × 100). Solutions are classified into true solutions (particle size less than 1 nm, transparent, do not scatter light), suspensions (particle size more than 100 nm, particles visible, settle on standing) and colloids (particle size 1–100 nm, appear homogeneous but actually heterogeneous, scatter a beam of light — known as the Tyndall effect). Common types of colloids include sol (solid in liquid, e.g. paint), gel (liquid in solid, e.g. jelly), aerosol (liquid/solid in gas, e.g. fog, smoke), foam (gas in liquid, e.g. shaving cream) and emulsion (liquid in liquid, e.g. milk).
The components of a mixture can be separated by physical methods that depend on differences in physical properties. Important methods are: evaporation (separates a soluble solid from its liquid solvent, e.g. salt from sea water), centrifugation (separates fine suspended particles using a centrifuge, e.g. cream from milk), separating funnel (separates two immiscible liquids, e.g. oil and water), sublimation (separates a sublimable solid from a non-sublimable solid, e.g. ammonium chloride and salt), chromatography (separates dissolved coloured substances, e.g. dyes in ink), distillation (separates a solvent from a soluble solid or two miscible liquids whose boiling points differ greatly), fractional distillation (separates miscible liquids whose boiling points differ by less than 25 K, e.g. crude oil refining, separation of gases of air) and crystallisation (purifies solids — better than evaporation as it removes impurities and prevents decomposition).
Changes are of two types. In a physical change, only the physical properties (state, shape, size) change and no new substance is formed (e.g. melting of ice, dissolving sugar in water). In a chemical change, one or more new substances with new properties are formed (e.g. rusting of iron, burning of magnesium). On the basis of their chemical composition, pure substances are of two kinds. Elements are basic substances which cannot be broken into simpler substances by ordinary chemical means; they are classified as metals (e.g. iron, copper, aluminium), non-metals (e.g. carbon, sulphur, oxygen) and metalloids (e.g. silicon, germanium, arsenic) which show properties of both. Compounds are substances formed when two or more elements combine chemically in a fixed mass ratio (e.g. water H2O, common salt NaCl). The properties of a compound are entirely different from those of its constituent elements, while the components of a mixture retain their original properties.
Textbook Question Answers
1-Mark Questions
Q1. What is a pure substance?
Answer: A pure substance consists of only one kind of particle and has a fixed composition and definite properties. Examples: distilled water, gold, oxygen.
Q2. Define a homogeneous mixture.
Answer: A mixture in which the components are uniformly mixed and no boundary of separation can be seen is called a homogeneous mixture. Example: salt solution, air.
Q3. Give one example of a heterogeneous mixture.
Answer: A mixture of sand and salt is a heterogeneous mixture, in which both components are visible separately.
Q4. What is the Tyndall effect?
Answer: The scattering of a beam of light by the colloidal particles of a colloid, making the path of light visible, is called the Tyndall effect.
Q5. Name the solute and the solvent in a sugar solution.
Answer: In a sugar solution, sugar is the solute and water is the solvent.
Q6. Name a metalloid.
Answer: Silicon (Si) is a metalloid; it shows properties of both metals and non-metals.
Q7. What method is used to separate cream from milk?
Answer: The method of centrifugation is used to separate cream from milk.
Q8. Give one example each of a sol and a gel.
Answer: Sol — paint or muddy water. Gel — jelly or cheese.
Q9. What is meant by a saturated solution?
Answer: A solution in which no more solute can be dissolved at a given temperature is called a saturated solution.
Q10. Name two physical changes.
Answer: Melting of ice and dissolving sugar in water are physical changes — no new substance is formed.
2-3 Mark Questions
Q1. Differentiate between a homogeneous and a heterogeneous mixture with examples.
Answer: In a homogeneous mixture the components are uniformly mixed and there is no visible boundary; the composition is the same throughout. Example: salt solution, air, brass. In a heterogeneous mixture the components are not uniformly mixed and clear boundaries between them can be seen; the composition is not the same throughout. Example: sand in water, oil in water, soil.
Q2. Calculate the mass percentage of a solution containing 20 g of salt dissolved in 80 g of water.
Answer: Mass of solute (salt) = 20 g; Mass of solvent (water) = 80 g. Mass of solution = 20 + 80 = 100 g. Mass percentage of solute = (mass of solute / mass of solution) × 100 = (20/100) × 100 = 20%.
Q3. Distinguish between a true solution, a suspension and a colloid.
Answer: A true solution is homogeneous, transparent, has particles smaller than 1 nm, and does not show the Tyndall effect (e.g. salt solution). A suspension is heterogeneous, particles are larger than 100 nm, visible to the naked eye, and they settle down on standing (e.g. chalk in water). A colloid appears homogeneous but is actually heterogeneous, particles are 1–100 nm in size, the mixture does not settle and shows the Tyndall effect (e.g. milk, fog).
Q4. Differentiate between a physical and a chemical change with one example each.
Answer: In a physical change, no new substance is formed; only physical properties such as state, shape or size change and the change is usually reversible. Example: melting of ice into water. In a chemical change, one or more new substances with different properties are formed and the change is generally irreversible. Example: rusting of iron, where iron reacts with oxygen and moisture to form iron oxide.
Q5. Why is crystallisation a better method than simple evaporation for purifying a solid?
Answer: (i) During simple evaporation, some solids decompose on direct heating, while crystallisation avoids this. (ii) Some impurities may remain dissolved in the solution after evaporation, but during crystallisation only the pure solid forms crystals while impurities stay in the mother liquor. Hence crystallisation gives a purer product than evaporation.
Q6. Differentiate between a mixture and a compound.
Answer: A mixture contains two or more substances mixed in any ratio without chemical combination; the components keep their original properties and can be separated by physical methods (e.g. air). A compound is formed when two or more elements combine chemically in a fixed ratio; it has properties entirely different from those of its components and can be separated only by chemical or electrochemical means (e.g. water).
5-6 Mark Questions
Q1. Explain how the components of the following mixtures can be separated: (a) salt and water (b) two miscible liquids with close boiling points (c) ammonium chloride and sodium chloride (d) different coloured dyes in ink.
Answer: (a) Salt and water can be separated by evaporation — the solution is heated and water vaporises, leaving solid salt behind. (b) Two miscible liquids with close boiling points are separated by fractional distillation using a fractionating column, where the more volatile liquid vaporises first and is condensed separately. (c) Ammonium chloride and sodium chloride are separated by sublimation — on heating, ammonium chloride directly changes from solid to vapour and is collected on a cool surface, while sodium chloride remains. (d) Different dyes in ink are separated by chromatography, in which the dyes travel different distances on filter paper as the solvent rises, separating into coloured spots.
Q2. Describe the properties of colloids and give the various types of colloids with examples.
Answer: Properties of colloids — (i) Particle size is between 1 nm and 100 nm. (ii) Colloids appear homogeneous but are actually heterogeneous. (iii) Particles are big enough to scatter light and show the Tyndall effect. (iv) The particles do not settle on standing; they can be separated only by centrifugation. (v) Colloids are stable.
Types of colloids — Sol (solid dispersed in liquid, e.g. paint, milk of magnesia); Solid sol (solid in solid, e.g. coloured gemstones); Aerosol (liquid or solid in gas, e.g. fog, clouds, smoke); Emulsion (liquid in liquid, e.g. milk, face cream); Foam (gas in liquid or solid, e.g. shaving cream, sponge); Gel (liquid in solid, e.g. jelly, cheese, butter).
Q3. What are elements? Classify them into metals, non-metals and metalloids with the properties and examples of each.
Answer: An element is a basic form of matter that cannot be broken down into simpler substances by ordinary chemical means. There are 118 known elements. They are classified into three groups:
(i) Metals — These are usually solid (except mercury), have a metallic lustre, are malleable, ductile, sonorous, and good conductors of heat and electricity. They have high melting and boiling points. Examples: iron, copper, aluminium, gold, silver.
(ii) Non-metals — These may be solid, liquid or gas, are non-lustrous (except iodine), brittle, non-malleable, non-ductile, non-sonorous, and bad conductors of heat and electricity (except graphite). Examples: carbon, sulphur, oxygen, hydrogen, chlorine.
(iii) Metalloids — These show some properties of metals and some of non-metals. Examples: boron, silicon, germanium, arsenic, antimony.
Q4. Give the differences between a compound and a mixture in tabular form.
| Compound | Mixture |
|---|---|
| Formed by chemical combination of elements. | Formed by simple mixing of substances. |
| Components are present in a fixed ratio. | Components may be present in any ratio. |
| Properties differ from those of its constituents. | Properties are the same as those of its constituents. |
| Components cannot be separated by physical methods. | Components can be separated by physical methods. |
| Always homogeneous. | May be homogeneous or heterogeneous. |
| Energy is absorbed or released during formation. | No energy change usually takes place. |
Q5. Explain the principle of fractional distillation. How is it used to separate the gases of air?
Answer: Fractional distillation is used to separate two or more miscible liquids whose boiling points differ by less than 25 K. The mixture is heated in a flask fitted with a fractionating column filled with glass beads which provide surfaces for the vapours to cool and condense again and again. The liquid with the lower boiling point boils first, rises up the column, and is condensed separately while the others remain.
For separating the gases of air — air is first compressed and then cooled to obtain liquid air. Liquid air is then allowed to warm up in a fractionating column. Different gases have different boiling points; nitrogen (b.p. −196 °C) escapes first, followed by argon (b.p. −186 °C), and then oxygen (b.p. −183 °C). In this way the gases of air are obtained separately.
Additional Multiple Choice Questions
Q1. Which of the following is a pure substance?
(a) Milk (b) Sea water (c) Distilled water (d) Air
Answer: (c) Distilled water.
Q2. Tyndall effect is shown by:
(a) Sugar solution (b) Salt solution (c) Colloid (d) Water
Answer: (c) Colloid.
Q3. Which one of the following is a heterogeneous mixture?
(a) Brass (b) Air (c) Sand and water (d) Salt solution
Answer: (c) Sand and water.
Q4. The size of particles in a colloid is:
(a) Less than 1 nm (b) 1–100 nm (c) More than 100 nm (d) More than 1000 nm
Answer: (b) 1–100 nm.
Q5. Which of the following is a metalloid?
(a) Iron (b) Carbon (c) Silicon (d) Sulphur
Answer: (c) Silicon.
Q6. Butter is an example of:
(a) Sol (b) Gel (c) Aerosol (d) Foam
Answer: (b) Gel.
Q7. Which method is used to separate two immiscible liquids?
(a) Distillation (b) Centrifugation (c) Separating funnel (d) Sublimation
Answer: (c) Separating funnel.
Q8. Which of the following is a chemical change?
(a) Melting of wax (b) Boiling of water (c) Burning of paper (d) Dissolving sugar
Answer: (c) Burning of paper.
Q9. Fog is an example of which type of colloid?
(a) Sol (b) Gel (c) Aerosol (d) Emulsion
Answer: (c) Aerosol.
Q10. The component present in larger quantity in a solution is called:
(a) Solute (b) Solvent (c) Mixture (d) Compound
Answer: (b) Solvent.
Fill in the Blanks
Q1. A solution that has more solute dissolved than at room temperature is a __________ solution.
Answer: supersaturated.
Q2. The scattering of light by colloidal particles is called the __________ effect.
Answer: Tyndall.
Q3. Air is a __________ mixture of gases.
Answer: homogeneous.
Q4. Brass is an alloy of copper and __________.
Answer: zinc.
Q5. The method used to separate dyes in black ink is called __________.
Answer: chromatography.
True or False
Q1. A solution is always a homogeneous mixture.
Answer: True.
Q2. A colloid is a heterogeneous mixture in nature.
Answer: True.
Q3. Distilled water is a mixture.
Answer: False — distilled water is a pure compound (H2O).
Q4. Iodine is a metal.
Answer: False — iodine is a non-metal.
Q5. Burning of magnesium is a chemical change.
Answer: True.
Glossary
| Term | Meaning |
|---|---|
| Pure substance | A substance made up of only one kind of particle with fixed composition. |
| Mixture | A material made of two or more substances physically combined in any ratio. |
| Homogeneous mixture | A mixture with uniform composition throughout. |
| Heterogeneous mixture | A mixture with non-uniform composition with visible boundaries. |
| Solution | A homogeneous mixture of two or more substances. |
| Solute | The component present in smaller quantity in a solution. |
| Solvent | The component present in larger quantity in a solution. |
| Concentration | The amount of solute present in a given amount of solvent or solution. |
| Saturated solution | A solution that cannot dissolve any more solute at a given temperature. |
| Suspension | A heterogeneous mixture in which solid particles are dispersed in a liquid and settle on standing. |
| Colloid | A mixture in which one substance is finely dispersed in another, with particle size 1–100 nm. |
| Tyndall effect | The scattering of light by particles of a colloid. |
| Sol | Colloid of solid particles in a liquid. |
| Gel | Colloid of liquid in a solid medium. |
| Aerosol | Colloid of liquid or solid particles in a gas. |
| Foam | Colloid of gas dispersed in a liquid or solid. |
| Emulsion | Colloid of one liquid dispersed in another liquid. |
| Evaporation | Method of separating a soluble solid from its liquid solvent by heating. |
| Centrifugation | Separation of fine particles using high-speed rotation. |
| Sublimation | Direct change of a solid into vapour on heating. |
| Chromatography | Method of separating components of a mixture using a stationary and a mobile phase. |
| Distillation | Separation of a solvent from a solution or two miscible liquids by heating and condensation. |
| Fractional distillation | Distillation using a fractionating column to separate liquids whose boiling points differ by less than 25 K. |
| Crystallisation | Process of obtaining pure crystals of a solid from its solution. |
| Physical change | A change in which no new substance is formed. |
| Chemical change | A change in which one or more new substances are formed. |
| Element | A pure substance that cannot be broken down into simpler substances by chemical means. |
| Metal | An element that is lustrous, malleable, ductile and a good conductor of heat and electricity. |
| Non-metal | An element that is brittle, non-lustrous and a bad conductor of heat and electricity. |
| Metalloid | An element that shows properties of both metals and non-metals. |
| Compound | A pure substance formed by chemical combination of two or more elements in a fixed ratio. |