Is Matter Around Us Pure? NCERT Class 9 Science Chapter 2 Solutions & Notes

A substance is a pure form of matter that consists of only a single type of constituent particle (atoms or molecules) having definite physical and chemical properties throughout.

Examples: Hydrogen ($H_2$), Oxygen ($O_2$), Sulphur ($S$), Pure Water ($H_2O$), Sodium Chloride ($NaCl$).

Note: A substance cannot be separated into other kinds of matter by any physical process of separation.

Given:

• Mass of solute ($\text{NaCl}$) = $36\text{ g}$
• Mass of solvent ($\text{H}_2\text{O}$) = $100\text{ g}$

Formula:

Answer: The concentration of the saturated sodium chloride solution is $26.47\%$ by mass.

Physical changes: Cutting of trees, Melting of butter in a pan, Boiling of water to form steam, Dissolving common salt in water, Making a fruit salad with raw fruits.

Chemical changes: Rusting of almirah, Passing of electric current through water (electrolysis) breaking down into hydrogen and oxygen gases, Burning of paper and wood.

Pure substances: Sugar, Common salt, Distilled water, Copper, Diamond, Gold.

Mixtures: Air, Wood, Coal, Milk, Soap, Soil, Steel, Tap water.

Method of Preparation of Tea:

1. Take some water as a solvent in a saucepan and heat it over a burner.
2. Add some sugar as a solute and heat it to dissolve the sugar completely because sugar is soluble in water. This forms a sweet solution.
3. Add tea leaves which are insoluble in the solution and boil the mixture to extract colour and flavour.
4. Now, add milk to the boiling solution and boil it again.
5. Filter the mixture through a tea strainer into a cup.
6. The clear prepared tea collected in the cup is the filtrate, and the used tea leaves collected on the tea strainer represent the residue.

(a) At $313\text{ K}$, $100\text{ g}$ of water dissolves $62\text{ g}$ of potassium nitrate ($\text{KNO}_3$).

(b) Solid crystals of potassium chloride ($\text{KCl}$) will separate out / precipitate at the bottom. Reason: The solubility of potassium chloride decreases as temperature decreases (from $54\text{ g}$ at $353\text{ K}$ to $35\text{ g}$ at $293\text{ K}$). The excess dissolved salt cannot remain in solution and crystallises out.

(c) Solubility of each salt at $293\text{ K}$ ($20^\circ\text{C}$) in $100\text{ g}$ of water:

• Potassium nitrate = $32\text{ g}$
• Sodium chloride = $36\text{ g}$
• Potassium chloride = $35\text{ g}$
• Ammonium chloride = $37\text{ g}$

Highest solubility: Ammonium chloride ($\text{NH}_4\text{Cl}$) with $37\text{ g}$ per $100\text{ g}$ of water.

(d) The solubility of a solid salt in water generally increases with an increase in temperature and decreases with a decrease in temperature.

(i) Saturated solution: The solution in which no more solute can be dissolved in the same amount of solvent at a given temperature is called a saturated solution. Example: Adding table salt to water until no more salt dissolves, leaving undissolved crystals at the bottom.

(ii) Pure substance: A substance which contains particles of only one chemical kind of matter with uniform properties throughout is called a pure substance. Example: Pure Gold, Distilled Water, Oxygen gas, Sodium chloride.

(iii) Colloid solution: A colloid is a heterogeneous mixture whose particles are bigger than that of a true solution ($> 1\text{ nm}$) but smaller than a suspension ($< 100\text{ nm}$). The particles scatter light (Tyndall effect). Example: Milk, Clouds, Ink, Blood, Shaving cream.

(iv) Suspension: It is a heterogeneous mixture whose particles are larger in size ($> 100\text{ nm}$) and are visible to the naked eye. The particles settle down when left undisturbed. Example: Chalk powder in water, Mixture of sand and water, Muddy water.

Homogeneous mixtures: Air, Soda water, Vinegar, Filtered tea.

Heterogeneous mixtures: Wood, Soil.

Take the given colourless liquid in a beaker and suspend a thermometer into it. Place the beaker on a wire gauze and heat it with a burner.

Note down the temperature at which the liquid begins to boil steadily. If the given liquid boils at exactly $100^\circ\text{C}$ ($373\text{ K}$) under $1\text{ atm}$ pressure, the liquid is confirmed to be pure water, as pure water has a sharp boiling point of $100^\circ\text{C}$.

The materials that are pure substances are:

• (a) Ice (compound, $H_2O$)
• (c) Iron (element, $Fe$)
• (d) Hydrochloric acid (compound, $HCl$)
• (e) Calcium oxide (compound, $CaO$)
• (f) Mercury (element, $Hg$)

(Milk, Brick, Wood, and Air are mixtures).

The solutions (homogeneous mixtures) are:

• (b) Sea water (homogeneous liquid solution of salts in water)
• (c) Air (homogeneous solution of gases)
• (e) Soda water (homogeneous solution of $CO_2$ gas in water)

(b) Milk and (d) Starch solution will show the Tyndall effect.

Reason: Both milk and starch solution are colloidal solutions. Colloidal particles are large enough ($1 - 100\text{ nm}$) to scatter a beam of light. Salt solution and copper sulphate solution are true solutions whose particles are too small ($< 1\text{ nm}$) to scatter light.

The following are chemical changes:

• (a) Growth of a plant (irreversible biochemical process)
• (b) Rusting of iron (formation of hydrated iron oxide)
• (d) Cooking of food (irreversible chemical change of molecules)
• (e) Digestion of food (enzymatic biochemical breakdown)
• (g) Burning of a candle (combustion of molten wax vapour producing $CO_2$ and $H_2O$)

(Mixing iron filings with sand and freezing of water are physical changes).

Explanation: Any substance that contains only one type of particle (molecules/atoms) is said to be a pure substance. Thus, distilled water ($H_2O$) is a pure substance, while air, steel, and brass are mixtures.

Explanation: Solution of hydrogen gas in metallic palladium is an example of a gas-in-solid solution in which hydrogen gas (solute) is absorbed into the solid metal palladium (solvent).

Explanation: Taste and colour can vary (tap water contains dissolved minerals) and do not provide chemical proof of compound nature. Fixed elemental proportion, sharp boiling point, and chemical separation prove water is a compound.

Explanation: $X_2Y$ is a compound. The properties of a compound are entirely different from those of its constituent elements ($X$ and $Y$). Hence statement (a) is false/incorrect.

Explanation: Generally non-metals are not sonorous as they are brittle and break or shatter instead of ringing/vibrating when struck.

Explanation: Brass is an alloy (solid solution) prepared by thoroughly mixing molten copper ($70\%$) and molten zinc ($30\%$).

Explanation: Mercury ($Hg$) and bromine ($Br_2$) are the only metal and non-metal respectively that exist in liquid form at normal room temperature.

Explanation: Silver, copper, and aluminium are pure chemical elements, whereas Bronze is an alloy (mixture of copper and tin).

Explanation: Substance 3 is lustrous like a metal but non-malleable and a semiconductor of electricity, which are intermediate characteristics of a metalloid (like Silicon or Germanium).

Explanation: A compound is always chemically combined in a strictly fixed proportion by mass according to the Law of Constant Proportions.

Explanation: Substance A has metallic properties (ductility, lustre, conductivity). B consists of immiscible liquid layers, which makes it a heterogeneous mixture. C is an alloy (homogeneous mixture, not a pure substance).

Explanation: Mixture 1 (sugar solution) has a single uniform phase throughout with no boundary of separation, making it homogeneous.

Explanation: Before heating, iron and sulphur retained their individual properties. Upon strong heating, a chemical reaction occurred: $\text{Fe} + \text{S} \rightarrow \text{FeS}$ (Iron Sulphide), which is a non-magnetic compound.

Explanation: Iodine is a sublimable substance which turns directly into violet vapours on heating and solidifies on the cooler walls of the funnel, leaving non-sublimable sodium chloride behind.

Explanation: Soda water contains carbon dioxide gas ($CO_2$, solute) dissolved in liquid water (solvent) under pressure.

Explanation: Tincture of iodine is a solution of solid iodine ($I_2$, solute) in liquid ethyl alcohol (solvent).

Explanation: In a suspension, solid insoluble particles ($> 100\text{ nm}$) remain suspended and settle to the bottom upon standing under gravity.

Explanation: By definition, a solution is saturated when it contains the maximum possible dissolved solute at that specific temperature.

Calculation: Mass of solution = $30\text{ g} + 270\text{ g} = 300\text{ g}$.
$$\text{Mass Percentage} = \frac{30}{300} \times 100 = \mathbf{10\%}$$

Explanation: Concentration is defined as the amount of solute present in a given quantity (mass or volume) of solution or solvent.

Calculation:
Solution 1: $25/(25+200) = 25/225 = 11.11\%$
Solution 2: $10/(10+100) = 10/110 = \mathbf{9.09\%}$ (Lowest)
Solution 3: $15/(15+50) = 15/65 = 23.08\%$
Solution 4: $50/(50+250) = 50/300 = 16.67\%$

Explanation: Colloidal particles ($1 - 100\text{ nm}$) pass through ordinary filter pores but are large enough to scatter light (Tyndall effect).

Explanation: Food colour in water is homogeneous (W is correct). Sand in water is a suspension (not colloid). Milk with dissolved sugar is a colloid (not suspension). Rice + flour is heterogeneous (Z is correct).

Explanation: Liquid 1 is a stable colloid (scatters light, filterable). Liquid 2 is an unstable suspension (scatters light, settles on standing).

Explanation: Salt solution is a true solution with particle size $< 1\text{ nm}$. Smoke (aerosol), Foam, and Jelly (gel) are all colloids that scatter light.

Explanation: Curdling of milk produces lactic acid and coagulated proteins, which is an irreversible chemical change.

Explanation: The fundamental defining characteristic of every chemical change is the synthesis/formation of chemically new substances with new properties.

Explanation: Biological decomposition and decay of wood alters the chemical composition of cellulose and lignin permanently, forming new gases and humus.

Explanation: Melting (change of state) is a physical change, whereas combustion, rusting, and enzymatic digestion are chemical changes.

Explanation: Salt dissolved in water is homogeneous because its composition is uniform throughout. A mixture with different composition in different parts is heterogeneous, not homogeneous. Hence Reason is false.

Explanation: Because pure substances consist of identical constituent particles having constant physical and chemical properties everywhere, they are inherently homogeneous.

Explanation: In air, constituent gases retain their individual chemical identities and are not chemically bonded in a fixed ratio, so they can be separated by physical techniques.

Explanation: Alloys are considered homogeneous mixtures because they show the properties of their constituent elements and can have variable composition.

Explanation: Tyndall scattering occurs when particle diameter is comparable to the wavelength of light. Colloids satisfy this condition.

Explanation: True solutions do not show Tyndall effect because their particles are extremely small and cannot scatter visible light rays. Hence Assertion is false.

Explanation: Airborne micro-particles of dust/smoke form a colloidal aerosol dispersion in air, which scatters light in all directions, illuminating the beam path.

Explanation: Both statements are true facts, but the name "chemical reaction" does not chemically explain why composition changes occur (which is due to bond breaking and new bond formation).

1. Technique: Centrifugation (অপকেন্দ্রণ). The principle is that denser particles are forced to the bottom and lighter particles stay at the top when spun rapidly.
2. Other Applications:
   • Used in washing machines to squeeze out water from wet clothes.
   • Used in diagnostic pathology labs for blood and urine tests.
   • Used in dairies to separate cream from milk.
3. Properties of a Colloid:
   • Colloids are heterogeneous mixtures that appear uniform.
   • Colloidal particles scatter a light beam (Tyndall effect) and do not settle down when left undisturbed.

   OR (Components of Solution): Solute is the component dissolved in smaller quantity (e.g., salt/sugar), and Solvent is the medium in larger quantity that dissolves the solute (e.g., water).

1. Solute and Solvent Identification:
   • (a) Tincture of iodine: Solute = Solid Iodine ($I_2$); Solvent = Liquid Alcohol (Ethanol).
   • (b) Soda water: Solute = Carbon dioxide gas ($CO_2$); Solvent = Liquid Water ($H_2O$).
2. Reason for Alloys: Alloys are homogeneous solutions because the metals mix uniformly at the atomic level, show the properties of their constituent metals, and can have variable compositions.
3. Aqueous vs Non-aqueous: An aqueous solution is a solution where water is the solvent (e.g., salt in water). A non-aqueous solution uses a liquid other than water as solvent (e.g., sulphur dissolved in carbon disulphide, or iodine in alcohol).

1. Air is the mixture, while Salt ($NaCl$), Water ($H_2O$), Sugar, and Alum are pure chemical compounds.
2. Three natural mixtures: Soil, Seawater, and Atmospheric Air (or Crude oil, Milk).
3. (a) Element: Iron / Sulphur / Oxygen. Compound: Water / Carbon dioxide. Mixture: Air / Brass.
   (b) Soil and soft drinks have no fixed chemical formula or fixed composition; their constituents can vary from sample to sample.
   OR (Characteristics of Mixtures):
   • A mixture has a variable composition with no fixed melting/boiling point.
   • The constituents of a mixture retain their individual chemical identities and can be separated by physical methods.

1. Phenomenon: Tyndall Effect (টিন্ডাল প্রভাব).
2. Natural Example: Sunlight streaming through mist/fog droplets in the canopy of a dense forest.
3. Reason for Salt Solution: In true salt solution, the dissolved sodium and chloride ions are $< 1\text{ nm}$ in diameter, which is much smaller than the wavelength of visible light. Hence, they cannot scatter light.
4. Chalk Powder Mixture: Chalk in water is a suspension. While the particles are suspended, it will scatter light strongly. But when left undisturbed, the chalk particles will settle to the bottom and light will pass through the clear liquid above.

(i) (a) Oxygen ($O_2$) is the non-metal essential for combustion.
(i) (b) Chlorine ($Cl_2$) is the non-metal that combines with sodium to form common salt ($\text{NaCl}$).

(ii) Two physical properties of metals:

• Metals are malleable (can be beaten into thin sheets) and ductile (can be drawn into wires).
• Metals possess a shining metallic lustre and are excellent conductors of heat and electricity.

(i) Classification:

• Elements: Chlorine gas ($Cl_2$), Potassium ($K$), Iron ($Fe$), Aluminium ($Al$), Iodine ($I_2$), Carbon ($C$), Sulphur powder ($S_8$).
• Compounds: Water ($H_2O$), Calcium carbonate ($\text{CaCO}_3$), Sodium chloride ($\text{NaCl}$).

(ii) Pure Substance vs Mixture: A pure substance contains particles of only one chemical kind that cannot be separated by physical means, whereas a mixture is a physical combination of two or more pure substances in any proportion that can be separated by physical methods.

(i) Difference: Mixtures have variable composition and constituent substances retain their individual properties. Compounds have fixed composition by mass and possess completely new properties distinct from their elements.

(ii) Classification:

• (a) Tincture of iodine: Homogeneous mixture (true solution)
• (b) Smoke: Heterogeneous mixture (colloidal aerosol of solid carbon in air)
• (c) Brass: Homogeneous mixture (solid alloy solution of Cu and Zn)
• (d) Sugar solution: Homogeneous mixture (true solution)

• (i) Dissolving salt in water: Physical change (reversible by evaporation, no new compound).
• (ii) Rusting of iron: Chemical change (formation of hydrated ferric oxide).
• (iii) Burning of paper: Chemical change (combustion produces ash, carbon dioxide, and water vapour).
• (iv) Melting of ice: Physical change (change of state from solid to liquid water).

(i) Concentration of a solution: It is the amount of solute dissolved in a given quantity (mass or volume) of solution or solvent.

(ii) Visibility in True Solution: The solute particles in a true solution are completely dissociated into individual molecules or ions with diameter less than $1\text{ nm}$ ($10^{-9}\text{ m}$), which is far smaller than the resolving limit of the human eye and visible light wavelength.

(i) Yes, suspensions exhibit the Tyndall effect when the particles are suspended in the medium because the large particles scatter the light rays passing through them.

(ii) Among the given samples, Starch solution (colloidal sol) and Smoke (colloidal aerosol) exhibit the Tyndall effect. Sodium chloride solution and Tincture of iodine are true solutions and do not show Tyndall effect.

(i) Components: The two components are Solute (substance dissolved in lesser amount) and Solvent (medium present in larger amount). Examples: Salt in water (aqueous solution), Tincture of iodine (iodine dissolved in alcohol).

(ii) Gold Alloy Qualities: Pure 24-karat gold is extremely soft and malleable, making it prone to bending, scratching, and losing shape. When alloyed with copper or silver (typically 22-karat gold = 22 parts gold + 2 parts copper/silver), it becomes much harder, mechanically stronger, and durable, making it ideal for crafting long-lasting jewellery ornaments.

In a chemical change, new chemical substances with completely different properties are formed. When metallic iron ($Fe$) is exposed to moist air, it reacts chemically with oxygen and water to form rust (hydrated ferric oxide, $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$).

Rust is non-magnetic, reddish-brown, and brittle, unlike lustrous, magnetic, malleable iron. The reaction is irreversible and accompanied by a mass change, proving that rusting is a chemical change.

(i) Classification:

• Diamond: An element (an allotropic crystalline form of pure carbon).
• Brass: A homogeneous mixture (an alloy of copper and zinc).

(ii) Difference: In a physical change, no new substance is formed and only physical states/shapes change reversibly (e.g. melting ice). In a chemical change, chemical bonds are broken and formed to create new substances with different compositions irreversibly (e.g. burning wood).

1. (i) Solid crystals of potassium chloride will separate out and deposit at the bottom because solubility decreases with a fall in temperature.
2. (ii) As water boils off completely, the remaining solid sugar gets charred and decomposes into black carbon with steam release (chemical decomposition).
3. (iii) The extra sugar will not dissolve and will simply settle down at the bottom of the beaker because the solution is already saturated.

Copper sulphate solution: It is a true solution where $Cu^{2+}$ and $SO_4^{2-}$ ions are smaller than $1\text{ nm}$ in diameter. These tiny particles cannot scatter light, so no Tyndall effect is shown.

Water and milk mixture: It is a colloidal emulsion where fat and protein particles range between $1\text{ nm}$ and $100\text{ nm}$. These particles are sufficiently large to scatter a beam of light, making the light path clearly illuminated (Tyndall effect).

• (i) Spreading of ink in water: Diffusion (ব্যাপন).
• (ii) Curdling of milk: Chemical change / Coagulation of protein.
• (iii) Beam illuminating dust particles: Tyndall Effect (টিন্ডাল প্রভাব).

Given:

• Mass percentage ($\text{Mass } \%$) = $20\%$
• Mass of solvent (water) = $100\text{ g}$
• Let mass of solute (sodium sulphate) = $x\text{ g}$

Formula:

Answer: $25\text{ g}$ of sodium sulphate is required.

1. (i) Drying of a shirt: Physical change because water only evaporates into vapour ($\text{Liquid} \rightarrow \text{Gas}$); the fabric and water molecules remain chemically unchanged.
2. (ii) Colour change of tea with lemon: Chemical change because citric acid in lemon reacts with tea polyphenols (thearubigins), altering their pH-sensitive chromophores.
3. (iii) Rusting of iron: Chemical change because iron oxidizes with moisture to form a new compound (hydrated iron oxide).
4. (iv) Burning of candle: Both physical and chemical change. Melting of wax is physical, and combustion of wax vapour producing $CO_2$ and $H_2O$ is chemical.

It is a chemical change because solid zinc reacts chemically with aqueous hydrochloric acid to form a new salt, zinc chloride, and releases hydrogen gas with heat.

Balanced Chemical Equation:

1. (i) Physical property: Steel is an alloy (homogeneous solid mixture). Describing the physical composition of mixed constituents involves no chemical reaction.
2. (ii) Physical property: Hardness and softness are physical mechanical properties of metals; cutting does not form a new substance.
3. (iii) Chemical property: Formation of new alkali hydroxides through reaction with water describes the chemical reactivity of basic metal oxides.

(i) Definition and Types of Elements:

An element is the purest, basic form of matter that consists of only one type of atom and cannot be broken down into simpler substances by chemical reactions.

The three main classes of elements are:

• 1. Metals (ধাতু): Lustrous, malleable, ductile, good conductors of heat and electricity (e.g., Gold, Copper, Aluminium, Iron).
• 2. Non-metals (অধাতু): Non-lustrous, non-malleable, brittle, poor conductors of heat and electricity (e.g., Carbon, Oxygen, Nitrogen, Sulphur).
• 3. Metalloids (উপধাতু): Elements with intermediate properties between metals and non-metals (e.g., Silicon, Germanium, Boron, Arsenic).

(ii) Dispersed Phase and Dispersion Medium:

(iii) Applications of Colloids:

• Medicines: Most medicines (like eye drops, antibiotics, milk of magnesia) are colloids for easier cellular absorption.
• Water Purification: Addition of alum ($\text{KAl(SO}_4)_2\cdot 12\text{H}_2\text{O}$) to muddy river water neutralizes colloidal clay particles (coagulation), making them settle down for drinking filtration.

(a) Sublimation (ঊর্ধ্বপাতন):

Principle: Used to separate a sublimable volatile solid (which changes directly from solid to vapour upon heating) from a non-sublimable impurity.

Applications: Separation of ammonium chloride, camphor, naphthalene, or iodine from common salt.

(b) Centrifugation (অপকেন্দ্রণ):

Principle: When a liquid mixture containing suspended micro-particles is spun rapidly at high speed, denser particles are forced outward to the bottom and lighter particles stay at the top.

Applications: Used in blood & urine clinical diagnosis, separating butter from curd/milk, and spinning wet clothes dry in washing machines.

(c) Chromatography (ক্রোমাটোগ্রাফি):

Principle: Based on the difference in solubilities of different dissolved solutes in the same solvent. As the mobile solvent travels up the stationary paper medium, the more soluble component travels faster and higher.

Applications: Separating colours in black dye/ink, extracting natural pigments from flower petals, and detecting drugs in blood by forensic toxicology.

(d) Separating Funnel (পৃথকীকরণ ফানেল):

Principle: Immiscible liquids (which do not mix together) separate into distinct layers according to their densities when left undisturbed.

Applications: Separating a mixture of kerosene oil and water, or extracting iron from its ore during metallurgical slag separation.