CBSE Class 9 Chemistry MCQs On Practical Skills Worksheet

Question 1. Which one of the following does not form true solution in water
(a) common salt
(b) alum
(c) albumin
(d) cane sugar
Answer: (c) albumin
Albumin forms a colloidal solution in water because of its large molecular size, whereas common salt, alum, and cane sugar form true solutions.
In simple words: Albumin forms a colloid, not a true solution.
Exam Tip: Remember that organic macromolecules like proteins (albumin, starch) form colloids in water.

Question 2. Which one of the following forms a colloid in water
(a) common salt
(b) alum
(c) albumin
(d) cane sugar
Answer: (c) albumin
Albumin disperses in water to form a colloidal system due to large particle dimensions.
In simple words: Albumin creates a colloidal mixture.

Exam Tip: True solutions are formed by low molecular weight ionic or covalent compounds.

Question 3. Which one of the following forms a colloid in water
(a) soil
(b) sand
(c) chalk powder
(d) none of these
Answer: (d) none of these
Soil, sand, and chalk powder form suspensions in water, not colloids.
In simple words: Soil, sand, and chalk make suspensions, so none of the options is correct.

Exam Tip: Suspensions have visible particles larger than 100 nm that settle down over time.

Question 4. Which one of the following is highly soluble in water
(a) common salt
(b) potash alum
(c) cane sugar
(d) starch
Answer: (c) cane sugar
Cane sugar has an exceptionally high solubility in water at room temperature compared to salts and insoluble polymers like starch.
In simple words: Cane sugar dissolves easily and in large amounts in water.

Exam Tip: Sugars dissolve readily due to extensive hydrogen bonding with water molecules.

Question 5. Which one of the following does not form true solution in water at room temperature
(a) common salt
(b) alum
(c) albumin
(d) cane sugar
Answer: (c) albumin
Albumin forms a colloidal dispersion, failing to produce a true homogeneous molecular solution.
In simple words: Albumin forms a colloid, not a true solution.

Exam Tip: Distinguish between true solutions (particle size \(< 1\) nm) and colloidal solutions (1 nm to 1000 nm).

Question 6. Which one of the following is least soluble in water
(a) common salt
(b) potash alum
(c) cane sugar
(d) starch
Answer: (d) starch
Starch is a macromolecule that is practically insoluble in cold water and only forms a colloidal suspension upon heating.
In simple words: Starch does not dissolve well in water.

Question 7. Which one of the following can be separated by filtration
(a) common salt and water
(b) sugar and water
(c) soil and water
(d) starch and water
Answer: (c) soil and water
Soil forms a suspension in water, and suspended particles are retained by filter paper during filtration.
In simple words: Soil particles are trapped by filter paper, separating them from water.
Exam Tip: Filtration works only for heterogeneous suspensions; true solutions and colloids pass through ordinary filter paper.

Question 8. In which one of the following the particle size is in the range of 0.1 nm to 1.0 nm
(a) true solution
(b) suspension
(c) colloid
(d) none of these
Answer: (a) true solution
True solutions contain solute particles (ions or small molecules) whose size is less than 1 nm (typically 0.1 nm to 1 nm).
In simple words: True solutions have the smallest particles.

Exam Tip: Learn the particle size ranges: True solution \(< 1\) nm, Colloid = 1 nm to 100 nm, Suspension \(> 100\) nm.

Question 9. Which one of the following forms a colloid in water
(a) sugar
(b) sand
(c) chalk powder
(d) none of these
Answer: (d) none of these
Sugar forms a true solution, while sand and chalk powder form suspensions. None of them form a colloid.
In simple words: None of the listed substances form a colloid.

Exam Tip: Common examples of colloids include milk, starch solution, and soap solution.

Question 10. Tyndall effect is observed in which one of the following
(a) true solution
(b) suspension
(c) colloid
(d) none of these
Answer: (c) colloid
Colloidal particles are large enough to scatter a beam of visible light, exhibiting the Tyndall effect.
In simple words: Light beams scatter when passing through a colloid.

Exam Tip: True solutions do not show the Tyndall effect because their particles are too small to scatter light.

Question 11. Sedimentation does not take place in which one of the following
(a) common salt and water
(b) sugar and water
(c) soil and water
(d) starch and water
Answer: (a) common salt and water (Note: sugar and water also do not sediment)
Both common salt and sugar form stable true solutions where particles never settle down under gravity.
In simple words: Salt and sugar dissolved in water do not settle down at the bottom.

Exam Tip: Stable mixtures like true solutions and stable colloids do not undergo sedimentation.

Question 12. Which one will boil at a constant temperature
(a) true solution
(b) colloid
(c) compound
(d) each one
Answer: (c) compound
A chemical compound has a definite fixed composition and boils at a sharp, constant temperature.
In simple words: Pure compounds boil at a fixed, constant temperature.

Exam Tip: Solutions and mixtures boil over a temperature range, not at a constant temperature.

Question 13. Which represents a constant composition
(a) true solution
(b) colloid
(c) suspension
(d) none of these
Answer: (d) none of these
Solutions, colloids, and suspensions are mixtures with variable compositions. Compounds and pure substances have constant compositions.
In simple words: Mixtures do not have a fixed composition, so none of the mixture types fit.

Exam Tip: Mixtures have variable composition whereas pure substances (elements and compounds) have fixed composition.

CLASS -IX ASSIGNMENT - JULY 2010
MULTIPLE CHOICE QUESTION ON PRACTICAL SKILLS
EXPERIMENT NO - 2

Question 1. Which one of the following is an example of heterogeneous mixture
(a) NaCl + water
(b) kerosene + water
(c) sugar + water
(d) alum + water
Answer: (b) kerosene + water
Kerosene and water are immiscible liquids forming a two-phase heterogeneous mixture.
In simple words: Kerosene floats on water, forming separate layers.

Exam Tip: Immiscible liquid mixtures and suspensions are always heterogeneous.

Question 2. Which one of the following is a mixture of two elements
(a) iron fillings + sulphur powder
(b) sugar + common salt
(c) water + milk
(d) air
Answer: (a) iron fillings + sulphur powder
Iron (Fe) and sulphur (S) are both pure elements physically mixed together.
In simple words: Iron and sulfur are both individual elements mixed together.

Exam Tip: Sugar and salt are compounds, water and milk contain compounds, air contains compounds like carbon dioxide.

Question 3. Which one is not the property of a mixture
(a) heterogeneous
(b) variable composition
(c) constant composition
(d) none of these
Answer: (c) constant composition
Mixtures have variable composition, not constant composition.
In simple words: Mixtures can have different proportions of their components.

Exam Tip: Constant composition is a defining characteristic of pure compounds, not mixtures.

Question 4. Correct method of separating iron fillings from sulphur powder
(a) heating in china dish
(b) heating in boiling tube
(c) heating in water
(d) moving magnet
Answer: (d) moving magnet
Iron is magnetic and can be physically attracted and separated from sulphur using a magnet.
In simple words: A magnet pulls out the iron filings from the mixture.

Exam Tip: Magnetic separation exploits the physical property of magnetism possessed by iron.

Question 5. In the formation of which one a large quantity of heat is involved
(a) compound
(b) colloid
(c) true solution
(d) each one
Answer: (a) compound
Chemical reactions forming compounds involve significant energy changes (absorption or release of heat), unlike physical mixing.
In simple words: Making a chemical compound involves a large energy change.

Exam Tip: Physical changes like dissolving or mixing involve very little heat change compared to chemical reactions.

Question 6. When a magnet is moved through of iron fillings and sulphur powder, then
(a) iron fillings will cling to magnet
(b) black mass is produced
(c) sulphur will cling to magnet
(d) none of these
Answer: (a) iron fillings will cling to magnet
Iron is ferromagnetic and sticks to the magnet, leaving sulphur behind.
In simple words: Iron sticks to the magnet while sulfur stays behind.

Exam Tip: This confirms that unheated iron and sulfur exist as a physical mixture retaining individual properties.

Question 7. When a mixture of iron fillings is heated with sulphur powder
(a) nothing happens
(b) black mass is formed
(c) yellow mass is formed
(d) white mass is formed
Answer: (b) black mass is formed
Heating iron and sulphur causes a chemical reaction forming iron sulphide (\(\text{FeS}\)), which is a black solid.
In simple words: Heating iron and sulfur creates a new black substance called iron sulfide.

Exam Tip: Formation of iron sulfide is a classic example of a chemical change.

Question 8. which one is a mixture
(a) salad of fruits
(b) air
(c) sea water
(d) all of the above
Answer: (d) all of the above
Fruit salad, air, and sea water are all physical combinations of different substances without fixed proportions.
In simple words: All listed items are mixtures.

Exam Tip: Air is a homogeneous mixture of gases; sea water is a homogeneous mixture of water and salts; fruit salad is heterogeneous.

Question 9. Name the process of conversion of solid to vapor
(a) evaporation
(b) fusion
(c) distillation
(d) sublimation
Answer: (d) sublimation
Sublimation is the direct transition of a substance from solid to gas (vapor) without passing through the liquid state.
In simple words: Turning directly from solid to gas is called sublimation.

Exam Tip: Examples of subliming substances include camphor, naphthalene, and ammonium chloride.

Question 10. Which one of the following is a pure substance
(a) milk
(b) cold drink
(c) carbon di oxide
(d) brass
Answer: (c) carbon di oxide
Carbon dioxide (\(\text{CO}_2\)) is a chemical compound with a fixed chemical formula and constant properties.
In simple words: Carbon dioxide is a pure chemical compound.

Exam Tip: Milk is a colloid/emulsion, cold drink is a solution/mixture, brass is an alloy (solid solution).

Question 11. What is observed when iron fillings and sulphur powder is heated in a china dish
(a) sulphur melts
(b) iron melts
(c) mixture evaporates
(d) yellow mass is formed (Correction: black mass is formed, but matching closest chemical outcome or option d as printed)
Answer: (d) yellow mass is formed / black mass is formed
Upon heating, iron and sulfur react to form iron sulfide (black mass).
In simple words: Heating iron and sulfur produces a black product.

Exam Tip: Note that printed options often have minor typos; the chemical reaction yields iron(II) sulfide.

Question 12. Mixture is always -
(a) homogenous
(b) heterogeneous
(c) homogenous or heterogeneous
(d) none of these
Answer: (c) homogenous or heterogeneous
Mixtures can be either homogeneous (uniform throughout, like saltwater) or heterogeneous (non-uniform, like oil and water).
In simple words: Mixtures can be either uniform or non-uniform in composition.

Exam Tip: Always check if the components are uniformly distributed when classifying mixtures.

Question 13. Compound is always -
(a) homogenous
(b) heterogeneous
(c) homogenous or heterogeneous
(d) none of these
Answer: (a) homogenous
Chemical compounds have a uniform composition throughout their mass, making them homogeneous substances.
In simple words: Compounds are always uniform throughout.

Exam Tip: Pure substances (elements and compounds) are homogeneous in nature.

Question 14. Which of the following is a compound
(a) blood
(b) air
(c) soil
(d) water
Answer: (d) water
Water (\(\text{H}_2\text{O}\)) is a chemical compound composed of hydrogen and oxygen in a fixed 2:1 ratio by atomic count.
In simple words: Water is a pure chemical compound.

Exam Tip: Blood, air, and soil are complex natural mixtures.

Question 15. On heating iron and sulphur in a china dish
(a) mixture is formed
(b) compound is formed
(c) both are formed
(d) no change
Answer: (b) compound is formed
Heating iron and sulphur causes a chemical reaction that creates iron(II) sulfide, a chemical compound.
In simple words: A chemical compound is formed when iron and sulfur are heated together.

Exam Tip: Compounds exhibit entirely different properties from their constituent elements.

CLASS -IX ASSIGNMENT - AUG 2010
MULTIPLE CHOICE QUESTION ON PRACTICAL SKILLS
EXPERIMENT NO - 3

Question 1. The color of copper sulphate solution is
(a) green
(b) blue
(c) white
(d) brown
Answer: (b) blue
Hydrated copper sulfate solution (\(\text{CuSO}_4 \cdot 5\text{H}_2\text{O}\) in water) has a characteristic deep blue color due to hydrated copper ions \([\text{Cu}(\text{H}_2\text{O})_6]^{2+}\).
In simple words: Copper sulfate solution is blue.

Exam Tip: Anhydrous copper sulfate is white, but in aqueous solution, it turns blue.

Question 2. The reaction is a -
\(\text{BaCl}_2 + \text{Na}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{NaCl}\)
(a) displacement reaction
(b) decomposition reaction
(c) double displacement reaction
(d) combination reaction
Answer: (c) double displacement reaction
In this reaction, the ions \(\text{Ba}^{2+}\) and \(\text{Na}^+\) exchange their partners (\(\text{SO}_4^{2-}\) and \(\text{Cl}^-\)), forming a white precipitate of barium sulfate.
In simple words: Ions exchange places between two compounds.

Exam Tip: Mutual exchange of ions is the hallmark of double displacement precipitation reactions.

Question 3. In the reaction, the colour of fumes of nitrogen di oxide is -
\(2\text{Pb(NO}_3)_2 \rightarrow 2\text{PbO} + 4\text{NO}_2 + \text{O}_2\)
(a) black
(b) orange
(c) blue
(d) reddish brown
Answer: (d) reddish brown
Nitrogen dioxide (\(\text{NO}_2\)) gas evolved during the thermal decomposition of lead nitrate is reddish-brown in color.
In simple words: Nitrogen dioxide gas gives off brown fumes.

Exam Tip: Memorize the color of \(\text{NO}_2\) gas as reddish-brown.

Question 3 (Second). The reaction is a
\(\text{Fe} + \text{CuSO}_4 \rightarrow \text{FeSO}_4 + \text{Cu}\)
(a) displacement reaction
(b) decomposition reaction
(c) double displacement reaction
(d) combination reaction
Answer: (a) displacement reaction
More reactive iron (Fe) displaces less reactive copper (Cu) from its aqueous copper sulfate solution.
In simple words: Iron replaces copper because iron is more reactive.

Exam Tip: Single displacement reactions involve a more active metal displacing a less active metal from a salt solution.

Question 4. In which of the following reaction, a more active metal displaces less reactive metal
(a) displacement reaction
(b) decomposition reaction
(c) double displacement reaction
(d) combination reaction
Answer: (a) displacement reaction
By definition, a single displacement reaction involves a reactive metal displacing a less reactive metal.
In simple words: Displacement reactions involve active metals replacing less active ones.

Exam Tip: Refer to the reactivity series of metals to predict if a displacement reaction will occur.

Question 5. The reaction is a -
(a) displacement reaction
(b) decomposition reaction
(c) double displacement reaction
(d) combination reaction
Answer: (b) decomposition reaction
(Referring to thermal decomposition of lead nitrate or similar decomposition context in the sequence).
In simple words: Breaking down a compound into simpler substances is a decomposition reaction.

Exam Tip: Decomposition requires energy input in the form of heat, light, or electricity.

Question 6. The gas evolved on adding dil. \(\text{H}_2\text{SO}_4\) in some pieces of Zn is
(a) oxygen
(b) hydrogen sulphide
(c) hydrogen
(d) sulphur di oxide
Answer: (c) hydrogen
Active metals like zinc react with dilute sulfuric acid to evolve hydrogen gas (\(\text{H}_2\)) with effervescence.
In simple words: Zinc reacts with acid to release hydrogen gas.

Exam Tip: Hydrogen gas burns with a pop sound when a burning splinter is brought near it.

Question 7. On mixing sodium sulphate and barium chloride
(a) yellow ppt is formed
(b) white ppt is formed
(c) black ppt is formed
(d) grey ppt is formed
Answer: (b) white ppt is formed
Reaction produces barium sulfate, which precipitates as an insoluble white solid.
In simple words: A white solid precipitate forms instantly.

Exam Tip: Barium sulfate (\(\text{BaSO}_4\)) precipitation is a standard test for sulfate ions.

Question 8. On burning magnesium ribbon
(a) white substance is formed
(b) black substance is formed
(c) yellow substance is formed
(d) grey substance is formed
Answer: (a) white substance is formed
Magnesium burns in air with a dazzling white flame to form magnesium oxide (\(\text{MgO}\)), which is a white powder.
In simple words: Burning magnesium creates a white powder of magnesium oxide.

Exam Tip: Magnesium ribbon must be cleaned with sandpaper before burning to remove any oxide coating.

Question 9. The reaction is a
\(\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2\)
(a) displacement reaction
(b) decomposition reaction
(c) double displacement reaction
(d) combination reaction
Answer: (a) displacement reaction
Zinc displaces hydrogen from dilute sulfuric acid.
In simple words: Zinc replaces hydrogen in the acid.

Exam Tip: This is a classic single metal displacement reaction.

Question 10. The color of residue left behind on heating lead nitrate (hot)
(a) green
(b) white
(c) reddish brown
(d) light yellow
Answer: (d) light yellow (or reddish brown when hot, yellow when cold)
Lead(II) oxide (\(\text{PbO}\)) formed as residue is reddish-brown when hot and turns yellow when cooled.
In simple words: The remaining solid residue is yellow when cool.

Exam Tip: Lead oxide (\(\text{PbO}\), litharge) is yellow when cold.

Question 11. The colour of nitrogen di oxide is
(a) black
(b) orange
(c) blue
(d) reddish brown
Answer: (d) reddish brown
\(\text{NO}_2\) is a characteristic reddish-brown toxic gas.
In simple words: Nitrogen dioxide gas is reddish-brown.

Exam Tip: Always identify \(\text{NO}_2\) by its pungent smell and reddish-brown fumes.

Question 12. The insoluble precipitate formed when barium chloride and sodium sulphate is mixed is of -
(a) sodium chloride
(b) barium sulphate
(c) barium sulphite
(d) none of these
Answer: (b) barium sulphate
Barium sulfate (\(\text{BaSO}_4\)) precipitates out as an insoluble white solid.
In simple words: The precipitate is barium sulfate.

Exam Tip: Sodium chloride remains dissolved in the aqueous solution.

Question 13. The colour of insoluble precipitate formed when barium chloride and sodium sulphate is mixed is
(a) blue
(b) white
(c) black
(d) yellow
Answer: (b) white
Barium sulfate precipitate is white in color.
In simple words: The precipitate formed is white.

Exam Tip: White precipitates are common for barium and silver salts in precipitation reactions.

Question 14. Burning of magnesium in oxygen is a
(a) displacement reaction
(b) decomposition reaction
(c) double displacement reaction
(d) combination reaction
Answer: (d) combination reaction
Two reactants (\(\text{Mg}\) and \(\text{O}_2\)) combine to form a single product (\(\text{MgO}\)).
In simple words: Two elements combine to make one compound.

Exam Tip: Synthesis or combination reactions result in a single product from multiple reactants.

Question 15. The reaction which takes place when magnesium burns in air is
A \(\text{Mg} + \text{O}_2 \rightarrow \text{MgO}\) (balanced: \(2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}\))
B. \(\text{Mg} + \text{N}_2 \rightarrow \text{Mg}_3\text{N}_2\)
C. both a and b
(d) none of these
Answer: (c) both a and b
When magnesium burns in air (which contains both oxygen and nitrogen), it forms magnesium oxide with oxygen and magnesium nitride with nitrogen.
In simple words: Magnesium reacts with both oxygen and nitrogen in the air.

Exam Tip: Air is not pure oxygen; burning in air yields both oxide and nitride products.

CLASS -IX ASSIGNMENT - AUG 2010
MULTIPLE CHOICE QUESTION ON PRACTICAL SKILLS
EXPERIMENT NO - 4

Question 1. Which one of the following can be separated by sublimation
(a) camphor
(b) sugar
(c) salt
(d) copper sulphate
Answer: (a) camphor
Camphor easily sublimes upon heating, separating it from non-volatile impurities.
In simple words: Camphor vaporizes directly when heated.

Exam Tip: Sublimation applies to mixtures where one component is sublimable.

Question 2. Which one of the following can be separated by sublimation
(a) iodine
(b) sugar
(c) salt
(d) iron sulphate
Answer: (a) iodine
Iodine crystals sublime readily on heating, producing violet vapors.
In simple words: Iodine sublimes directly when heated.

Exam Tip: Iodine vapor condenses back to shiny dark crystals on cooler surfaces.

Question 4. Name the process of conversion of solid into vapor
(a) sublimation
(b) evaporation
(c) condensation
(d) fusion
Answer: (a) sublimation
Solid directly turning into vapor is termed sublimation.
In simple words: The solid-to-gas transition is sublimation.

Exam Tip: Remember ammonium chloride, camphor, naphthalene, and dry ice as common examples.

Question 5. If sand is added to water -
(a) it dissolves
(b) remains insoluble
(c) dissolves slightly
(d) none of these
Answer: (b) remains insoluble
Sand (\(\text{SiO}_2\)) does not dissolve in water and settles down.
In simple words: Sand does not dissolve in water.

Exam Tip: Insoluble solids form heterogeneous suspensions in water.

Question 6. The mixture of sand and ammonium chloride is separated by
(a) sedimentation
(b) sublimation
(c) evaporation
(d) both a and b (Correction: sublimation separates ammonium chloride from sand)
Answer: (b) sublimation
Ammonium chloride sublimes on heating leaving sand behind.
In simple words: Heating vaporizes ammonium chloride while sand remains behind.

Exam Tip: Sublimation is ideal for separating a volatile solid from a non-volatile solid.

Question 7. Choose the homogenous mixture
(a) sand and water
(b) sugar and water
(c) chalk and water
(d) none
Answer: (b) sugar and water
Sugar dissolves completely in water to form a uniform (homogeneous) true solution.
In simple words: Sugar and water make a uniform mixture.

Exam Tip: Homogeneous mixtures have a uniform composition throughout.

Question 8. Salt can be separated from water by
(a) evaporation
(b) distillation
(c) condensation
(d) both a and b
Answer: (d) both a and b
Salt can be recovered by evaporating water or via distillation if you want to collect both salt and water vapor.
In simple words: Both evaporation and distillation can separate salt from water.

Exam Tip: Distillation recovers both solvent and solute; evaporation recovers only the solid solute.

Question 9. The sand during filtration
(a) remains on filter paper
(b) passes through filter paper
(c) both a and b
(d) none
Answer: (a) remains on filter paper
Sand particles are too large to pass through filter paper pores and stay behind as residue.
In simple words: Sand gets trapped on the filter paper.

Exam Tip: The liquid passing through is called the filtrate, and the solid retained is the residue.

Question 10. Which of the following sublimes during heating
(a) dry salt
(b) dry ice
(c) ice
(d) all of them
Answer: (b) dry ice
Solid carbon dioxide (dry ice) sublimes directly into carbon dioxide gas at atmospheric pressure.
In simple words: Dry ice turns straight into gas without melting.

Exam Tip: Solid carbon dioxide is called dry ice because it does not melt into liquid.

CLASS -IX ASSIGNMENT - SEPSS 2010
MULTIPLE CHOICE QUESTION ON PRACTICAL SKILLS
EXPERIMENT NO - 5

Question 1. What is the melting point of ice
(a) 100 K
(b) 273 K
(c) 373 K
(d) 0 K
Answer: (b) 273 K
Ice melts at \(0^\circ\text{C}\), which corresponds to \(0 + 273.15 = 273\) K.
In simple words: The melting point of ice is 273 Kelvin (\(0^\circ\text{C}\)).

Exam Tip: To convert Celsius to Kelvin, add 273.

Question 2. When does ice and water together exist at atmospheric pressure
(a) at \(0^\circ\text{C}\)
(b) below \(0^\circ\text{C}\)
(c) above \(0^\circ\text{C}\)
(d) never
Answer: (a) at \(0^\circ\text{C}\)
At the melting point temperature (\(0^\circ\text{C}\) or 273 K), solid ice and liquid water exist in thermal equilibrium.
In simple words: Ice and water coexist at exactly \(0^\circ\text{C}\).

Exam Tip: At this temperature, added heat is used as latent heat of fusion rather than raising temperature.

Question 3. Which one is true about kinetic energy of molecules of ice and water at \(0^\circ\text{C}\)
(a) it is high in ice
(b) it is high in water
(c) it is same in both
(d) none
Answer: (c) it is same in both
Kinetic energy depends solely on temperature. Since both ice and water are at the same temperature (\(0^\circ\text{C}\)), their average molecular kinetic energy is identical.
In simple words: Molecules in ice and water at the same temperature have the same kinetic energy.

Exam Tip: Water has higher potential energy due to latent heat, but kinetic energy depends only on temperature.

Question 4. What is the boiling point of pure water
(a) 273 K
(b) 373 K
(c) 173 K
(d) 200 K
Answer: (b) 373 K
Pure water boils at \(100^\circ\text{C}\), which equals \(100 + 273 = 373\) K.
In simple words: Water boils at 373 Kelvin (\(100^\circ\text{C}\)).

Exam Tip: Boiling point is the temperature at which vapor pressure equals atmospheric pressure.

Question 5. Which of the following will increase the rate of evaporation of water
(a) surface area
(b) temperature
(c) wind energy
(d) all of the above
Answer: (d) all of the above
Evaporation rate increases with larger surface area, higher temperature, and stronger wind speed.
In simple words: Surface area, temperature, and wind speed all speed up evaporation.

Exam Tip: All these factors increase the escape rate of liquid molecules into the vapor phase.

Question 6. The heat energy needed to convert 1 kg of water into steam is called
(a) latent heat of vaporization
(b) latent heat of fusion
(c) molar heat
(d) specific heat
Answer: (a) latent heat of vaporization
The heat required to change liquid to gas at its boiling point without temperature change is the latent heat of vaporization.
In simple words: Turning water into steam requires the latent heat of vaporization.

Exam Tip: Latent heat of vaporization for water is approximately \(2.26 \times 10^6\) J/kg.

Question 7. Heat is liberated when
(a) water boils
(b) ice melts
(c) vapor condenses
(d) both a and c (Correction: boiling absorbs heat; condensation liberates heat. Therefore, only c is correct or check context. Wait, condensation liberates heat.)
Answer: (c) vapor condenses
Condensation is an exothermic phase change where gas turns to liquid, liberating heat.
In simple words: Heat is released when gas turns into liquid (condensation).

Exam Tip: Phase changes from gas to liquid and liquid to solid release heat (exothermic).

Question 8. During solidification process of water into ice
(a) temperature is decreased
(b) temperature is increased
(c) heat is liberated
(d) heat is absorbed
Answer: (c) heat is liberated
Solidification (freezing) releases latent heat while the temperature remains constant at \(0^\circ\text{C}\).
In simple words: Freezing water releases heat.

Exam Tip: Freezing is an exothermic process.

Question 9. When ice converts into water
(a) temperature is decreased
(b) temperature is increased
(c) heat is liberated
(d) heat is absorbed
Answer: (d) heat is absorbed
Melting is an endothermic process where ice absorbs latent heat of fusion.
In simple words: Melting ice absorbs heat from its surroundings.

Exam Tip: Endothermic phase changes absorb heat without raising the temperature.

Question 10. On supplying heat to a solid, its temperature may
(a) decrease
(b) increase
(c) remain constant
(d) increase then decrease
Answer: (b) increase (or remain constant during phase change)
Supplying heat to a solid generally increases its temperature until it reaches its melting point, where temperature remains constant during melting.
In simple words: Adding heat raises the temperature of a solid until it melts.

Exam Tip: Temperature rises when sensible heat is added; temperature stays constant when latent heat is added during phase change.

CLASS: IX CHEMISTRY
ASSIGNMENT NO: 1
MATTER IN OUR SURROUNDING - MONTH: JULY - 2010

Question 1. What is matter? What are the physical states of matter?
Answer: Anything that occupies space and has mass is called matter. The three primary physical states of matter are solid, liquid, and gas (with plasma and Bose-Einstein condensate as higher energy/special states).
In simple words: Matter is anything that has mass and takes up space, existing as solids, liquids, or gases.

Exam Tip: Mention mass and volume/space occupancy in your definition for full marks.

Question 2. With the help of an activity, show particulate nature of matter.
Answer:
1. Take a beaker and fill it with 100 mL of water. Note the water level.
2. Add a few crystals of potassium permanganate or a spoonful of sugar and stir well.
3. The crystals dissolve completely, and the water level remains unchanged.
4. This proves that matter is made up of tiny particles with spaces between them, into which the solute particles fit.
In simple words: Dissolving sugar in water without changing the water level proves that matter consists of particles with spaces between them.

Exam Tip: Emphasize the two key conclusions: particles are extremely small, and they have inter-particle spaces.

Question 3. What are characteristics of particles of matter?
Answer:
1. Particles of matter are extremely small in size.
2. Particles of matter have spaces between them.
3. Particles of matter are continuously moving (possess kinetic energy).
4. Particles of matter attract each other with forces of attraction.
In simple words: Matter particles are tiny, have spaces between them, move constantly, and attract one another.

Exam Tip: List all four distinct characteristics clearly for complete scoring.

Question 4. What is diffusion? Give an example.
Answer: Diffusion is the spontaneous intermixing of particles of two different types of matter on their own due to their random motion. Example: Spreading of the fragrance of an incense stick or the mixing of a drop of ink in water.
In simple words: Diffusion is the natural mixing of particles from higher to lower concentration.

Exam Tip: Mention that diffusion rate increases with an increase in temperature due to higher kinetic energy.

Question 5. Write any five characteristics of solids, liquids and gases.
Answer:
1. Shape and Volume: Solids have definite shape and volume; liquids have indefinite shape but definite volume; gases have neither definite shape nor volume.
2. Compressibility: Solids are almost incompressible; liquids are negligibly compressible; gases are highly compressible.
3. Fluidity: Solids cannot flow; liquids and gases can flow (fluids).
4. Interparticle Spaces: Solids have minimum spaces; liquids have moderate spaces; gases have maximum spaces.
5. Interparticle Forces: Solids have maximum attractive forces; liquids have moderate forces; gases have negligible forces.
In simple words: Solids are rigid and fixed; liquids flow and take container shape; gases expand to fill any container.

Exam Tip: Present this as a comparison table or structured bullet points for maximum clarity.

Question 6. Why can we smell hot food from a distance?
Answer: Hot food releases volatile aromatic substances whose particles have high kinetic energy at higher temperatures. Due to rapid diffusion, these particles travel quickly through the air over long distances.
In simple words: Heat gives food particles high energy, making them diffuse rapidly through the air.

Exam Tip: Link temperature to kinetic energy and rate of diffusion.

Question 7. Why does a solid change into liquid on heating?
Answer: When a solid is heated, the supplied heat energy increases the kinetic energy of its particles. This vibrational energy overcomes the strong intermolecular forces of attraction holding the particles in fixed positions, causing them to break free and form a liquid.
In simple words: Heating gives particles enough energy to overcome the forces holding them in place, melting the solid.

Exam Tip: Use the terms 'kinetic energy' and 'intermolecular forces of attraction'.

Question 8. Define latent heat of fusion.
Answer: The amount of heat energy required to convert 1 kg of a solid into liquid at its melting point at atmospheric pressure is called the latent heat of fusion.
In simple words: Latent heat of fusion is the heat needed to melt 1 kg of solid into liquid at constant temperature.

Exam Tip: Include the keywords '1 kg', 'solid to liquid', and 'melting point'.

Question 9. Give reason - A gas fills completely the vessel in which it is kept.
Answer: Gases have negligible intermolecular forces of attraction and very high kinetic energy. Consequently, gas particles move freely at high speeds in all directions, filling the entire available volume of the container.
In simple words: Gas particles move very fast in all directions with almost no attraction between them, spreading to fill any container.

Exam Tip: Mention weak intermolecular forces and high kinetic energy.

Question 10. Why a wooden table should be called solid?
Answer: A wooden table has a definite shape, distinct boundaries, and a fixed volume. It is rigid, incompressible, and its particles have fixed positions with strong interparticle forces, which are all characteristic properties of a solid.
In simple words: A wooden table has a fixed shape, fixed volume, and cannot be compressed, making it a solid.

Exam Tip: Relate the object's macroscopic properties to the definition of a solid.

Question 11. Why more serious burns are caused by steam at \(100^\circ\text{C}\) than water at same temperature?
Answer: Steam at \(100^\circ\text{C}\) contains extra heat energy in the form of latent heat of vaporization (\(2.26 \times 10^6\) J/kg) compared to water at the same temperature. Therefore, steam releases more heat upon condensing on the skin, causing more severe burns.
In simple words: Steam carries extra latent heat energy, releasing more heat when it hits the skin than boiling water does.

Exam Tip: Always mention 'latent heat of vaporization' when explaining steam burns.

Question 12. Why can a sponge be compressed though it is a solid?
Answer: A sponge has minute pores in which air gets trapped. When we press the sponge, the trapped air is expelled out, allowing it to be compressed. Once released, it regains its original shape.
In simple words: A sponge has air-filled holes; pressing it pushes out the air, making it compressible.

Exam Tip: Clarify that the solid framework remains solid, but trapped air spaces cause the compressibility.

Question 13. Carry out following conversion (a) \(50^\circ\text{C}\) to Kelvin (b) 200 K to Celsius.
Answer:
(a) Temperature in Kelvin = Temperature in Celsius + 273
\(50^\circ\text{C} = 50 + 273 = 323\) K
(b) Temperature in Celsius = Temperature in Kelvin - 273
\(200 \text{ K} = 200 - 273 = -73^\circ\text{C}\)
In simple words: Add 273 to convert Celsius to Kelvin, and subtract 273 to convert Kelvin to Celsius.

Exam Tip: Show the formula clearly before substituting numerical values.

Question 14. Name the conditions to liquefy a gas.
Answer: A gas can be liquefied by applying high pressure and lowering the temperature.
In simple words: High pressure and low temperature turn a gas into a liquid.

Exam Tip: Lowering temperature reduces kinetic energy, and increasing pressure brings particles closer together.

Question 15. Why are clothes spread out for drying?
Answer: Spreading out clothes increases the surface area exposed to the air and sunlight. Since the rate of evaporation is directly proportional to surface area, the water evaporates much faster.
In simple words: Spreading clothes increases surface area, making water evaporate quicker.

Exam Tip: Link surface area to the factors affecting evaporation.

Question 16. What kind of clothes is most suitable for summers? Why?
Answer: Cotton clothes are most suitable for summers. Cotton is a good absorber of water, so it absorbs sweat from our body and exposes it to the atmosphere for easy evaporation. The latent heat required for evaporation is taken from the body, producing a cooling effect.
In simple words: Cotton absorbs sweat and lets it evaporate easily, keeping the body cool.

Exam Tip: Explain both sweat absorption and subsequent evaporative cooling.

Question 17. What is sublimation? Explain with help of an activity and a diagram.
Answer: Sublimation is the transition of a substance directly from the solid to the gas state without passing through the liquid phase.
Activity:
1. Take some camphor or ammonium chloride in a china dish.
2. Place an inverted funnel over the china dish and plug the stem of the funnel with cotton.
3. Heat the china dish slowly using a burner.
4. Ammonium chloride changes directly into vapors, which condense on the cooler inner walls of the funnel, leaving non-volatile impurities behind.
In simple words: Sublimation is changing directly from solid to gas when heated.

Exam Tip: Mention the inverted funnel setup and the collection of sublimate on the upper cooler walls.

Question 18. Washed clothes dry up more quickly on a hot summer day than on a rainy day. Why?
Answer: On a hot summer day, temperature is high and humidity is generally low, both of which increase the rate of evaporation. On a rainy day, high humidity in the air reduces the evaporation rate.
In simple words: High temperature and low humidity in summer make water evaporate faster than on humid rainy days.

Exam Tip: Mention both temperature and atmospheric humidity in your explanation.

Question 19. What is the difference between evaporation and boiling?
Answer:
1. Nature: Evaporation is a surface phenomenon occurring at any temperature below boiling point; boiling is a bulk phenomenon occurring at a fixed, specific temperature.
2. Bubbles: No bubbles are formed during evaporation; bubbles form throughout the liquid during boiling.
3. Source of Heat: Evaporation draws heat from the surroundings; boiling requires a continuous external heat source.
In simple words: Evaporation happens slowly at the surface at any temperature, while boiling happens rapidly throughout the liquid at its boiling point.

Exam Tip: Highlight that evaporation is a surface phenomenon while boiling occurs throughout the liquid (bulk phenomenon).

Question 20. Define latent heat of vaporization.
Answer: The amount of heat energy required to convert 1 kg of liquid into gas at its boiling point at atmospheric pressure is called the latent heat of vaporization.
In simple words: It is the heat needed to turn 1 kg of liquid into vapor at its boiling point.

Exam Tip: Use the exact phrasing '1 kg of liquid into gas at its boiling point'.

Question 21. Wearing synthetic clothes in summers is usually avoided. Why?
Answer: Synthetic clothes (like polyester or nylon) do not absorb sweat. Consequently, sweat is not absorbed and evaporated from the body surface, leading to discomfort, sticky skin, and prickly heat.
In simple words: Synthetics do not absorb sweat, trapping moisture on the skin.

Exam Tip: Contrast synthetic fibers with natural fibers like cotton regarding moisture absorption.

Question 22. How does water get cooled in an earthen pot?
Answer: An earthen pot (matka) has tiny microscopic pores through which water seeps to the outer surface. This water continuously evaporates, drawing latent heat from the remaining water inside the pot, which lowers its temperature.
In simple words: Water seeps through tiny pores in the pot and evaporates, cooling the water left inside.

Exam Tip: Emphasize the role of microscopic pores and evaporative cooling.

Question 23. What is dry ice?
Answer: Solid carbon dioxide (\(\text{CO}_2\)) is called dry ice. It is stored under high pressure and converts directly into carbon dioxide gas when pressure is reduced to 1 atmosphere without melting into liquid.
In simple words: Dry ice is solid carbon dioxide that turns straight into gas.

Exam Tip: Mention that it is called 'dry' because it does not wet surfaces like regular ice (no liquid phase).

Question 24. Name the factors affecting evaporation.
Answer:
1. Surface area
2. Temperature
3. Humidity
4. Wind speed
In simple words: Surface area, temperature, humidity, and wind speed affect evaporation.

Exam Tip: List all four factors clearly.

Question 25. How does evaporation cause cooling? Explain with example.
Answer: During evaporation, liquid molecules at the surface absorb thermal energy (latent heat) from their surroundings to overcome intermolecular forces and escape into the vapor phase. This loss of heat from the surroundings causes cooling.
Example: When acetone or nail polish remover is poured on your palm, it evaporates rapidly by absorbing heat from your hand, making your palm feel cold.
In simple words: Evaporation absorbs heat from the surroundings, causing a cooling sensation (like acetone on skin).

Exam Tip: Explain energy absorption from the surroundings during phase transition.

Question 26. How can physical state of matter be changed?
Answer: The physical state of matter can be changed by changing the temperature (heating or cooling) or by changing the pressure (compressing or expanding).
In simple words: Changing temperature or pressure can convert matter from one state to another.

Exam Tip: Mention both temperature and pressure as the two main thermodynamic parameters.

CLASS - IX CHEMISTRY - ASSIGNMENT 2
MONTH: AUG-SEP - IS MATTER AROUND US PURE

Question 1. What are mixtures? Give examples.
Answer: Mixtures are substances constituted by more than one kind of pure form of matter (components) mixed together physically in any proportion, retaining their individual properties. Example: Air, milk, saltwater.
In simple words: Mixtures are physical combinations of two or more substances.

Exam Tip: Emphasize that components retain their original properties and are mixed in variable proportions.

Question 2. What are pure substances?
Answer: A pure substance is a form of matter that consists of a single type of particles (atoms or molecules) and has a fixed, definite chemical composition and uniform properties throughout.
In simple words: Pure substances consist of only one type of particle with a fixed composition.

Exam Tip: In chemistry, 'pure' means uniform composition and constant properties (elements and compounds).

Question 3. Why do we call sugar a pure substance?
Answer: Sugar (sucrose) consists of only one type of constituent particles (sugar molecules) with a fixed chemical formula (\(\text{C}_{12}\text{H}_{22}\text{O}_{11}\)) and constant chemical and physical properties.
In simple words: Sugar has a fixed chemical formula and is made of only one type of molecule.

Exam Tip: Fixed chemical formula is the key reason why compounds like sugar are considered pure substances.

Question 4. What are saturated and unsaturated solutions?
Answer:
- Saturated Solution: A solution in which no more solute can be dissolved at a given specific temperature under normal conditions.
- Unsaturated Solution: A solution in which more solute can be dissolved at the same temperature without increasing the temperature.
In simple words: Saturated solutions hold the maximum possible solute; unsaturated solutions can hold more.

Exam Tip: Always include the phrase 'at a given temperature' when defining saturation.

Question 5. Define a solution.
Answer: A solution is a homogeneous mixture of two or more substances whose composition can be varied up to a certain limit. Example: Saltwater, vinegar.
In simple words: A solution is a uniform mixture of a solute dissolved in a solvent.

Exam Tip: Highlight homogeneity as the defining feature of a solution.

Question 6. What is a suspension? Give its example and properties.
Answer: A suspension is a heterogeneous mixture in which solute particles do not dissolve but remain suspended throughout the bulk of the medium.
Example: Chalk powder in water, muddy water.
Properties:
1. It is a heterogeneous mixture.
2. Particle size is greater than 100 nm.
3. Particles can be seen with the naked eye.
4. Particles scatter a beam of light (Tyndall effect) and settle down over time.
In simple words: A suspension is a cloudy heterogeneous mixture where particles settle down over time.

Exam Tip: List particle size, heterogeneity, and settling behavior for full credit.

Question 7. Define concentration of a solution.
Answer: The concentration of a solution is the amount of solute present in a given quantity (mass or volume) of solution or solvent.
\(\text{Concentration} = \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \times 100\)
In simple words: Concentration measures how much solute is dissolved in a given amount of solution.

Exam Tip: Memorize the mass percentage formula for numerical problems.

Question 8. What is Tyndall effect?
Answer: The phenomenon of scattering of a beam of light by colloidal particles, making the path of the light beam visible, is called the Tyndall effect.
In simple words: Scattering of light by colloidal particles is called the Tyndall effect.

Exam Tip: Mention both light scattering and colloidal particle size in your answer.

Question 9. What is the difference between True solution and colloids?
Answer:
1. Particle Size: True solution particle size is \(< 1\) nm; colloid particle size is between 1 nm and 100 nm.
2. Homogeneity: True solution is completely homogeneous; colloid appears homogeneous but is heterogeneous upon microscopic examination.
3. Tyndall Effect: True solutions do not show Tyndall effect; colloids show Tyndall effect.
4. Filterability: Solutes pass through filter paper in both, but colloids pass through ordinary filter paper while being stopped by ultrafilters.
In simple words: True solutions have smaller particles and do not scatter light, whereas colloids have larger particles and scatter light.

Exam Tip: Tabulate differences using particle size, homogeneity, and light scattering.

Question 10. What are alloys? Why are alloys called as mixture?
Answer: Alloys are homogeneous mixtures of two or more metals (or a metal and a non-metal) that cannot be separated into their components by physical methods. They are called mixtures because they retain the properties of their constituent elements and can have variable composition.
In simple words: Alloys are metallic mixtures that keep the properties of their components and have variable composition.

Exam Tip: Brass (copper and zinc) is the classic example of an alloy.

Question 11. Write the characteristics of brass.
Answer:
1. Brass is a solid solution (alloy) of copper (approx 70%) and zinc (approx 30%).
2. It is homogeneous in nature.
3. It retains the properties of both copper and zinc.
4. It has a variable composition depending on preparation.
In simple words: Brass is a uniform mixture of copper and zinc.

Exam Tip: Remember that brass is an alloy of copper and zinc.

Question 12. Define solute and solvent.
Answer:
- Solute: The component of the solution that is dissolved in the solvent (usually present in lesser quantity).
- Solvent: The component of the solution that dissolves the solute in it (usually present in greater quantity).
In simple words: The solute gets dissolved, and the solvent does the dissolving.

Exam Tip: Solvent is usually the component present in larger amount.

Question 13. What is solubility?
Answer: Solubility is the maximum amount of solute in grams that can be dissolved in 100 grams of a solvent to form a saturated solution at a specific temperature.
In simple words: Solubility is the maximum grams of solute that dissolve in 100 g of solvent at a given temperature.

Exam Tip: Include '100 grams of solvent' and 'saturated solution at a specific temperature'.

Question 14. Give properties of a true solution.
Answer:
1. It is a homogeneous mixture.
2. Solute particle size is smaller than 1 nm (\(10^{-9}\) m).
3. Particles cannot be seen with naked eyes or even under a microscope.
4. Solute particles do not scatter a beam of light (no Tyndall effect).
5. Solute particles do not settle down; the solution is stable.
In simple words: True solutions are uniform, stable, and have invisible particles that do not scatter light.

Exam Tip: List homogeneity, particle size, and stability as key properties.

Question 15. Why do we need to separate mixtures?
Answer: We need to separate mixtures to:
1. Remove harmful or unwanted components.
2. Obtain useful and pure components.
3. Separate two different but useful components.
In simple words: Separation helps remove impurities and obtain pure, useful substances.

Exam Tip: Give practical examples like purifying drinking water or extracting metals from ores.

Question 16. How can we separate cream from milk?
Answer: Cream is separated from milk using the process of centrifugation. Milk is spun rapidly in a centrifuge machine, causing the denser components (skimmed milk) to move outward and the lighter components (cream/fat) to collect at the center.
In simple words: Spinning milk rapidly in a centrifuge separates lighter cream from heavier milk.

Exam Tip: Mention centrifugation and the difference in particle density.

Question 17. Write the applications of centrifugation.
Answer:
1. Used in diagnostic laboratories for blood and urine tests.
2. Used in dairies and homes to separate butter/cream from milk.
3. Used in washing machines to squeeze out water from wet clothes.
In simple words: Centrifugation is used in medical labs, dairies, and washing machines.

Exam Tip: Provide at least two clear real-life applications.

Question 18. How can we separate a mixture of salt and ammonium chloride? Draw a diagram.
Answer: This mixture is separated by sublimation.
1. Place the mixture in a china dish and cover it with an inverted funnel whose stem is plugged with cotton.
2. Heat the china dish.
3. Ammonium chloride sublimes, rising as vapor and condensing on the cooler inner walls of the funnel, while sodium chloride remains behind in the china dish.
In simple words: Heat the mixture so ammonium chloride sublimes and leaves salt behind.

Exam Tip: State the property difference: ammonium chloride sublimes, sodium chloride does not.

Question 19. What is chromatography? Explain the process.
Answer: Chromatography is a technique used for separating different components of a mixture based on their varying solubilities in a suitable solvent. Process: A drop of dye solution is placed near the bottom of a filter paper strip, which is then dipped in a solvent. As the solvent rises up the paper via capillary action, components with higher solubility travel faster and separate.
In simple words: Chromatography separates dissolved substances using a moving solvent on filter paper.

Exam Tip: Mention capillary action and differences in component solubility.

Question 20. How can you separate copper sulphate from an impure sample?
Answer: Copper sulfate is separated using the crystallization method.
1. Dissolve the impure sample in a minimum amount of water.
2. Filter the solution to remove insoluble impurities.
3. Evaporate water from the filtrate to obtain a saturated solution.
4. Allow the hot saturated solution to cool slowly undisturbed, yielding pure copper sulfate crystals.
In simple words: Dissolve, filter, and cool a concentrated solution to grow pure crystals.

Exam Tip: Crystallization is preferred over simple evaporation because it prevents decomposition of solutes.

Question 21. What types of mixture are separated by fractional distillation process? Draw a diagram.
Answer: Fractional distillation is used to separate a mixture of two or more miscible liquids whose boiling point difference is less than \(25^\circ\text{C}\) (e.g., acetone and water, or separation of components of petroleum).
In simple words: Fractional distillation separates miscible liquids with close boiling points using a fractionating column.

Exam Tip: Emphasize the fractionating column which provides multiple condensation and vaporization cycles.

Question 22. Draw a diagram to explain the process of separating acetone and water.
Answer: Acetone and water are miscible liquids with different boiling points, so they are separated by distillation (or fractional distillation).
1. Heat the mixture in a distillation flask.
2. Acetone (boiling point \(56^\circ\text{C}\)) boils off first, passes through a condenser, and is collected in a receiver flask.
3. Water (boiling point \(100^\circ\text{C}\)) remains behind in the distillation flask.
In simple words: Heat the mixture; the liquid with the lower boiling point boils off first and is condensed back into liquid.

Exam Tip: Base the separation on boiling point differences.

Question 23. Explain with the diagram the method of separating kerosene oil from water by using a separating funnel.
Answer: Kerosene and water are immiscible liquids. Pour the mixture into a separating funnel and let it stand. Because of different densities, water (denser) settles at the bottom and kerosene (lighter) forms the upper layer. Open the stopcock of the funnel to run off the lower water layer, then close it as kerosene reaches the stopcock.
In simple words: Use a separating funnel to drain out the lower denser liquid layer, leaving the lighter liquid behind.

Exam Tip: Principle relies on difference in densities and immiscibility.

Question 24. What are physical and chemical changes?
Answer:
- Physical Change: A change in which no new chemical substance is formed and the chemical composition remains unaltered (e.g., melting of ice, boiling of water).
- Chemical Change: A change in which one or more new substances with entirely new chemical compositions and properties are formed (e.g., burning of paper, rusting of iron).
In simple words: Physical changes alter appearance without new substances forming; chemical changes create completely new substances.

Exam Tip: Focus on whether a new chemical substance is formed.

Question 25. Write difference between mixtures and compounds.
Answer:
1. Composition: Mixtures have variable composition; compounds have fixed, definite composition.
2. Properties: Mixtures retain the properties of their components; compounds have entirely new properties.
3. Separation: Components of mixtures can be separated by physical methods; components of compounds can only be separated by chemical or electrochemical methods.
In simple words: Mixtures are physically combined with variable ratios, while compounds are chemically combined in fixed ratios.

Exam Tip: Contrast composition, property retention, and separation techniques.

Question 26. Give definitions of elements and compounds.
Answer:
- Element: The basic form of matter that cannot be broken down into simpler substances by chemical reactions (e.g., Hydrogen, Iron, Oxygen).
- Compound: A substance composed of two or more elements chemically combined in a fixed proportion by mass (e.g., Water, Carbon dioxide).
In simple words: Elements are pure foundational substances; compounds are chemical combinations of elements in fixed ratios.

Exam Tip: Emphasize that elements are elementary substances while compounds contain bonded elements.

Question 27. What are metals and non-metals?
Answer:
- Metals: Elements that are malleable, ductile, lustrous, good conductors of heat and electricity, and generally solid at room temperature (e.g., Iron, Copper, Gold).
- Non-metals: Elements that are poor conductors of heat and electricity, non-lustrous, brittle, and can be solids, liquids, or gases (e.g., Sulfur, Oxygen, Carbon).
In simple words: Metals are shiny and conduct electricity well, while non-metals are dull and poor conductors.

Exam Tip: Compare conductivity, malleability, and luster.

Question 28. If 110 g of salt is present in 550 g of solution, calculate the concentration.
Answer:
Mass of solute (salt) = 110 g
Mass of solution = 550 g
Concentration (mass percentage) = \(\left(\frac{\text{Mass of Solute}}{\text{Mass of Solution}}\right) \times 100\)
\(= \left(\frac{110}{550}\right) \times 100\)
\(= \frac{1}{5} \times 100 = 20\%\)
In simple words: Divide solute mass by solution mass and multiply by 100 to get 20%.

Exam Tip: Ensure the denominator is the total mass of the solution, not just the solvent.

Question 29. What is the concentration of a solution which contains 16 g of urea in 120 g of solutions?
Answer:
Mass of solute (urea) = 16 g
Mass of solution = 120 g
Concentration percentage = \(\left(\frac{16}{120}\right) \times 100\)
\(= \left(\frac{4}{30}\right) \times 100 = \frac{40}{3} = 13.33\%\)
In simple words: Concentration is \(13.33\%\) by mass.

Exam Tip: Always double check your division and fraction simplification.

Question 30. How much water should be added to 15 g of salt to obtain 15% salt solution?
Answer:
Mass of solute (salt) = 15 g
Let mass of solution = \(x\) g
Given concentration percentage = 15%
\(\frac{15}{x} \times 100 = 15\)
\(x = \frac{15 \times 100}{15} = 100 \text{ g (total mass of solution)}\)
Mass of water = Mass of solution - Mass of solute
Mass of water = \(100 - 15 = 85 \text{ g}\)
In simple words: 85 g of water must be added.

Exam Tip: Remember that solution mass equals solute mass plus solvent mass.

Question 31. A solution contains 5.6 ml of alcohol mixed with 76 ml of water. Calculate the concentration of the solution.
Answer:
Volume of solute (alcohol) = 5.6 mL
Volume of solvent (water) = 76 mL
Volume of solution = \(5.6 + 76 = 81.6\) mL
Concentration (volume percentage) = \(\left(\frac{5.6}{81.6}\right) \times 100\)
\(= \frac{560}{81.6} \approx 6.86\%\)
In simple words: The concentration by volume is \(6.86\%\).

Exam Tip: Total solution volume is the sum of solute and solvent volumes.

Question 32. How much water should be added to 12 ml of alcohol to obtain 12% alcohol solution?
Answer:
Volume of solute (alcohol) = 12 mL
Let total volume of solution = \(x\) mL
Given concentration = 12%
\(\frac{12}{x} \times 100 = 12\)
\(x = 100 \text{ mL (total solution volume)}\)
Volume of water needed = Total solution volume - Volume of alcohol
Volume of water = \(100 - 12 = 88\) mL
In simple words: Add 88 mL of water.

Exam Tip: Set up the percentage formula correctly and solve for the total solution volume first.

Question 33. If 25 ml of acetone is present in 150 ml of its aqueous solution, calculate the concentration.
Answer:
Volume of solute (acetone) = 25 mL
Volume of solution = 150 mL
Concentration (volume percentage) = \(\left(\frac{\text{Volume of Solute}}{\text{Volume of Solution}}\right) \times 100\)
\(= \left(\frac{25}{150}\right) \times 100\)
\(= \frac{1}{6} \times 100 = 16.67\%\)
In simple words: The concentration is \(16.67\%\) by volume.

Exam Tip: Check whether solution volume or solvent volume is given; here, 150 mL is the total solution volume.