CBSE - Class 9 - CH2 - Is Matter Around Us Pure

Q1: On the basis of composition, how matter is classified?

Answer:

• Pure Subtance
• Mixture

Q2(NCERT): What is meant by a pure substance?

Answer: A sample of matter containing only one substance is called a pure substance. In other words all constituents of the substance are same in their chemical nature.



Q3: What are the characteristics exhibited by a pure substance?

Answer:

1. A pure substance contains only one kind of atoms or molecules.
2. It is perfectly homogenous
3. It has definite composition which does not vary with time.
4. It has definite melting point, boiling point, density etc.

Q4: What are different categories of pure substance?
Answer:

• Elements
• Compounds

Q5: Name two properties of a substance to check its purity?

Answer: A pure substance has a fixed melting point or boiling point at constant pressure. The purity of a substance can be tested by checking its melting point or boiling point. If a substance is impure i.e. it contains traces of another substance, the melting and boiling point of that substance will change.

Q6: Define mixture.

Answer: If two or more substances (elements or compounds) are mixed together in any proportion, do not undergo any chemical change but retain their properties, the resulting substance is called mixture.

Q7: What are the kinds of mixture?

Answer:

• Homogenous mixture
• Hetrogenous mixture

Q8: What are the characteristics of mixture?

Answer:

1.  Variable composition: The constituents of a mixture are present in any ratio. Example: A mixture of sand and salt can be in a ratio of 1:2 or 1:3 by weight.

2. Only Physical change: The mixture is a result of physical change. The constituents of a mixture do not bind each other by chemical bonds. Example: In air the main constituents, i.e., oxygen, nitrogen and carbon dioxide, do not bind each other with chemical bonds.

3. No specific properties: The properties of a mixture are the average of the properties of its constituents. Example: The properties of air are average common properties of nitrogen and oxygen.

4.  Homogeneity: Most of the mixtures are heterogeneous, i.e., their constituents are not spread evenly throughout. However, some mixtures are homogeneous i.e. constituents are uniformly spread out. Example: In the mixture of iron and sulphur, at some places iron is more and at some places sulphur is more.

5.  Separation Methods: In general, the constituents of mixture can be separated by applying suitable physical methods. E.g. Iron can be separated from the mixture of iron and sulphur with the help of a magnet.

6.  Energy changes: In general, no energy is released or absorbed during the formation of a mixture.  Example: On mixing iron and sulphur, heat energy is neither absorbed nor evolved.

Q 9: List the points of differences between homogeneous and heterogeneous mixtures.

Answer:

Homogenous Mixture	Heterogeneous Mixture
1. Constituents have uniform composition throughout the mixture.	1. Constituents have non-uniform composition throughout mixture.
2. Air, blood, saturated sugar, water solution, alloys are examples of homogenous mixture.	2. Rock, oil and water, Mixture of Iron filling and Sulphur.
3. Particles Not visible to unaided eye.	3. Particles may be visible to unaided eye.
4. All solutions are homogenous mixture.	4. All suspensions are heterogeneous mixture.

Q10: Identity which of the following is homogenous mixture or heterogeneous one. Also identify the type of constituents in mixture (e.g. gas in gas, gas in liquid, gas in solid etc.)
a. Air
b. Water and Oil (N₂ + O₂ )
c. Hydrogen in Palladium
d. Aerated Water (CO₂ + H₂O)
e. Chalk in water
f. Ethyl Alcohol in Water
g. Alloys e.g. brass
h. Dust (e.g. fine sand) in water
i. Sand + iron fillings
j. Sand + ammonium chloride
h. milk
i. Mercury in amalgamated Zinc

Answer:

Type of Mixture	Homogenous Mixture	Heterogeneous Mixture
1. Gas in gas	Air
2. Gas in liquid	Aerated water
3. Gas in solid	Hydrogen in palladium
4. Liquid in liquid	Ethyl alcohol + Water	Water + Oil, Milk
5. Liquid in solid	Mercury in amalgamated Zinc
6. Solid in liquid	Sugar in water	Chalk in water, Dust in water
7. Solid in solid	Alloys e.g. brass	Sand + Iron Fillings, Sand + ammonium chloride

Q11: What are the constituents of brass?

Answer: Brass is an alloy and is a mixture of Zinc(30%) and Copper (70%).

Q12: Alloys cannot be separated by physical means, though it is considered mixture, Why?

Answer:  Alloys are homogeneous mixtures of metals and cannot be separated into their components by physical methods. But still, an alloy is considered as a mixture because it shows the properties of its constituents and can have variable composition.

Q13: What are elements?

Answer: Elements are substances that cannot be chemically broken down into simpler substances. So an element is made up of only one kind of atoms. For example, silver is an element which is made up of only silver atoms. Eiements are the building blocks of all matter.

Q14: How elements are further classified?

Answer:  Metals, Non-metals, metalloids.

Q15: What is a compound? Give an example.

Answer: A Compound is u substance made up of two or more elements chemically combined in a fixed ratio by weight. They are homogeneous and exhibit definite physical and chemical properties E.g water is a compound. It is made up of two elements hydrogen and oxygen, which combine chemically in a fixed ratio of 1:8 by weight. It possesses properties entirely different from the properties of hydrogen and oxygen.

Q16: State the differences between compounds and mixtures.

Answer:

Sno.	Compounds	Mixtures
1.	It is made up of  two or more elements that are chemically combined.	It is made up of two or more pure substances that are mixed physically.
2.	A compound has definite melting and boiling points and density.	A mixture has no definite melting or boiling points and density.
3.	The properties of a compound are entirely different from those of its constituents.	A mixture retains the properties of the components.
4.	A compound is always homogeneous.	A mixture is heterogeneous, and some are homogeneous.
5.	The constituents of a compound cannot be separated by physical means.	The components of a mixture can be separated by simple physical means,(dissolving, magnetic separation, heating, and filtration)

Q17: What is a solution? What are the properties of a solution?

Answer: A homogeneous mixture of two or more substances is called a solution.
Properties of a solution

1. A solution is a homogeneous mixture i.e. the solute and solvent molecules cannot be distinguished even under a microscope.
2. The particles of a solution are smaller than 1 nm in diameter. So, they cannot be seen by naked eyes.
3. A true solution is clear and transparent ( A few may be coloured one but clear. e.g. Copper Sulphate solution)
4. Because of very small particle size, they do not scatter a beam of light passing through the solution. So, the path of light is not visible in a solution.
5. The solute particles cannot be separated from the mixture by the process of filtration.
6. The solute particles do not settle down when left undisturbed, that is, a solution is stable.

Q18: What is meant by solute and solvent? 

Answer: A solution is obtained when a substance is dissolved in another substance. The substance which is dissolved to obtain a solution is called solute, whereas the substance in which the solute is dissolved is called solvent.
For example when sugar is dissolved in water to obtain sugar solution, sugar is the solute and water is solvent.

Q19: Identify solute and solvent in the following solutions. Also mention the physical state of solute and solvent.
(a) Sugar in water
(b) Urea in water
(c) Ammonium chloride in water
(d) Ethyl alcohol in water
(e) Carbon Di-Oxide in water (soda water)

Answer:

Solution	Solvent		Solute
		Name	Physical State	Name	Physical State
Sugar in water	Water	Liquid	Sugar	Solid
Urea in water	Water	Liquid	Urea	Solid
Ammonium Chloride in water	Water	Liquid	Ammonium Chloride	Solid
Ethyl Alcohol in water	Water	Liquid	Ethyl Alcohol	Liquid
Carbon Di-Oxide in water	Water	Liquid	Carbon Di-Oxide	Gas

Q20: What is meant by Solubility?

Answer: The maximum amount of the solid that can be dissolved in a given amount of the  solvent (water) is termed its solubility at that temperature.
Numerically, it is the number of grams of a solute that dissolves in 100g of a solvent to form a saturated solution at a given temperature and pressure. Solubility is the maximum weight of a solute that can be dissolved in 100g of a solvent at a given temperature and pressure.

Q21: What factors affect the solubility of solvent and solute?

Answer:

• Temperature: For majority of solutions of solid-in-liquid and liquid-in-liquid types, solubility increases with temperature. However for solutions of gases-in-water type, solubility decreases with increase in temperature.
• Pressure: It is applicable to gas-in-liquid solutions. An increase in pressure increases the solubility of a gas. For example, aerated drinks contain carbon dioxide gas under pressure.
• Mechanical Stirring: Mechanical stirring increases solubility. For example, sugar dissolves faster on stirring with a spoon.
• Size of Solute Particles: Smaller the particle size of solute, greater is the solubility. For example, it is easier to dissolve powdered sugar than granules of sugar.

Q22: Why do fish go in deep waters during day light?

Answer: During day time, the shallow water is warmer and hence it has less dissolved oxygen. Therefore fish tend to go in deep water during day time.

Q23: Based on the type of solvent, how solutions are classified?

Answer:

• Aqueous solution
• Non-aqueous solution

Q24: Based on the amount of solute in the given solution, how solutions are classified?

Answer:

• Unsaturated solution
• Saturated solution
• Super saturated solution

Q25: What are aqueous solutions?

Answer: Solutions in which water is the solvent are called aqueous solutions. E.g. sugar solution i.e. sugar dissolved in water.

Q26: What are non-aqueous solutions?

Answer: The solution obtained by dissolving solute in any liquid other than water is called non-aqueous solution. (non-aqueous mean without water).
e.g. if solute is dissolved in solvent like benzene, alcohol, ether, carbon disulphide, acetone etc. forms a non-aqueous solution.

Q27: When we open the cap of a cola drink (or any carbonated beverage), why does excess of bubbles come out?

Answer: A cola (or carbonated) beverage is produced by dissolving carbon dioxide in the drink
solution under pressure. Increase in pressure increases the solubility of gases. Therefore, more carbon dioxide dissolves at the higher pressure. When this pressure is suddenly released, e.g. by removing the cap of the bottle, carbon dioxide is less soluble, and it comes out in as bubbles from the solution.

Q28: Why air is a mixture not a compound? Give reasons.

Answer: Air is a mixture for of the following reasons:

1. Air does not have a fixed composition. Its composition varies from place to place.
2. The components of air can be separated by a physical method such as fractional distillation of liquid air.
3. Liquid air does not have a definite boiling point.
4. We can prepare air artificially by mixing the various components of air in the same proportions in which they occur at a place. This is a physical process because no energy changes occur.

Q29: What are the advantages of preparing solutions?

Answer: In a solution, solute and solvent may be present in the form of ions or molecules or both. Solutions are obtained for the following reasons:

1. To carry out reactions. For a reaction to take place, reactant molecules must come close together. In solutions, reactions take place faster because reactants are in ionic or molecular form an dare close to each other.
2. As dissolving medium like medicines. Certain medicines can only be administered to patients in solution form only. e.g. saline glucouse solution is given to patients suffering from dehydration.

Q30: What is an unsaturated solution?

Answer: A solution in which some more solute can be dissolved at any fixed temperature is called an unsaturated solution.

Q31(NCERT): What is saturated solution? Explain with an example.

Answer: A solution in which no more solute can be dissolved at any fixed temperature is called a saturated solution. In saturated solution, dissolved and undisolved solutes are in equilibrium with each other. Any more solute added will settle down at the bottom of the container as a precipitate.

e.g. if 500 g of a solvent can dissolve a maximum of 150 g of a particular solute at 40°C. Then, the solution obtained by dissolving 150 g of that solute in 500 g of that solvent at 300 K is said to be a saturated solution at 300 K.


Q32: What is supersaturated solution?

Answer: Any solution containing more solute than required to prepare a saturated solution at any fixed temperature is called supersaturated solution.

Supersaturated solution is a meta-stable state i.e. it will remain in supersaturated state so long it is left undisturbed. A slight disturbance (e.g. slight rise or fall of temperature) will turn it into a saturated solution.

Q33(NCERT): What are suspensions? Explain with an example.

Answer: A suspension is a heterogeneous mixture in which very fine particles (about 10^-5 m size) of solid are dispersed in any medium (liquid or gas). In suspension, the solute particles are visible to the naked eye, and remain suspended throughout the bulk of the medium. The Tyndall effect is observed in this mixture.

Muddy pond water, smoke coming out of a chimney, chalk-water mixture, coarse lime particles in water are examples of suspension.

Q34: What are the properties of  suspensions?

Answer: Following are the properties or important features of  suspensions:

1. Heterogeneous nature: Suspension is a cloudy heterogeneous mixture.
2. Particle Size: solute particles are of size of order 10^-5 m ( larger than 100 nano-m)
3. Visibility: The particles can be seen with unaided eye or under simple microscope. Particles scatter a beam of light passing through it so path is visible.
4. Sedimentation:  The larger particles have tendency to settle down while the very fine particles remain suspended in the medium.
5. Separation by filtration: Larger particles of suspension can be filtered out from fluid medium.

Q35(NCERT): Explain with an example what is a colloid?

Answer: A colloid is intermediate between suspension and solution. A homogenous-looking heterogeneous mixture in which particles have size between 1 nm to 100nm (1 nano-m = 10^-9 m) and are dispersed in a continuous medium is called a colloid.

A Colloidal solution is also called a sol. The continuous medium in a sol is called the dispersion medium and the particles form the dispersed phase or colloidal particles.

Examples of colloids are: milk, blood, toothpaste, jelly, fog, cloud etc.

Q36: What are the physical states of dispersed phase and dispersion medium of a cloud?

Answer:
Dispersed phase: Liquid
Dispersion medium: Gas

Q36: What are the physical states of dispersed phase and dispersion medium of a fog?

Answer:
Dispersed phase: Liquid (water droplets)
Dispersion medium: Gas (air)


Q37: What are the properties of a colloid?

Answer: Properties of a colloid are:

1. Heterogeneous nature: Colloid or Sol is heterogeneous in nature i.e. it can be seen only with a powerful microscope.
2. Particle Size: Colloid particle size lies between 1nm (10^-9 m) to 100 nm (10^-7 m)
3. Separation of colloidal particles by filtration: Colloidal particles cannot be separated from the
   mixture by the process of filtration. But, a special technique of separation known as centrifugation.
4. Brownian Motion: When seen under microscope, the colloidal particles in a colloid are seen to be moving in a random fashion, called Brownian motion.
5. Stability: They do not settle down when left undisturbed, that is, a colloid is quite stable.
6. Tyndall effect: Colloid exhibit Tyndall  effect. When a light is passed through sol kept in dark room, the path of light beam is visible. It is due to scattering of light by colloidal particles and this effect is called Tyndall effect.
7. Electrophoresis: Man colloidal particles may have electrical charge (+ve or -ve) on them. When an electric current passes through a sol, colloid particles move towards opposite charged electrodes. This process is called electrophoresis.

Q38(NCERT/CBSE 2011): How are sol, solution and suspension different from each other?

Answer:

Property	Solution	Sol(Colloid)	Suspension
1. Particle Size (1 nano-m = 10-9 m)	less than 1 nm	1 nm to 100 nm	More than 100 n-m
2. Appearance	Transparent	Translucent	Opaque
3. Particles Visibility	Not visible even under powerful microscope	Visible under powerful microscope	Visible to the naked eye
4. Nature	Homogeneous	Heterogeneous	Heterogeneous
5. Diffusion of particles	diffuses rapidly	diffuses slowly	diffusion does not occur
6. Scattering of Light	Does not scatter light	It scatters light	It scatters light

Q39(CBSE 2010): (a) What is Tyndall effect? Doe true solution exhibit Tyndall effect.

Tyndall Effect
(credits: wikipedia)

(b) ‘‘Tyndall effect can be observed when sunlight passes through the canopy of dense forest. or we see a rich red sunset.’’ "Explain how this occurs

Answer: (a)  The phenomenon by which colloidal particles scatter light is called Tyndall effect. The particles of a colloids scatter a beam of light passing through it and make its path visible. True solutions DO NOT show Tyndall effect. Scattering of light or Tyndall Effect is exhibited by Colloidal and Suspension particles.

Red Sunset

(b) When sunlight passes through a canopy of forest, the colloidal misty air particles scatter the sunlight. Similarly, the seattering of light by smoke or dust particles (colloid) produce a rich red sunset.

Watch the following you tube video explaining about Tyndall effect:

Q40: What do you mean by strength of the solution?

Answer: The amount of solute dissolved in a unit quantity (mass or volume) of a solution is called the concentration or strength of the solution.

i.e. Concentration of solution = Amount of solute/Amount of solution


Q41: What are the various methods to express concentration of a solution?

Answer: There are many ways to express strength of a solution. A fee are:

1. Percentage by mass
2. Percentage by volume

 1. Percentage by mass: It is defined as number of parts by  mass of the solute dissolved per hundred parts by mass of the solution. i.e.

2. Percentage by volume: It is defined as the number of parts by volume of the solute dissolved in hundred parts by volume of the solution.

Similarly, for liquid in liquid solutions, the strength of a solution can be expressed as

Note Mass percentage concentration of solution is independent of temperature. While mass by volume percentage strength of a solution is temperature dependent, since volume changes with temperature.

Q42(NCERT): To make a saturated solution, 36 g of sodium chloride is dissolved in 100 g of water at 293K. Find its concentration at this temperature.

Answer:  Concentration of a solution (mass %) = mass of solute × 100 / mass of solution.
mass of solute (NaCl) = 36g
mass of solvent (Water) = 100g
mass of solution = mass of solute + mass of solvent = 100 + 36 = 136g
mass by % concentration = 36 × 100 / 136 = 26.47%

Q43: Calculate the mass of glucouse and mass of water required to make 200g of 25% solution of glucouse.

Answer:  Given mass of solution(M) = 200g
Concentration of solution = 25%
Since, Mass by Mass percentage of solution = mass of solute(m) × 100 / mass of solution(M).
⇒ 25 = m × 100/200g
⇒ m = 25 × 200/100 = 50g

∴ mass of solute = 50g
mass of solvent (water) = M - m = 200g - 50g = 150g

Q44: A solution contains 40 mL of ethyl alcohol mixed with 100mL of water. What is the concentration of the solution in terms of volume by volume percentage?

Answer: Volume of solute (ethyl alcohol) = 40mL
Volume of solvent (water) = 100 mL
Total volume of the solution = 40 + 100 = 140 mL

Volume by Vol. Percentage = Volume of solute × 100 / volume of solution
⇒ v/v % = 40 × 100 /140 = 28.57%


Q45: What are different ways to separate solid mixtures?

Answers:

1. Hand Picking
2. Sieving
3. Magnetic Separation Method
4. Sublimation

Sieve
credits:wikipedia

Q46: Describe sieving method.

Answer: When the sizes of the components of a mixture are big enough, they can be separated with the help of sieve. A sieve is a simple mechanical device in which a mesh is attached to a frame. When the mixture is placed on the mesh and is stirred, particles of smaller size pass through the mesh while the bigger particles of the other component remain above the mesh.
E.g. gram can be separated wheat, sieving of sand at construction site etc.

Q47: Winnowing works on what property?

Answer: Winnowing is based on the property that grains are heavier than husk and hay.


Q48: What are the reasons for separating the constituents of a mixture?

Answer:

1. To remove any harmful or undesirable constituent.
2. To obtain a pure sample of a substance.
3. To obtain useful constituent from a mixture.

Q49: (a) When it is useful to apply sedimentation and decantation?
(b) Give an example where these methods are used?
(c) Explain the process.

Answer: (a) Sedimentation and Decantation are used to separate coarse particles of a solid which is insoluble in the liquid. Sedimentation works on the property that heavy particles settle down due to gravity.

(b) e.g. sand in muddy water can be separated using sedimentation and decantation technique.
Another example is to remove dust from rice.  When water is added, rice grains being heavy settle down while the dust remains floating on the water.

(c) The particles of sand particles settle down to the bottom of the beaker. The settling down of heavier particles is called sedimentation. We can transfer the clear water in the upper layer (supernatent) carefully to another beaker without disturbing the solid partciles. This is called decantation.



Separating solid-liquid mixtures

Q 50: What principle is applied in centrifugation? Give examples where this method is applied to separate mixtures.

Answer: Centrifugation works on the principle that that the denser particles are forced to the
bottom and the lighter particles stay at the top when spun rapidly.

Examples are:

1. Separating cream from milk (cream comes as top layer)
2. Used in diagnostic laboratories for blood and urine tests.
3. Used in washing machines to squeeze out water from wet clothes.
4. Used in preparing lactic cultures to prepare cheese (paneer) from milk in dairies.

Q51: How crystallization is better than evaporation?

Answer: The process of evaporation gives residue which may contain impurities. When there is requirement to obtain solid without impurity, crystallization method is preferred.

In general practice, impure solids are obtained by evaporation and are further purified by crystallization. e.g. salt obtained from sea is further purified by this method.

In some cases e.g. sugar crystal cannot be obtained by simple evaporation technique because sugar gets charred on heating to dryness. In such cases, crystallization technique is applied.

Q52(NCERT): Name the technique to separate
(i) butter from curd
(ii) salt from sea-water
(iii) camphor from salt

Answer:
(i) butter from curd : centrifugation
(ii) salt from sea-water: evaporation followed by crystallization.
(iii) camphor from salt: sublimation

Q53: What is Chromatography?

Answer: Chromatography is a technique used to separate those solutes of a mixture which are soluble in the same solvent. It works on the principle of adsorption. It is used to separate the coloured components (dyes) in black ink.

Q54: A good method to separate alum (phitkari) from impure samples is
(a) Filtration
(b) Sedimentaion
(c) Crystallization
(d) Sublimation

Answer: (c) Crystallization

Q55: A boy buys common salt from the market which is contaminated with Ammonium Chloride (NH₄Cl) and sand. The procedure he should adopt to obtain pure NaCl is the following :

(a) to mix the sample in water and evaporate the solution
(b) to mix the sample in water and evaporate the decanted solution
(c) to mix the sample in acetone and evaporate the decanted solution
(d) to heat the sample, then mix in water and evaporate the decanted solution

Answer: (d) to heat the sample, then mix in water and evaporate the decanted solution
Ammonium chloride sublimes, it can be separated from the mixture of salt and sand by sublimation method. Sand can be separated from the mixture of sand and salt by dissolving salt in water and then by filtration. Salt can be separated through the process of evaporation.

Separating liquid-liquid mixtures

Q56(NCERT): Which separation techniques will you apply for the separation of the following?(a) Sodium chloride from its solution in water.
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride.
(c) Small pieces of metal in the engine oil of a car.
(d) Different pigments from an extract of flower petals.
(e) Butter from curd.
(f) Oil from water.
(g) Tea leaves from tea.
(h) Iron pins from sand.
(i) Wheat grains from husk.
(j) Fine mud particles suspended in water.

Answer:
(a) Sodium chloride from its solution in water.  (Evaporation)
(b) Ammonium chloride from a mixture containing sodium chloride and ammonium chloride. (Sublimation)
(c) Small pieces of metal in the engine oil of a car.(Filtration)
(d) Different pigments from an extract of flower petals. (Chromatography)
(e) Butter from curd. (Centrifugation)
(f) Oil from water. (Separating Funnel)
(g) Tea leaves from tea. (Filtration)
(h) Iron pins from sand. (Magnetic Separation)
(i) Wheat grains from husk. (Winnowing)
(j) Fine mud particles suspended in water.(Sedimentation →Loading →Filtration)
(Note: for item 'j' Fine mud particles suspended in water., Refer to diagram 'Water purification system in water works' in NCERT book).

Q57: Define Brownian movement in colloids.

Answer: The colloidal particles move at random in a zigzag motion in all directions. This type of zig-zag motion of colloidal particles is called Brownian movement. It is caused due to the collision of the colloidal particles with the molecules of the dispersion medium.



Q58(CBSE 2010): Name the following :
(a) a lustrous liquid metal.
(b) a liquid non-metal
(c) a metal which can be cut with a knife
(d) a non-metal which is good conductor of electricity.
(e) an element which melts when kept on the palm.
(f) the best conductor of heat.

Answer:  (a) Mercury
(b) Bromine
(c) Sodium
(d) Graphite
(e) Gallium and Cesium
(f) Metals are the best conductors of heat.


Q59: How many elements are there which are in gaseous state at room temperature?

Answer: Eleven(Hydrogen, Helium, Neon, Argon, Krypton, Xenon, Radon, Fluorine, Chlorine, nitrogen and oxygen.)

 Q60: Name the elements are in liquid state at room temperature.

Answer: Mercury and bromine.

Q61: Who used the term 'element' first time?

Answer: Robert Boyle was the first scientist to use the term element in 1661.


Q62: Who gave the first explanatory definition of 'element'?

Answer: French scientist Lavoisier (1743-94).
"Element is the basic form of a substance or a basic unit of substance. It cannot be obtained in other simple substances by its chemical decomposition."


Q63: Identify the following as mixture or compound.
(i) blood
(ii) common salt
(iii) sugar
(iv) brass

Answer:
(i) blood - colloid
(ii) common salt - compound
(iii) sugar - compound
(iv) brass - alloy mixture


Q64: Sasha heats a container carrying Nitrogen and Oxygen. After heating at very high temperature, it gives Nitric oxide. Identify what are mixtures and/or compounds before and after the reaction.

Answer: Before heating, the container has mixtures of nitrogen and oxygen. After heating, a new compound is formed i.e. nitric oxide.

Q65: In beaker A, sugar cubes are dissolved into water while in beaker B, crushed cubes are taken. In which beaker the rate of dissolution is faster?

Answer: Beaker B. Crushed cubes will have larger area of solute and solute size is smaller. It will increase the rate of solubility.


Q66(CBSE 2010): (A) Identify solute and solvent in the following solutions :
(i) aerated drinks
(ii) tincture of iodine
(iii) lemon water

(B) State the principle of each of the following methods of separation of mixtures.
(i) centrifugation method.
(ii) separation using separating funnel.

Answer: (A) (i) aerated drinks: Solute = Carbon Dioxide, Solvent=Water
(ii) tincture of iodine: Solute= Iodine, Solvent = Alcohol
(iii) lemon water: Solute: Sugar, Citric Acid, Solvent: Water

(B) (i) Centrifugation Principle: The denser particles are forced to the bottom and the lighter particles stay at the top when spun rapidly.
(ii) Separation using separating funnel: Immiscible liquids separate out in layers depending on their
densities.

Q67: Why solutions do not exhibit Tyndall effect?

Answer: The solution particles are too small (< 1nm) to scatter the light. Therefore, true solution does not exhibit Tyndall effect.

Q68: What is an emulsion? Give examples

Answer: Emulsion is a colloidal solution where both dispersed phase and dispersed medium are liquids. Examples are: Milk, face cream

Q69:  What are the differences and similarities between concentration and solubility?

Answer:

1. Concentration is amount of solute dissolved in a unit quantity (mass or volume) of a solution. While solubility is the maximum amount of the solute that can be dissolved in a given amount of the  solvent.
2. Concentration is  expressed as percent by weight or volume, mole fraction, molarity etc. Solubility is the ability of a solute to dissolve in a solvent at given temperature and pressure. It is expressed as grams/litre or moles/litre.
3. Based on concentration, a solution can be saturated, unsaturated or supersaturated. Based on solubility, the solutes are soluble or insoluble, miscible or immiscible.
4. (Similarity) Both depends on temperature.

Q70: Give examples of liquids that are
         (i) completely miscible
         (ii) partially miscible
         (iii) practically immiscible

Answer:
(i) Liquids completely miscible: mix in all proportions. e.g. alcohol and water
(ii) Liquids partially miscible: dissolve in each other to a limit. e.g. ether and water
(iii) Liquids practically immiscible: e.g. benzene and water