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1. Classify matter on the basis of chemical composition?
Ans.: On the basis of chemical composition, the matter exists either as a single substance or, as a mixture containing two or more substance.
(i). Substance: A kind of matter that cannot be separated into other kinds of matter by any physical process is called a substance. For example, iron, water etc.
(ii). Mixtures: A mixture is that form of matter in which two or more substances are present in any proportion. For example, air is a mixture of mainly oxygen, nitrogen, smaller Amounts of CO2, dust particles etc., are also present in the air.
2. Write the properties of a pure substance.
Ans.: The properties of a pure substance are:
(i). a pure substance contains only one kinds of atoms or molecules.
(ii). a pure substance is perfectly homogenous.
(iii). a pure substance has a definite composition which does not change with time.
(iv). a pure substance has definite melting point, boiling point, density etc.
3. Write the properties of a mixture.
Ans.: The properties of a mixture are:
(i). In mixtures, the constituents may be present in any proportion.
(ii).Mixtures show the properties of all the constituents present therein. For example, a mixture containing sugar and salt will be salty-sweet.
(iii). During the preparation of a mixture, no energy such as, heat, light and electricity is absorbed or evolved.
(iv). Mixtures are heterogeneous.
(v). The components of a mixture can be separated by simple physical methods.
4. Classify mixtures.
Ans.: A mixture may be homogeneous or heterogeneous depending upon the nature of the substances present therein.
(i). Homogeneous mixture: A mixture which is perfectly uniform in its composition and properties throughout is called homogeneous mixture. Homogeneous mixtures are called solution. E.g., Kerosene is a homogeneous mixture of many hydrocarbons, solution of common salt in water, some common alloys etc.
(ii). Heterogeneous mixture: A mixture which has different compositions and different properties at different locations in the mixture is called heterogeneous mixture. For example, muddy water from a pond, a mixture containing sugar, and sand are heterogeneous mixtures.
5. What is gun powder?
Ans.: A mixture containing potassium nitrate, sulphur and charcoal is called gun powder.
6. What is a solution?
Ans.: A homogeneous mixture of two or more substance is called a solution. Homogeneous means physically and chemically uniform throughout. For example, when a small amount of sodium chloride is dissolved in water, a homogeneous mixture so obtained is called a solution of sodium chloride in water.
7. What is a true solution?
Ans.: A solution in which size of the solute particles is about 10-10 m is called a true solution. In a true solution, the solute particles and the solvent molecules cannot be distinguished even under a microscope.
Properties of true solution:
(i).a true solution is a homogeneous mixture.
(ii). a true solution is clear and transparent.
(iii). a true solution does not scatter light.
(iv). the components of a true solution, cannot be separated by filtration.
(v). the solute particles in a true solution don’t settle down.
8. What is meant by solute and solvent?
Ans.: The substance which is dissolved to obtain a solution is called solute, whereas the substance in which the solute is dissolved in called solvent.For example, when sugar is dissolved in water to obtain a sugar solution, sugar is the solute and water is the solvent.
In other words, the component present in larger amount in a solution is called the solvent, whereas the component present in smaller amount is called the solute.
9. What are aqueous solutions?
Ans.: Solutions in which water is the solvent are called aqueous solution. For example, when sugar is dissolved in water, the solution obtained is called an aqueous solution of sugar.
10. What are non-aqueous solutions?
Ans.: The solutions obtained by dissolving a solute in any solvent other than water are called non-aqueous solution. For example, if a solute is dissolved in any solvent such as benzene, alcohol, ether, carbon disulphide, carbon tetrachloride, acetone etc., the solution obtained is called no - aqueous solution.
11. Why is water called universal solvent?
Ans.: Water can dissolve a wide variety of substance. That is why, water is called universal solvent. Plants use this property of water for taking minerals from the soil.
12. Write the advantages of using solutions.
Ans.: The advantages of using solutions are:
(i). For carrying out reaction: For a reaction to take place, the reacting molecules or ions must come closer to each other. In solutions, the reactants can come closer easily.
(ii). For dissolving medicines: Certain medicines are given to patients in the form of their solutions in suitable aqueous or non-aqueous solvents, such as alcohol. For example, saline glucose is administered to a patient suffering from dehydration.
13. What is the concentration of a solution?
Ans.: The quantity of solute dissolved in a given mass or volume of the solution or the solvent is called concentration of the solution.
14. What is mass percent of solute in the solution?
Ans.: The mass of solute in grams per 100g of the solution is called mass percent of solute in the solution. For example, a 5% solution of sodium chloride means that 5 grams of sodium chloride are present in 100 grams of the solution.
Therefore, Mass by mass percent of solute in a solution= mass of solute/mass of solution x 100.
However, when a solute is present in trace quantity, the concentration is expressed in the units of parts per million or ppm.
15.What is a suspension?
Ans.: A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium. Particles of a suspension are visible to the naked eye. Muddy pond water, smoke coming out a chimney of a factory, chalk-water mixture and coarse lime particles in water are examples of suspensions.
16. Write the properties of a suspension.
Ans.: The properties of a suspension are:
(i). suspension is a heterogeneous mixture.
(ii). the particles of a suspension can be seen by the naked eye.
(iii). the particles of a suspension scatter a beam of light passing through it and make its path visible.
(iv). the solute particles settle down when a suspension is left undisturbed, that is, a suspension is unstable. They can be separated from the mixture by the process of filtration.
17. What is a colloidal solution?
Ans.: A homogeneous-looking heterogeneous mixture in which particles having size between 1 and 1000 nm are dispersed in a continuous medium is called a colloidal solution. Examples: Milk, Office paste, blood, Toothpaste, Jelly etc. Fog is a common colloid, in which water droplets are dispersed in air.
18. What are the properties of a colloid?
Ans.: (i). Heterogeneous nature: A colloid is heterogeneous in nature.
(ii). Particle size: The size of particles in a colloid lies between 10-9m and 10-7m.
(iii). Separation of colloidal particles by filtration: Colloidal particles can easily pass through the pores of a filter paper.
(iv). Brownian movement of colloidal particles: When seen under a microscope, the colloidal particles in a colloid are seen to be moving in a random fashion.
19. What is an unsaturated solution?
Ans.: A solution in which some more solute could be dissolved at any fixed temperature is called an unsaturated solution. For example, a solution of sugar in which some more sugar could be dissolved without changing its temperature is called an unsaturated solution of sugar.
20. What is a saturated solution?
Ans.: A solution in which no more solute can be dissolved at any fixed temperature is called a saturated solution.
21. What is tyndall effect?
Ans.: The scattering of a beam of light in a colloidal solution is called Tyndall effect. Tyndall effect can also be observed when a fine beam of light enters a room through a small hole. This happens due to the scattering of light by the particles of dust and smoke in the air.
22. Write the name of some methods which are commonly used to separate the constituents of a mixture.
Ans.: Some most commonly used methods for separating the constituents of a mixture are:
(i). Handpicking, (ii). Sieving, (iii). Winnowing, (iv). Sedimentation, (v). Evaporation,
(vi). Crystallization, (vii). Filtration, (viii). Sublimation, (ix). Gravity method, (x)j. Distillation,
(xi). fractional distillation, (xii). Magnetic separation method, (xiii). Centrifugation etc.
23. What are the purpose of separating the constituents of a mixture?
Ans.: The constituents of a mixture are separated for the following purposes:
(i). To remove any harmful or undesirable constituent: For example, small pieces of stones and undesirable seed grains are removed from rice and dals before cooking.
(ii). To obtain a pure sample of a substance: Impurities present in any substance are removed to obtain a pure sample of the substance.
(iii). To obtain useful constituents: A mixture may contain one or more useful constituents. These should be separated for their proper use.
24.What is centrifugation?
Ans.: It is a method used for separating cream from milk. When milk is rotated at high speed, the cream collects in the center and is removed through an outlet.
Applications of centrifugation:
(i). It is used in washing machines to squeeze out water from the wet clothes.
(ii). It is used for separating cream from milk and butter from curd in dairies or at home.
(iii). It is used in diagnostic laboratories for testing blood/urine.
24.What is sublimation?
Ans.: Sublimation is a method to used for separating substances which on heating change directly into vapour. The vapour on cooling gives the pure substance in solid form. Substances like Camphor, Ammonium chloride, Naphthalene, Benzoic acid and Iodine on heating directly change into vapour.
25. What is winnowing?
Ans.: The process of separating grains from husk and hay with the help of wind is called winnowing.
26. Write the method of separating insoluble solids from liquids.
Ans.: The insoluble solids can be separated from liquids by the following methods:
(i). Sedimentation: During sedimentation, the heavier coarse particles settle down quickly. Fine particles of clay settle down very slowly. This settling down of heavier particles is called sedimentation.
(ii). Decantation: When we transfer the clearsupernatant water carefully to another beaker. This process of separating clear liquid without disturbing the settled solid particles is called decantation.
27. Write about filtration method.
Ans.: This method is used for separating fine insoluble solid particles from any liquid. In this method, the mixture is allowed to pass through a filter. The insoluble solid particles are retained by the filter, and the clear liquid is collected as the filtrate. Filtering tea by using a strainer is an example of filtration.
28. Write about crystallization method.
Ans.: Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. Crystallisation technique is better than simple evaporation technique as:
(i). Some decompose or some, like sugar, may get charred on heating to dryness.
(ii). some impurities may remain dissolved in the solution even after filtration. On evaporation these contaminate the solid.
29. Write the applications of Crystallisation.
Ans.: (i). Purification of salt that we get from sea water.
(ii). Separation of crystals of alum from impure samples. Thus, by choosing one of the above methods according to the nature of the components of a mixture, we get a pure substance.
30. Write some methods to purify city water.
Ans.: It is a complete chain method in which many methods are used. These are used step by step.
(i). Sedimentation: In this method the raw water from the river is pumped into the settling tank. The supernatant water is then sent to another settling tank.
(ii). Loading: Here, alum is added to make the finer suspended particles of clay settle down.
(iii). Filtration: The supernatant water from the second tank is then allowed to pass through a sand and gravel filter. This removes all suspended impurities from the water.
(iv). Aeration: The filtered water is then aerated to kill micro-organisms and saturate water with oxygen.
(v). Chlorination: The aerated water is then mixed with a calculated amount of chlorine. Chlorination kills harmful micro-organisms. This process is called sterilization of water.
31. What is a physical change?
Ans.:A change in which only the physical properties of the substance get changed, while its chemical composition remains unchanged, is called a physical change. For example, when sugar is added to water, it disappears to form its solution. In solution, both water and sugar retain their properties. Sugar can be recovered from its solution by evaporating water.
32. Write the properties of a physical change.
Ans.: (i). In a physical change only the physical properties of the substances undergo change.
(ii). In a physical change can be reversed easily, i.e., the original form of the substance can be obtained easily by simple methods.
(iii). In a physical change, no new substance is formed.
(iv). Physical changes are temporary.
33. What is a chemical change?
Ans.: A change in which chemical composition and chemical properties of the reacting substances undergo a change is called a chemical change. Thus, due to a chemical change, the nature of the reacting substance is completely changed, and a new substance is formed. The burning of carbon is a chemical change.
34. Write the properties of chemical changing.
Ans.: (i).In a chemical change, the chemical composition and chemical properties of the reacting substance are completely changed. For example, during the burning of coal, the dark coloured solid coal gets converted into a gaseous substances called carbon dioxide.
(ii). Whenever a chemical change take place, one or more new substances are formed. For example, during the burning of coal, the new product formed is carbon dioxide.
(iii). A chemical change cannot be reversed easily. For example, it is not possible to get back milk after it is converted into curd.
(iv). A chemical change is a permanent change. For example, a grown tree cannot be converted back into a small plant.
(v). During a chemical change a certain amount of energy in the form of heat, light or sound energy, is either absorbed or evolved. For example, when a cracker bursts, heat, light and sound energies are evolved.
35.Differentiate between physical changing and chemical changing.
Ans.: The differences between physical changing and chemical changing are:
(i). In a physical change, only physical properties such as colour, physical state, volume, etc., change. | (i). In a chemical change, the chemical composition and chemical properties of the reactants undergo a change. |
(ii). No new substance is formed. | (ii). A new substance is formed. It has properties different from those of the reacting substance. |
(iii). No energy in the form of heat, light or sound energy is absorbed or evolved in a physical change. | (iii). A chemical change is always accompanied by the absorption or evolution of energy in the form of heat, light, or sound energy. |
(iv). The original form of the substance can be obtained easily by simple physical methods. | (iv). The original substance cannot be obtained by simple physical methods. |
(v). Physical change is generally temporary, in the sense that it can be easily reversed by reversing the conditions. | (v). Chemical change is a permanent change in the sense that it cannot be reversed by just reversing the condition. |
36. What is an element?
Ans.: An element is the simplest form of matter, which cannot be split into simpler substance by any chemical or physical method. There are 115 elements known to us at present, out of which 92 are naturally occurring, while 23 have been prepared artificially.
37. Classify elements on the basis of chemical and physical properties.
Ans.: On the basis of chemical and physical properties, the elements are further classified intometals, nonmetals and metalloids.
38. What is a chemical compound?
Ans.: A compound is a pure substance made up of two or more elements combined chemically in a definite proportion by mass, is called a chemical compound. For example, what is a chemical compound made up of hydrogen and oxygen combined in the ration 2:1 by volume, or 1:8 by mass.
39. Write the properties of a chemical compound.
Ans.: A chemical compound has the following properties:
(i). A chemical compound is obtained by the chemical combination of two or more elements in a definite proportion by mass.
(ii). Compounds are homogeneous, i.e., their properties are the same throughout.
(iii).The chemical and physical properties of a compound are entirely different from those of the constituent elements. For example, hydrogen is a combustible gas, while oxygen is a supporter of combustion.
(iv). During the formation of a compound from its elements, a certain amount of energy in the form of heat, light, and electricity is either absorbed or evolved. For example, when carbon burns, it reacts with oxygen to form carbon dioxide, and heat is liberated.
40. Differentiate between mixtures and compounds.
Ans.:
Mixtures | Compound |
(i). A mixture is heterogeneous, except solutions which are homogeneous. | (i). A chemical compound is always homogeneous. |
(ii). The components of a mixture may be present in any proportion. | (ii). The components of a compound are present in a fixed proportion. |
(iii). Properties of a mixture are the average properties of the components | (iii). Properties of a compound are entirely different from those of the component elements. |
(iv). The components of a mixture can be separated by simple physical methods. | (iv). The components of a compound cannot be separated by simple physical methods. |
(v). During the preparation of a mixture, no heat, light or electricity is absorbed or evolved. | (v). Formation of a compound involves the absorption or evolution of energy, such as, heat, light or electrify. |
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