FREQUENTLY ASKED QUESTIONS CHEMISTRY PRACTICAL CLASS XI
Address: Q. 1 What do you mean by the term ‘Crystallization’?
Ans. Solid particles having well-defined geometrical shapes are called crystals. These are formed when a hot saturated salt solution is allowed to cool down gradually and undisturbed. This process is known as crystallization.
Q. 2 How will you remove insoluble impurities from a substance?
Ans. These can be removed by dissolving the impure substance in water and then by filtration.
Q. 3 Why is the solution not concentrated too much while preparing crystals?
Ans. Excessive concentration is avoided because it will form only poor quality of crystals or a powder.
Q. 4 What is solubility?
Ans. It is the amount of the solute dissolved in 100g of the solvent to make a saturated solution.
Q. 5 What is blue vitriol?
Ans. It is hydrated or crystalline copper (II) sulphate (CuSO4.5H2O).
Q. 6 Why hot saturated solution is not cooled suddenly to get crystals?
Ans. By allowing saturated solution to cool slowly, crystals formed are of bigger size and better quality. It helps in their better separation as units rather than giving a massy substance of no proper geometry.
Q. 7 Define pH value.
Ans. The pH value of a solution is the negative logarithm of activity of hydronium ion concentration.
pH = -log [H3O?] = log 1/[H3O?]
Q. 8 What is pH value of pure water?
Ans. The pH value of pure water is 7.
Q. 9 If the pH value of a solution is 6, what is its hydronium ion H?O? concentration is the solution acidic or basic?
Ans. Its hydronium ion (H3O?) concentration = 1.0 x10?? moles/litre. It is an acidic solution.
Q. 10 Name the common acid-base indicators.
Ans. Phenolphthalein and Methyl orange.
Q. 11 What is the effect of addition of NH?CL to a solution of NH?OH?
Ans. The pH value of the solution decreases.
Q. 12 What is meant by a Cation and an anion?
Ans. Cations are positively charged ion while anion is a negatively charged ion.
Q. 13 What is a radical?
Ans. It is an atom or group of atoms carrying either positive or negative charge and which behaves as a single unit in chemical reaction.
Q. 14 What is meant by acid and basic radicals?
Ans. In an ionic compound, e.g., Na? Cl?, the part (Na?) which has come from the base (NaOH) is called basic radical and the part (Cl?) which has come from the acid (HCl) is called the acid radical.
Q. 15 Name the coloured ions.
Ans. Cu??, Ni??, Mn??, Fe??, Fe??.
Q. 16 Name the basic radicals which are absent, if the given mixture is not coloured.
Ans. Cu??, Ni??, Mn??, Fe??, Fe??, Co??.
Q. 17 NO? and Br? both are brown in colour. How will you distinguish between them?
Ans. The gas which turns FeSO? solution black is Br?.
Q. 18 Which burner is used in the laboratory?
Ans. Bunsen burner.
Q. 19 Why alums and phosphates swell on heating?
Ans. On heating, the molecular structure changes which results in increase in volume. Increase in volume cause swelling.
Q. 20 Why do we use conc. HCL for preparing a paste of the mixture fro flame test?
Ans. In order to convert the metal salts into their chlorides which are more volatile as compared to the other salts of the metals?
Q. 21 Why is platinum wire preferred to other metal wires for performing flame test?
Ans. Because it is not attacked by chemicals and does not impart any colour to the flame test.
Q. 22 Can we use a glass rod in place of platinum wire for performing flame test?
Ans. No, the glass contains sodium silicate as one of its constituents which imparts a golden yellow colour to the flame. Hence, it will interfere with the identification of basic radicals.
Q. 23 What type of flame is used to perform the flame test?
Ans. Non-luminous (oxidizing)
Q. 24 What is the chemistry of flame test?
Ans. Certain metallic salts (especially chlorides) when heated strongly in an oxidizing flame get ionized. The electrons I the ions get excited to higher energy levels on gaining energy from the flame. When the electron drops back to its original level, it gives out extra energy in the form of light radiations. In case of metals such Cu??, Ba??, Ca??, the amount of energy is small and the light radiations emitted fall in the visible region, i.e. in the form of coloured flame.
Q. 25 Name the radicals which are detected by dilute acid test.
Ans. Carbonate, sulphide, nitrite and sulphite.
Q. 26 Name the radicals which are detected by conc. H?SO? test.
Ans. Chloride, bromide, iodide, nitrate, oxalate and acetate
Q. 27 Why is dil. HCL sometimes preferred to dil. H?SO? in preliminary tests. >?
Ans. It reacts faster. Moreover, H?SO? forms an insoluble sulphate layer if Pb, Sr and Ca salts are present in the mixture. This insoluble sulphate layer prevents further reaction.
Q. 28 While performing conc. H?SO? test, why should the reaction mixture not be heated to boiling?
Ans. On boiling, sulphuric acid itself may decompose to produce sulphur dioxide.
Q. 29 Which gas is evolved by the action of dilute acids on carbonates?
Ans. Carbon dioxide.
Q. 30 What happens when CO? is passed through lime water?
Ans. Lime water turns milky due to the formation of insoluble CaCO?. However, on passing excess of the gas, milkiness disappears due to the conversion of insoluble calcium carbonate into soluble calcium bicarbonate, Ca(HCO?)?.
Q. 31 Why H?S turns lead acetate paper black?
Ans. It is due to the formation of PbS.
Q. 32 What is lime water?
Ans. A solution of Ca (OH)?.
Q. 33 Why is lime water stored in stoppered bottles?
Ans. Because otherwise, atmospheric CO? reacts with lime water to form insoluble calcium carbonate.
Q. 34 Why the use of platinum wire should be avoided for testing lead salts?
Ans. Because lead salts form a brittle alloy with platinum and the wire gets corroded.
Q. 35 Why do we add copper chips or paper pellet, in conc. H?SO? acid test for nitrates?
Ans. Because Cu or carbon of paper readily reduces the HNO? formed in the reaction mixture, to reddish brown NO? gas.
Q. 36 What is the cause of formation of brown ring in the test for nitrates?
Ans. The brown ring is due to the formation of a complex having the formula
Q. 37 Instead of lime water, what we can use for the detection of CO??
Ans. Mg(OH)? solution.
Q. 38 Can we use dil. HNO? in place of dil. HCl or H?SO? in dilute acid test?
Ans. No, because HNO? will oxidize H?S to sulphur if S?? is present in the mixture.
Q. 39 Why Br? and I? are evolved instead of HBr and HI in conc. H?SO? test?
Ans. Conc. H?SO? being an oxidizing agent oxidizes HBr or HI formed to Br? or I?, in case of conc. H?SO? test for chlorides, HCl produced is not oxidized to Cl?.
Q. 40 What is a group reagent?
Ans. It is a reagent which helps in identifying a group of radicals.
Q. 41 What is chromyl chloride?
Ans. Red vapours having the formula CrO?Cl?.
Q. 42 Why do bromides and iodides not give the chromyl chloride test?
Ans. Because chromyl bromide and chromyl iodide are not stable.
Q. 43 Can we use Ba(NO?)? instead of BaCl? in the detection of sulphate radical?
Ans. Yes, since we need only Ba?? ions to combine with SO??? ions to form BaSO?.
Q. 44 Why is silver nitrate solution stored in coloured bottles?
Ans. Silver nitrate solutions photosensitive. Sunlight decomposes it into its oxide.
Q. 45 Can we use H?SO? to prepare original solution for faction analysis?
Ans. No, this is because Pb??, Ba?? and Ca?? ions, if present in the mixture, form white precipitates with H?SO?.
Q. 46 When original solution is prepared in HNO? or aqua regia, why is it essential to remove HNO? completely before carrying out the analysis?
Ans. Otherwise HNO? oxidizes H?S in group II producing colloidal sulphur which interferes in analysis.
Q. 47 If a mixture dissolves in hot HCL and on cooling, a white ppt. is formed, what is the reason for it?
Ans. It is due to the presence of Pb?? ions. PbCl? is soluble in hot but separates out on cooling.
Q. 48 What is the function of HCl in second group?
Ans. This prevents the precipitation of IV group cations as their sulphides along with II group metals.
Q. 49 What is the function of conc. HNO? in third group?
Ans. To oxidize ferrous ion into ferric ion.
Q. 50 What is the function of NH?Cl in III group analysis?
Ans. It prevents the precipitation of cations of subsequent groups as their hydroxides by suppressing the dissociation of NH?OH by common ion effect.
Q. 51 What is the function of NH4OH in fourth group analysis?
Ans. It increases the S?? ion conc. in solution by increasing the degree of dissociation of H?S. Consequently. IV group cations, which have high solubility products of their sulphides, also get precipitated.
Q. 52 Can N?Cl be replaced by any other ammonium salt in III group?
Ans. Yes, NH?NO?can be used because we need only NH?? ions. However, (NH?)?SO? cannot be used because it will precipitate group V cations as their sulphates.
Q. 53 Can we add NH?OH in group III before adding NH?Cl ?
Ans. No, then purpose of adding NH?Cl will be forfeited and cations of IV, V and VI groups will also get precipitated along with III group.
Q. 54 Can we use NaOH instead of NH?OH in group III?
Ans. No, NaOH being a strong base, its ionization cannot be suppressed by common ion effect. Moreover, it dissolves precipitates of Al(OH)?.
Q. 55 Why is it essential to oxidize ferrous salts to ferric salt in group III analysis?
Ans. Because ferrous salts are not completely precipitated as Fe(OH)? on adding NH?OH in presence of NH?Cl.
Q. 56 Can we use Na?CO? as group reagent precipitate of (NH?)?CO? in group V analysis?
Ans. No, because being highly ionized it causes the precipitation of Mg?? as MgCO? by providing more of CO??? ions.
Q. 57 What is the function of NH?OH in group V analysis?
Ans. It converts the impurity of ammonium bicarbonate present in ammonium carbonate, into ammonium carbonate. In its absence, ammonium bicarbonate will change cations of group G into their bicarbonates which are soluble. Hence, these will not get precipitated.
Q. 58 What is the function of NH?Cl in group V?
Ans. To prevent precipitation of Mg?? as MgCO? by suppressing the dissociation of (NH?)?CO? by common ion effect.
Q. 59 Name a cation which is not obtained from a metal.
Q. 62 Why is lead placed both in the first as well as in the second group?
Ans. This is due to the fact that lead is not completely precipitated in group I as PbCl?. A part of it goes into the filtrate meant for groupie and is precipitated there as PbS.
Q. 64 Why only acetic acid is used for dissolving group V precipitates?
Ans. Acetic acid does not interfere in group V analysis. If dil. H?SO? is used it will precipitate Ba?? and Sr?? as their insoluble sulphates. On the other hand, if dil. HCl or HNO? is used for this purpose, calcium will not be precipitated on adding ammonium oxalate. This is because calcium oxalate formed is soluble in both dil. HCl and HNO?.
Q. 68 What is water extract and how is it prepared?
Ans. A small quantity of the salt is heated with distilled water. The solution is cooled and filtered. The filtrate is labeled as water extract.
Q. 71 What is common ion effect?
Ans. The suppression of degree of ionization of a weak electrolyte by adding a common ion, i.e. the ion already present due to the dissociation of the weak electrolyte, is known as common ion effect.
Q. 73 Why is original solution prepared for the detection of basic radicals?
Ans. The scheme of analysis is based on the characteristics of ions in solution. Therefore, for performing tests, the mixture is dissolved to bring the metallic radicals in solution.
Q. 75 What is the purpose of fusion of the organic compound with sodium metal for the preparation of Lassaigne’s extract?
Ans. When the organic compound is fused with sodium metal, the elements such as nitrogen, sulphur and halogens, if present in the compound are converted into sodium salts which are soluble in water. The aqueous solution is then used to identify these elements.
Q. 76 Why is sodium metal kept under kerosene oil?
Ans. Sodium metal reacts with oxygen and moisture of the air. Kerosene oil prevents the contact of sodium and air and therefore protects it from the action of oxygen and moisture.
Q. 77 Why is sodium metal dried up before ignition?
Ans. To avoid explosion, because sodium metal is very sensitive towards moisture.
Q. 78 Can potassium metal b used in place of sodium in Lassaigne’s test?
Ans. No, potassium can’t be used since it is too reactive and hence its dangerous to use.