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5651.

Give structural formulae of the following compounds using their IUPAC names : 1. 2-Chloropropane 2. 1-Bromopentane 3. 2-Bromo-2-methylpropane 4. 1-Pentanol 5. Ethanol 6. Methanol 7. Methanal 8. Ethanal 9. 3-Pentanone 10. Propanone 11. Methanoic acid 12. Pentanoic acid 13. Ethene 14. 2-Butene 15. 1-Pentyne

Answer»

SOLUTION :
Discussion
5652.

Give some applications of solar energy .

Answer»

SOLUTION :(i)In Making solar HEATER devies .
(II) In making solar cookers.
Discussion
5653.

Give scientific reasons for the statement :Ionic compounds are non-conductor in the solid state, but they conduct electricity in the molten state or in an aqueous solution.

Answer»

Solution :Conduction of electricity : The conduction of electricity through a solution involves the movement of charged PARTICLES. A solution of an ionic compound in water contains IONS, which move to the opposite electrodes, when electricity is passed through the solution. Ionic compounds in the solid state do not conduct electricity because the movement of ions in the solid is not possible due to their rigid structure. But, ionic compounds conduct electricity in the molten state. Since the ELECTROSTATIC forces of attraction between the oppositely charged ions are overcome and free ions are OBTAINED due to heat. As a result, these ions move freely and conduct electricity.
Discussion
5654.

Give scientific reasons for the statement :Melting points and boiling points of ionic compounds are high.

Answer»

Solution : Melting and BOILING POINTS: A considerable LARGE amount of energy is required to break the STRONG INTER ionic attraction . Hence ionic compounds have high melting and boiling points
Discussion
5655.

Give scientific reasons for the statement :Hydrogen gas is not evolved when Al reacts with nitric acid.

Answer»

Solution :When Al reacts with nitric acid, HYDROGEN GAS is not evolved because `HNO_3`is a STRONG OXIDISING agent. It oxidises the H, produced with water and itself is reduced to any of the nitrogen oxide.
Discussion
5656.

Give scientific reason : "The magnetic field produced by a current carrying conductor increases as the number of turns in the coil increases”.

Answer»

Solution :ACCORDING to law of electromagnetic induction, if current is PASSED through a conductor then MAGNETIC FIELD is produced around the conductor, if the number of turns in a coil are INCREASED then the magnetic field of conductor also increases.
Discussion
5657.

Give scientific reasons for the following statements : (1) Candle burns with a yellow flame.

Answer»

SOLUTION :Candle is made-up of wax, which consists of saturated hydrocarbons containing 18 to 20 carbon atoms.
When a candle is lighted (ignited), the wax melts which rises up the wick and GETS covered by vapour. As a result, there is no proper MIXING of `O_(2)` of air and wax vapours.
Therefore, here incomplete combustion of wax takes place, and COMPLETE combustion of wax does not take place.
Furthermore, incomplete combustion of wax produces unburnt carbon particles, which rise up in the flame, get heated and imparts yellow COLOUR to the flame.
Discussion
5658.

Give reasons why is copper used to make hot water tanks and not steel (an alloy of iron).

Answer»

Solution :(i) Copper is GOOD conductor of heat than the steel.
(II) Copper will not be corroded by hot water while the steel MAY be corroded.
Discussion
5659.

Give reasons why copper is used to make hot water tanks but not steel (an alloy or iron).

Answer»

SOLUTION :Copper is a MUCH better conductor of heat than steel. ALSO, copper does not REACT with water at all but steel (an alloy of IRON) reacts with steam. So, copper is used to make hot water tanks but not steel (an alloy of iron).
Discussion
5660.

Give reason: why do chips manufactures usually flush bags of chips with gas such as nitrogen ?

Answer»

SOLUTION :To prevent the oil and FATS of the chips from being oxidized or BECOME rancid.
Discussion
5661.

Give reasons why copper is used to make hot water tanks and not steel (an alloy of iron).

Answer»

SOLUTION :Electrical conductivity of a METAL is decreased when it is alloyed with another metal or non-metal.Thus, the electrical conductivity of STEEL (which is an alloy) is much LESS than that of pure copper. That is why copper is used to MAKE hot water tanks and not steel.
Discussion
5662.

Give reasons Sodium, potassium and lithium are stored under oil.

Answer»

Solution : SODIUM, potassium and LITHIUM are highly reactive metals. They release hydrogen gas, when they react with air or moisture in air. Hydrogen gas is highly COMBUSTIBLE and it catches FIRE. To prevent accidental fire, lithium, sodium and potassium are stored under oil
Discussion
5663.

Give reasons : Sodium, potassium and lithium are stored under oil.

Answer»

SOLUTION : Reaction of sodium, potassium and LITHIUM with oxygen is so VIOLENT that they catch fire. Alsowe cannot store them under WATER because there is a BRISK reaction. Hence, they are storedunder oil.
Discussion
5664.

Give reasons: Platinum, gold and silver are used to make jewellery. (ii)Sodium, potassium and lithium are stored under oil. (iii)Aluminium is a highly reactive metal, yet it is used to make utensils for cooking. (iv)Carbonate and sulphide ores are usually converted into oxides during the process of extraction.

Answer»

Solution :Platinum , gold and silver are used to make jewellary because of their bright shiny surface and high resistance to corrosion. Also, they have high malleability and ductility.
(ii)Sodium, potassium and LITHIUM are stored under oil to prevent their reaction with oxygen, moisture and carbon dioxide of air so as to protect them because they are very reactive metals.
(iii)Aluminium metal forms a THIN layer of aluminium oxide all over its surface, under the action of moist air. This layer prevents the metal underneath from further corrosion. It is cheap, easily available,malleable,ductile and a good conductor of heat. Therefore, it is used to make utensils for cooking.
(IV)It is easier to obtain a metal from its oxides as compared to its sulphides and carbonates. So prior to reduction, metal CARBONATE and sulphides must be converted into metal oxides. A carbonate ORE is converted into oxide by calcination, whereas a sulphide ore is converted into oxide by roasting.
Discussion
5665.

Give reasonsPlatinum, gold and silver are used to make jewellery.

Answer»

Solution :Platinum, gold and silver are used to make JEWELLERY, because these METALS POSSESSES LUSTRE. They are MALLEABLE and ductile, as a result, jewellery of different shapes can be made. Moreover, these metals do not chemically react with water or air. Due to these properties platinum, gold and silver are used to make jewellery.
Discussion
5666.

Give reasons :Platinum, gold and silver are used to make jewellery.

Answer»

SOLUTION : These metals are unreactive. They do not react with oxygen and other gases PRESENT in AIR and with moisture. Thus, their shine is MAINTAINED. That is why these metals are USED to make jewellery
Discussion
5667.

Give reasons : (i) Sodium metal is stored under kerosene oil. (ii) Inspite of being highly reactive, aluminium is still used for making utensils.

Answer»

Solution :(i) Sodium is a highly reactive metal. It can catch fire if kept in OPEN. Hence to PROTECT it an prevent ACCIDENTAL fire, it is stored under kerosene oil.
(ii) DUE to high reacticity of ALUMINIUM, a thin layer of aluminium oxide is formed on it. This protective layer of aluminium oxide prevents the metal from further corrosion. Hence, aluminium is anodizing.
Discussion
5668.

Give reasons :(i) Silver metal does not easily combine with oxygen but silver jewellary tarnishes after sometime. (ii) Iron grills are frequently painted. (iii) Gold ornaments retain their luster even after several years of use.

Answer»

SOLUTION : (i) Silver metal reacts with the H S GAS present in traces in the atmosphere to form silver SULPHIDE which tarnishes the metal. (ii) Iron forms a layer of RUST `(Fe_2O_3.xH_2O)`on combination with oxygen and moisture present inthe atmosphere. This weakens the metal. To prevent this loss iron grills are frequently painted. (iii) Gold is an unreactive metal. It does not react with atmospheric oxygen and acids. So the lustreof gold jewellery is retained even after SEVERAL years
Discussion
5669.

Give reasons : i) lonic compounds in solid state do not conduet electricity, whereas in molten state are good conductors of electricity. (ii) ii) Silver articles when exposed to air gradually turn blackish. ii) Chenmical reaction does not take place when copper is added to iron sulphate solution.

Answer»

SOLUTION :(i) In the solid state ionic COMPOUNDS do not conduct electricity because movement of ions in the solid is not possible due to their RIGID structure.
In solid state, they are hard because of the strong force of attraction between the positive and negative ions.
In molten state, electrostatic forces of attraction between the oppositely charged ions are OVERCOME due to the heat.
thus the ions nnove freely and conduct electricity. (ii) Silver reaucts with sulphur in the air to fornm a coating of silver sulphide.
Reactivity of copper is LESS than that of iron.
Discussion
5670.

Give reasons: (i) "Alloys of iron are more useful when compared to pure iron." (ii) Copper loses its brown layer gradually when exposed to air. (iii) Aluminium oxide is called amphoteric oxide.

Answer»

Solution :(i) PURE iron is very soft.
Streches easily when hot
Alloys are HARD.
The propertics of iron can be changed if it is MIXED with other substances.
(ii) Copper reacts with moist carbon dioxide in the air and SLOWLY loses its shiny brown surlace and gains a green coat.
(iii) Aluminium oxide `(Al_(2)O_(3))` reacts with both acids as well as bases to produce salt and water.
Discussion
5671.

Give reasons for the observations: Use of synthetic detergents causes pollution of water.

Answer»

Solution :Synthetic detergents are GENERALLY non-biodegradable, that is, they are not DECOMPOSED by microorganisms like bacteria. Hence, use of synthetic detergents causes WATER POLLUTION in lakes and rivers.
Discussion
5672.

Give reasons for the observations: Air holes of a gas burner have to be adjusted when the heated vessels get blackened by the flame.

Answer»

SOLUTION :We NEED to adjust AIR holes of gas burner so that sufficient oxygen-rich mixture is BURNT to give a clean blue flame for complete combustion.
Discussion
5673.

Give reasons for the following : Why do metals deposit at the cathode and not at the anode when their fused salts are electrolysed ?

Answer»

Solution :Because METAL CATIONS are POSITIVELY charged and MOVE towards cathode and not towards ANODE .
Discussion
5674.

Give reasons for the following The blue colour of aqueous copper sulphate fades when it is electrodes.

Answer»

Solution :During the electrolysis of an AQUEOUS solution of CuSO4, using platinum ELECTRODES, `Cu^(2+)` ions are discharged at cathode.
i.e. `Cu^(2+) + 2E^(-) to Cu`
therefore` Concentration of `Cu^(2+)` ions goes on decreasing. Hence, the BLUE colour of the solution slowly fades away.
Discussion
5675.

Give reasons for the following observation: (a)The element carbon forms a very large number of compounds. (b)Air holes of a gas burner have to be adjusted when the heated vessels get blackened by the flame. c)Use ofsynthetic detergents causes pollution of water.

Answer»

Solution :a)Carbons forms LARGE numbers of compounds SINCE carbon is SMALL in size can form stable covalent bonds and it shows tetravalency.
(b)Air holes of gas BURNER are made open.So that air can pass through,which is needed for complere COMBUSTION,so that heated vessels so do not ger blackened.
c)Some synthetic detergents are non-boidegradable ,therefore,cause pollution of water.
Discussion
5676.

Give reasons for the following Lead bromide undergoes electrolytic dissociation in the molten state but is a non-electrolyte in the solid state.

Answer»

Solution :In `PbBr_2` in the solid STATE, there are STRONG electrostatic FORCES of attraction between `Pb^(2+)` and `Br^(-)` ions. The ions can.t move. Hence, it is a non- electrolyte. In the molten state, the electrostatic forces of attraction between `Pb^(2+)` and `Br^-`ions are very much decreased. They undergo electrolytic dissociation easily.
Discussion
5677.

Give reasons for the following In the electrolysis of acidified water dilute sulphuric acid is preferred to dilute nitric acid for acidification

Answer»

Solution :This is because DILUTE nitric ACID is an oxidising agent and may ALSO PRODUCE NO.
Discussion
5678.

Give reasons for the following:(i) Zinc oxide is considered an amphoteric oxide. (ii) Non-metals in general do not displace hydrogen from dilute acids. (iii) Metals conduct electricity.

Answer»

SOLUTION :(i) Zinc oxide reacts with both ACIDS and alkalis. Therefore, it is an AMPHOTERIC oxide.

(ii) Non-METALS cannot provide ELECTRONS to convert H+ ions into hydrogen gas.(iii) Metals conduct electricity with the help of free or mobile electrons.
Discussion
5679.

Give reasons for the following : (i) Zinc can displace copper from copper sulphate solution. (ii) Silver articles become black after sometime when exposed to air. (iii) A metal sulphide is converted to its oxide to extract the metal from sulphide ore.

Answer»

Solution :(i) Zinc is placed above copper in the activity series. Therefore, it can easily displace copper from copper SULPHATE solution
`Zn(s) + CuSO_(4)(aq) rarrZnSO_(4)(aq) + Cu(s)`
(ii) Air contains traces of hydrogen sulphide `(H_(2)S)`gas. Silver has a tendency to combine with `H_(2)S` to form silver sulphide `(Ag_(2)S)` which is black in colour. On ACCOUNT of this, silver articles become black after some time when kept exposed to air.
(iii) A metal sulphide is normally converted into OXIDE by heating with excess of air or oxygen. This process is called roasting. Actually, the oxide of metal which is formed can be easily reduced to the metallic form by reduction with carbon or some electropositive element.
`underset("Zinc sulphide")(2ZnS + 3O_(2)) overset("heat")rarr2ZnO + 2SO_(2)`
`ZnO + C overset("heat")rarrZn + CO`
Discussion
5680.

Give reasons for the following: (i) There is either a convergence or a divergence of magnetic field lines near the ends of a current carrying straight solenoid. (ii) The current carrying solenoid when suspended freely rests along particular direction. (iii) The burnt out fuse should be replaced by another fuse of identical rating.

Answer»

Solution :(i) The ENDS of a current carrying solenoid behave as magnetic poles. One end behaves as north magnetic pole and field lines diverge from it. Other end of solenoid behaves as south magnetic pole and field lines CONVERGE there.
(II) As a current carrying solenoid behaves as a bar magnet, it rests along a particular direction (nearly north-south direction) when suspended freely.
(iii) A FUSE has a definite current as well as POWER rating, hence the burnt out fuse should be replaced by another fuse of identical rating only.
Discussion
5681.

Give reasons for the following:(i) Platinum, gold and silver are used to make jewellery (ii) To make hot water tanks, copper is used and not steel (an alloy of iron).(iii) Lemon is used for restoring the shine of tarnished copper decorations.

Answer»

Solution : (i) Platinum, gold and silver are used to make jewellery because these are the least reactive metals.They are least corroded and maintain their shine when exposed to ATMOSPHERE. (ii) Copper is better conductor of heat than steel. Steel is an alloy of IRON and carbon. Alloys are poor conductors of heat, that is why copper is used to make hot water tanks. (iii) Copper articles wear a green coating of copper carbonate. This layer of copper carbonate is removed by rubbing the surface with lemon. CITRIC acid PRESENT in lemon reacts with coppercarbonate to FORM soluble copper citrate and the shine of the copper article is restored.
Discussion
5682.

Give reasons for the following : (i) Metal oxides generally react with acids. However, zinc oxide reacts even with sodium hydroxide. (ii) Heat of neutralisation value for the reaction of HCl and NH_(4)OH is less than 13.7 kcal/mol (iii) All indicators are not suitable for all titrations

Answer»

Solution :(i) Zinc oxide is amphoteric in nature. Hence, it can REACT both with acids as well as bases.
(ii) `INH_(4) OH` is a weak BASE and hence, some heat is utilised to ionise the weak base
(III) Indicator that undergoes COLOUR change in the pH range of the titration is only SUITABLE for a particular titrations
Discussion
5683.

Give reasons for the following : (i) Metals are good conductors of electricity whereas non-metals are not. (ii) Ionic compounds have usually high melting and boiling points.

Answer»

SOLUTION :(i) Metals have FREE electrons to move so they conduct electricity.
(ii) A considerable amount of energy is REQUIRED to BREAK the strong inter-ionic attraction.
Discussion
5684.

Give reasons for the following:(i) Ionic compounds have generally high melting points. (ii) Hydrogen is not a metal, but it has been assigned a place in the reactivity series of metals. (iii) The galvanised iron article is protected against rusting even if the zinc layer is broken. (iv) The wires carrying current in homes have a coating of PVC.

Answer»

Solution : (i) There are ELECTROSTATIC forces of ATTRACTION between the cations and anions in ionic compoundswhich are difficult to break. Therefore ionic compounds have high melting points. (ii) Metals above hydrogen evolve hydrogen gas and metals below hydrogen do not evolve hydrogen when treated with an acid. That is when hydrogen has been ASSIGNED a place in the reactivity series of metals. (iii) Even if zinc layer is broken, it will preferentially oxidised because it is more REACTIVE than iron. (iv) The wires CARRYING current in homes have a coating of PVC. This is because PVC is an insulatingsubstance and protects from electric shock.
Discussion
5685.

Give reasons for the following : (i) Ionic compounds have high melting point and boiling point. (ii) Ionic compounds conduct electricity in molten state. (iii) Ionic compounds are solids at room temperature and are somewhat hard.

Answer»

Solution :(i) A large amount of energy is required to BREAK the strong inter-ioni attraction.
(ii) When ionic compounds is PRESENT in molten state, crystal structure deforms and they can easily conduct electricity with the MOBILE ions.
(III) Due to the strong force of attraction between the positive and negative ions.
Discussion
5686.

Give reasons for the following: (a) Unsaturated hydrocarbons show addition reactions but not saturated hydrocarbons. (b) Carbon forms only covalent bonds.

Answer»

Solution :(a) Unsaturated hydrocarbons contain either one/more double bonds `(gtC=Clt)` or triple bonds `(-C-=C-)` in their molecules. These are unstable and reactive in nature. The saturated hydrocarbons contain only C-C bonds in their molecules and are of stable nature. Therefore, unsaturated hydrocarbons have urge or desire to become saturated or stable. to achieve this, they take part in the addition reactions. for example.
`underset("Ethyne")(H-C-=C-H+H_(2))underset(473K)overset("Nickel (NI)")tounderset("Ethene")(H-overset(H)overset(|)(C)=overset(H)overset(|)(C)-H), H-overset(H)overset(|)(C)=overset(H)overset(|)(C)=overset(H)overset(|)(C)-H+H_(2)underset(473K)overset("Nickel (Ni)")tounderset("Ethane")(H-underset(H)underset(|)overset(H)overset(|)(C)-underset(H)underset(|)overset(H)overset(|)(C)-H)`
(b) Carbon (Z=6) has electronic distribution 2,4. It is not in a POSITION to either LOSE four electrons or gain four electrons since ENERGY involved is very high. It shares its four valence electrons with the electrons of other atoms ad takes part in the covalent BOND formation.
Discussion
5687.

Give reasons for the following: a] Metals have a tendency to form cations. b] Fluorine atom is smaller than chlorine atom. c] Non-metals have a tendency to gain electrons.

Answer»

Solution :a) Since metals have 1-3 valence electrons, it is easier to lose and attain a noble gas configuration rather than gain electrons. HENCE, they tend to form cations.
b] Fluorine atom has only two electron shells while chlorine atom has three. Thus, Fluorine atom is SMALLER.
c] Non-metals have 5-8 valence electrons (EXCEPT carbon which has 4 valence electrons). It is easier for them to gain additional electrons and attain noble gas configuration while bonding rather than lose electrons. Hence, NONMETALS tend to gain electrons.
Discussion
5688.

Give reasonsCarbonate and sulphide ores are usually converted into oxides during the process of extraction.

Answer»

Solution : It is EASIER to obtain a metal from its oxide. HENCE, it is ESSENTIAL to CONVERT carbonate and sulphide ores into oxide ores during the extraction of metal. Extraction of metal by REDUCTION of oxide ores is easier than extraction from its carbonate or sulphide ores.
Discussion
5689.

Give reasons Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.

Answer»

Solution :Aluminium is highly reactive metal. It reacts with OXYGEN of air to form aluminium oxide, which gets deposited as thin layer on the SURFACE of aluminium. This layer acts as PROTECTIVE layer and prevents further reaction of aluminium with oxygen. Moreover it is light in weight and a good conductor of heat. Also, it.s manufacturing cost is very low in comparison to other metals. Therefore, most of the UTENSILS for cooking are made from aluminium.
Discussion
5690.

Give reasons : Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.

Answer»

Solution :This is because aluminium is a GOOD conductor of electricity. Aluminium FORMS a layer ofaluminium oxide at high temperature after which it resists CORROSION and does not react with FOOD.
Discussion
5691.

Give reasons:a) Pure ethanoic acid is known as glacial acetic acid. b) Ethanol is used as an antifreeze in car batteries. c) Ethanol mixed with methanol is known as denaturedalcohol. d) Oxyacetylene flame is used for welding. e) Gasoline is used as a fuel in western countries.

Answer»

Solution :a] Pure ethanoic acid freezes at `27^@C` during WINTER and resembles a glacier, HENCE the name glacial acetic acid.
b] Ethanol when mixed with water freezes at a HIGHER temperature than water, hence it is used in radiators of cars in cold countries.
c] Methanol is poisonous, hence when mixed with ethanol, it makes the latter unfit for consumption.
d] Oxyacetylene gives out a large amount of HEAT when burnt.
e] Alcohol added to PETROL increases the efficiency of the fuel.
Discussion
5692.

Give reasons (a)Platinum, gold and silver are used to make jewellery. (b) Sodium, potassium and lithium are stored under oil. (c ) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking. (d) Carbonate and sulphide ones are usually converted into oxides during the process of extracting.

Answer»

Solution :(a) It is because, they do not corrode. Hence, do not LOSE their shine and lustre.
(b) The reaction of these SUBSTANCES with oxygen is highly exothermic. This may cause fire, ALSO they react with water to form base so they are kept in oil.
(c ) This is because aluminium is good conductor of heat and ELECTRICITY. To prevent its reaction with food at high temperature, a protective layer of aluminium oxide `(Al_(2)O_(3))` is FORMED which prevent further corrosion.
(d)Carbonate and sulphide ores are usually converted into oxides during the process of extraction because it is easier to obtain metal from its oxide as compared with its sulphides or carbonates ore.
Discussion
5693.

Give reasons : (a) Platinum, gold and silver are used to make jewellery. (b) Sodium, potassium and lithium are stored under oil. ( c) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking. (d) Carbonate and sulphide ores are usually converted into oxides during the process of extraction.

Answer»

Solution :(a) Platinum, gold and silver are used to make jewellery because these are not attacked by air and moisture. They don.t undergo corrosion and retain their lustre for a long time. Also these metals are malleable and ductile.
(b) Sodium, potassium and lithium are stored under oil becasue in contact with moist air containing carbon dioxide, these are covered with a CARBONATE layer.
e.g. `4Na+O_(2)to2Na_(2)O`
`Na_(2)O+H_(2)Oto2NaOH`
`2NaOH+CO_(2)toNa_(2)CO_(3)+H_(2)O`
Also they react with water.
e.g. `2Na+2H_(2)Oto2NaOH+H_(2)`.
HENCE, these metals are stored under oil.
(c ) This is due to the reason that a thin sticking oxide layer of aluminium is FORMED on its surface which prevents further reaction. Also aluminium is a good conductor of heat and the oxide layer `(Al_(2)O_(3))` is stable EVEN at HIGH temperatures.
(d) This is because it is easier to obtain a metal from its oxide as compared to its sulphide and carbonate ore.
Discussion
5694.

Give reasons: (a) Platinum gold and silver are used to make jewellary. (b) Sodium, potassium and lithium are stored under oil. (iii) Aluminium is a highly reactive metal, yet it is used to make utensils for cooking.

Answer»

Solution :(a) PLATINUM gold and silver are used to make jewellary because of their bright shiny surface and high resistance to CORROSION. Also they have high malleability and ductility.
(b) Sodium, potassium and lithium are stored under OIL to PREVENT their reaction with oxygen, moisture and carbon dioxide of air so as to protect them as they are highly reactive metals.
(c ) Aluminium metal forms a thin layer of aluminium oxide all over its surface under the action of moist air. This layer prevents the metal underneath from further corrosion. It is cheap, easily avaialable , malleable and ductile. Therefore, it is used to make utensils for cooking.
Discussion
5695.

Give reason : Why copper is used to make hot water tanks and not steel (an alloy of iron.)?

Answer»

Solution : Copper does not REACT with COLD and HOT water nor it reacts with steam of water. Hence copper can be USED to make hot water tanks. But, steel is an alloy of an IRON and it reacts with steam of water, hence, iron in steel is slowly corroded. Therefore, copper is used for making hot water tanks and not steel.
Discussion
5696.

Give reason: Sodium, potassium and lithium are stored under oil.

Answer»

SOLUTION :Sodium , potassium and lithium are very reactive metals and REACT very vigorously with AIR as WELL as water. Therefore, they are kept immersed in kerosene oil in order to PREVENT their contact with air and moisture.
Discussion
5697.

Give reason : Iron grills are frequently painted.

Answer»

SOLUTION :To PREVENT RUSTING
Discussion
5698.

Give reason: (i) Reactivity of Al decreases if it is dipped in HNO_3 (ii) Carbon cannot reduce the oxides of Na or Mg. (iii) NaCl is not a conductor of electricity in solid state whereas it does conduct electricity in aqueous solution as well as in molten state. (iv) Metals like Na,K,Ca and Mg are never found in their free state in nature.

Answer»

Solution :(i) DUE to the formation of a layer of oxide i.e., `Al_2O_3` it becomes less REACTIVE .
(ii) Na or Mg are more reactive metals as compared to carbon, so their oxides are more stable.
(iii) In solid NaCl, the movement of ions is not possible due to its rigid structure but in aqueous solution or MOLTEN state, the ions can move freely. These free ions are responsible for conduction of electricity.
(iv) Na,K,Ca and Mg and highly reactive metals and thus never found in their free state in NATURE.
Discussion
5699.

Give reason : i)Element in a group have similar chemical properties. ii)Elements of groups I form ions with a charge of + 1.

Answer»

Solution :i) Elements in a group have same NUMBER of valence electrons and same valency THEREFORE have similar CHEMICAL properties.
ii) It is because elements of group 1 lose one electron to acquire +1 charge and become STABLE.
Discussion
5700.

Give reason : (i) "Alloys of iron are more useful when compared to pure iron" (ii) Alumunium oxide is called amphoteric oxide

Answer»

Solution :(i) Pure IRON is very soft
Stretches, when easily wher HOT
Alloys are HARD
The properties of iron can be changed if it is MIXED with other substances.
(ii) Aluminium oxide `(Al_(2)O_(3))` reacts with both acids as well as bases to produce salt and water.
Discussion