Chapter 3 Some Common Gases Long Answer Type Questions
Question 1. Describe burette and pipettes. Why pipettes and burettes are used in the laboratory?
Answer:
Filter Paper
Filter paper is used to separate solid, insoluble particles from a liquid. It is a thick, porous, circular paper. Filter papers having different pore sizes are available.
The filter paper is chosen on the basis of the size of the insoluble, solid particles which are to be separated from the liquid.
After getting some idea about some common laboratory equipment and apparatus, we can now discuss two common gases which are very come to know about their physical and chemical usage.
One of them is oxygen and the other is Properties, their sources, and their preparation of hydrogen. During this brief discussion, we will procedures and their uses.
Read And Learn More WBBSE Solutions For Class 8 School Science Long Answer Type Questions
Question 2. Discuss briefly the laboratory method of preparation of oxygen from potassium chlorate.
Answer:
Laboratory Preparation Of Oxygen From Potassium Chlorate
Oxygen is usually prepared in the laboratory by heating carefully a mixture of potassium chlorate (KCIO3) and manganese dioxide (MnO2).
Four parts of solid potassium chlorate are intimately mixed with one part of solid manganese dioxide and taken in a hard glass test tube.
The test tube is fitted in such a way that it is tilted downwards. A delivery tube is fixed at the mouth of the test tube with the help of the bore of the cork.
The other end of the delivery tube is Potassium introduced into the gas jar filled with water. chlorate + The test tube is then heated strongly by a dioxide Bunsen burner.
Potassium chlorate melts and decomposes, evolving oxygen. The gas is collected in the gas jar by the downward displacement of water.
⇒ \(2 \mathrm{KClO}_3+\left[\mathrm{MnO}_2\right] \stackrel{\text { heat }(\Delta)}{\longrightarrow} 2 \mathrm{KCl}+3 \mathrm{O}_2+\left[\mathrm{MnO}_2\right]\)
- The gas is collected by the downward displacement of water, because,
- The solubility of oxygen in water is low
Oxygen is almost as heavy as air, so it cannot be collected by the downward displacement of air
In this reaction, MnO2 acts as a catalyst. If KCIO3 is heated alone, oxygen is produced at a temperature higher than 610°C.
In presence of a little amount of MnO2, KCIO3 decomposes at about 250°C to produce oxygen.
Actually, when KCIO3 is heated alone, it melts at 357°C and rapidly gives off oxygen at 380°C.
But the mass becomes pasty as the reaction proceeds due to the formation of potassium perchlorate (KCIO4 )whose melting point is 610°C.
⇒ \(4 \mathrm{KClO}_3 \rightarrow 3 \mathrm{KClO}_4+\mathrm{KCl}\)
When heated above 610°C, it decomposes to produce oxygen and a residue of KCI is left.
⇒ \(\mathrm{KClO}_4 \rightarrow \mathrm{KCl}+2 \mathrm{O}_2\)
In presence of a little amount of MnO2, KCIO3 smoothly decomposes at about 250°C to produce oxygen without the formation of KCIO4 in the intermediate stage.
West Bengal Class 8 Science Gases Solutions
Thus MnO2 accelerates the reaction and acts as a true catalyst. Moreover, both KCIO3 and KCIO4 are explosive substances. Any probable danger of explosion in the act of heating them to high temperatures is avoided by performing the reaction at lower temperatures with the help of a catalyst.
Precaution
- KCIO3 and MnO2 should be mixed intimately
- MnO2 must not be contaminated with charcoal or antimony sulphide
- The hard glass test tube must be tilted downwards
- Heating should be done slowly and should continue from the front to the back side of the test tube.
Question 3. Discuss a laboratory method for the preparation of oxygen at room temperature.
Answer:
Preparation of Oxygen from Sodium Peroxide at room temperature
Oxygen is produced easily when water is added to solid sodium peroxide. No heating is required for this process.
⇒ \(2 \mathrm{Na}_2 \mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow 4 \mathrm{NaOH}+\mathrm{O}_2\)
Materials and Apparatus Required: Solid sodium peroxide (Na2O2), distilled water, a conical flask, a cork with two holes in it, a dropping funnel, a bent delivery tube, and a gas jar.
Experiment: Solid Na2O2 is taken in the conical flask and the mouth of the conical flask is fitted with a cork. Through one of the holes, a dropping funnel is attached and through another hole, one end of the bent delivery tube is inserted.
The other end of the bent delivery tube is introduced into the gas jar filled with water. Now water is added
Observation: Oxygen gas is evolved. The gas dropwise to solid Na2O2 through the dropping is collected in the gas jar by downward funnel displacement of water.
Common Gases Chapter 3 WBBSE
Question 4. How oxygen can be produced by the electrolysis of water?
Answer:
Preparation of Oxygen by Electrolysis of Water
Electrolysis of water acidified with dilute sulphuric acid can produce hydrogen at the cathode and oxygen gas at the anode. A platinum electrode is used as an anode and cathode in a rectangular tank.
High voltage is passed through the tank to carry out the electrolysis. Here, hydrogen gas is obtained as a by-product.
Manufacture of Oxygen by Fractional Distillation of air Industrially oxygen is produced in bulk quantity by a process known as a fractional distillation of liquid air.
Air is composed of nitrogen and oxygen, in which oxygen forms about 21% by volume. The two gases can be separated from one another by liquefaction of air followed by fractional distillation.
Removal of water vapour, CO2 & dust particles: Air is first freed from water vapour and carbon dioxide by passing them over fused calcium chloride and slaked lime, respectively. Dust particles are removed from the air by passing them through an electric precipitator.
Question 5. Discuss briefly the physical properties of oxygen.
Answer:
Physical Properties of Oxygen
- Oxygen is a colourless, odourless and tasteless gas.
- It is slightly heavier than air. The density of oxygen at normal temperature and pressure is 1.428 grams per litre.
- It condenses to a pale blue liquid, which freezes to a blue solid if cooled in liquid hydrogen. The freezing point of liquid oxygen is – 218°C and the boiling point of liquid oxygen is – 183°C.
- Oxygen is slightly soluble in water. The solubility of oxygen at 0°C and 1 atmospheric pressure is 1438 mg/lit. The ‘ dissolved oxygen sustains the life of aquatic plants and animals.
- The respiration of aquatic animals is dependent on the dissolved oxygen in the water. Since oxygen is more soluble in water than nitrogen, water is richer in oxygen than ordinary air.
Oxygen has three naturally occurring isotopes, \({ }_8^{16} \mathrm{O},{ }_8^{17} \mathrm{O} \text { and }{ }_8^{18} \mathrm{O}\) But the natural abundance of the last two is very low.
| Isotope | 168 O | 178O | 188O |
| Natural abundance | 0.99763% | 0.00037% | 0.002% |
Oxygen exhibits allotropy. Its allotropic modification is ozone (03).
Question 6. Discuss briefly the reaction of oxygen with non-metals.
Answer:
The reaction of oxygen with non-metals:
Oxygen reacts with non-metals such as carbon, sulphur, phosphorous, etc. to produce oxides. Generally, most oxides of non-metals are acidic. Their aqueous solution produces acid. A few examples are given below.
When a piece of glowing charcoal is introduced in a jar of oxygen, charcoal burns more brightly throwing sparks. The product of this reaction is carbon dioxide.
⇒ \(\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2\)
When a small quantity of sulphur is heated in a flame and then introduced in a jar filled with oxygen, it is observed that the burning takes place brilliantly producing a blue flame and sulphur dioxide (SO2) is produced.
\(\mathrm{S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2\)When a piece of white phosphorous is introduced in a jar of oxygen, it burns brightly with white flames and forms white fumes of phosphorous pentoxide (P2O5) which solidifies on cooling.
⇒ \(4 \mathrm{P}+5 \mathrm{O}_2 \rightarrow 2 \mathrm{P}_2 \mathrm{O}_5\)
WBBSE Class 8 Gases Practice Questions
Question 7. Oxides of non-metals are generally acidic—Why? Generally, most oxides of non-metals are acidic. Their aqueous solution produces acid.
Answer:
Oxides of non-metals are generally acidic:
For example, CO2 SO2 and P2O5 are three oxides of non-metal. When they are dissolved in water, they form acids (e.g. carbonic acid, sulphurous acid and phosphoric acid, respectively) and turn blue litmus paper red, indicating that their solution is acidic.
⇒ \(\begin{array}{ll}
\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2 ; & \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3 \\
\mathrm{~S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2 ; & \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{SO}_3 \\
4 \mathrm{P}+5 \mathrm{O}_2 \rightarrow 2 \mathrm{P}_2 \mathrm{O}_5 ; & \mathrm{P}_2 \mathrm{O}_5+3 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{H}_3 \mathrm{PO}_4
\end{array}\)
Question 8. Discuss briefly the reaction of oxygen with metals.
Answer:
The reaction of oxygen with metals:
Some metals burn in oxygen on heating producing metal oxides. Metal oxides are mostly basic oxides. Some examples are given below.
When a piece of hot, dry sodium is introduced in a jar of oxygen, the metal burns spontaneously producing a golden yellow flame and forming sodium oxide (Na2O).
⇒ \(4 \mathrm{Na}+\mathrm{O}_2 \rightarrow 2 \mathrm{Na}_2 \mathrm{O}\)
When a burning magnesium ribbon is introduced in a gas jar filled with oxygen, it burns brightly producing blinding white light. The white powdery substance left after burning is magnesium oxide (MgO).
⇒ \(2 \mathrm{Mg}+\mathrm{O}_2 \rightarrow 2 \mathrm{MgO}\)
In a similar way, potassium, calcium, etc. react with oxygen to form basic oxides
\(\begin{gathered}2 \mathrm{Ca}+\mathrm{O}_2 \rightarrow 2 \mathrm{CaO} \\
4 \mathrm{~K}+\mathrm{O}_2 \rightarrow 2 \mathrm{~K}_2 \mathrm{O}
\end{gathered}\)
Question 9. Oxides of metals are generally basic—Explain.
Answer:
Oxides of metals are generally basic:
Metal oxides are mostly basic oxides. Some basic oxides form hydroxides when they react with water. Hydroxides of some metals such as sodium, magnesium, calcium, potassium etc.
Are soluble in water and turn red litmus paper blue, indicating that the aqueous solution of those metal oxides is basic. Some examples are given below.
⇒ \(\begin{array}{ll}
\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2 ; & \mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3 \\
\mathrm{~S}+\mathrm{O}_2 \rightarrow \mathrm{SO}_2 ; & \mathrm{SO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{SO}_3 \\
4 \mathrm{P}+5 \mathrm{O}_2 \rightarrow 2 \mathrm{P}_2 \mathrm{O}_5 ; & \mathrm{P}_2 \mathrm{O}_5+3 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{H}_3 \mathrm{PO}_4
\end{array}\)
Question 10. Give two examples of amphoteric oxides. Why are they so-called? Explain.
Answer:
Examples of amphoteric oxides:
Aluminium oxide (Al2O3) and zinc oxide (ZnO) are two examples of amphoteric oxide. They are called amphoteric oxides because they have the properties of acidic and basic oxides.
They undergo a neutralization reaction with both acids and bases. They act as weakly basic oxides towards a strong acid and as weakly acidic oxides towards a strong base.
1. For example, the aluminium metal reacts with oxygen to form aluminium oxide (Al2O3).
⇒ \(4 \mathrm{Al}+3 \mathrm{O}_2 \rightarrow 2 \mathrm{Al}_2 \mathrm{O}_3\)
It reacts with an acid to form aluminium chloride (salt) and water.
⇒ \(\mathrm{Al}_2 \mathrm{O}_3+6 \mathrm{HCl} \rightarrow 2 \mathrm{AlCl}_3+3 \mathrm{H}_2 \mathrm{O}\)
It reacts with a base (such as sodium hydroxide) to produce sodium aluminate and water.
⇒ \(\mathrm{Al}_2 \mathrm{O}_3+2 \mathrm{NaOH} \rightarrow 2 \mathrm{NaAlO}_2+\mathrm{H}_2 \mathrm{O}\)
2. Zinc oxide (ZnO) is an amphoteric oxide. Zinc metal reacts with oxygen to form zinc oxide (ZnO).
⇒ \(2 \mathrm{Zn}+\mathrm{O}_2 \rightarrow 2 \mathrm{ZnO}\)
It reacts with hydrochloric acid to form zinc chloride (salt) and water.
⇒ \(\mathrm{ZnO}+2 \mathrm{HCl} \rightarrow \mathrm{ZnCl}_2+\mathrm{H}_2 \mathrm{O}\)
It reacts with sodium hydroxide to produce sodium zincate and water.
⇒ \(2 \mathrm{ZnO}+4 \mathrm{NaOH} \rightarrow 2 \mathrm{Na}_2 \mathrm{ZnO}_2+2 \mathrm{H}_2 \mathrm{O}\)
West Bengal Board Class 8 Science Revision
Question 11. Stannic oxide and lead monoxide are amphoteric oxides—Explain.
Answer:
Stannic oxide and lead monoxide are amphoteric oxides:
Stannic oxide and lead monoxide are called amphoteric oxide because they have properties of the acidic and basic oxide.
They undergo a neutralization reaction with both acids and bases.
They act as weakly basic oxides towards a strong acid and as weakly acidic oxides towards a strong base.
Stannic oxide (SnO2) is prepared by burning tin at white heat in the air. It dissolves in concentrated H2SO4 to produce stannic sulphate (which is unstable). On fusion with sodium hydroxide, it forms sodium stannate, which is soluble in water.
⇒ \(\mathrm{Sn}+\mathrm{O}_2 \rightarrow \mathrm{SnO}_2\)
Reaction with acid: \(\mathrm{SnO}_2+2 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Sn}\left(\mathrm{SO}_4\right)_2+2 \mathrm{H}_2 \mathrm{O}\)
Reaction with base: \(\mathrm{SnO}_2+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_2 \mathrm{SnO}_3+\mathrm{H}_2 \mathrm{O}\)
Lead monoxide is obtained by heating lead in the air. It reacts with HNO3 forming lead nitrate. It dissolves in a hot sodium hydroxide solution forming sodium plumbite.
⇒ \(2 \mathrm{~Pb}+\mathrm{O}_2 \rightarrow 2 \mathrm{PbO}\)
Reaction with acid: \(\mathrm{PbO}+2 \mathrm{HNO}_3 \rightarrow \mathrm{Pb}\left(\mathrm{NO}_3\right)_2+\mathrm{H}_2 \mathrm{O}\)
Reaction with base: \(\mathrm{PbO}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_2 \mathrm{PbO}_2+\mathrm{H}_2 \mathrm{O}\)
Question 12. Discuss briefly the uses of oxygen in industries.
Answer:
The uses of oxygen in industries:
Oxygen is a very important element in the chemical industry. It is used to remove impurities from crude iron and pure steel Is produced.
Impurities present in crude Iron enhance rusting of iron.
During the preparation of H2SO4, oxygen Is utilized. SO2 Is oxidized to SO3 by reaction with oxygen which is then converted to HSO2 In subsequent steps.
H2SO4 is an essential Component in car batteries, and storage cells and is used for making paints and fertilizers and for refining petroleum and metals like zinc and copper.
Oxygen is used during the industrial production of HNO3 by the Ostwald process. HNO3 is essential for producing fertilizers like ammonium nitrate (NH4NO3) and for preparing explosives.
For welding and cutting of metals, oxy-hydrogen flame and oxy-acetylene flame are produced in which temperature as high as approximately 3000°C is achieved.
These flames are produced by the exothermic reactions between oxygen and hydrogen and between oxygen and acetylene.
Question 13. Show with an experiment that hydrogen is lighter than air.
Answer:
An experiment that hydrogen is lighter than air:
With the help of an experiment, it can be shown that hydrogen is lighter than air.
Apparatus and chemicals required: Two gas jars with lids – one filled with hydrogen gas and the other filled with air, a taper.
Experiment: Two gas jars marked A and B are taken. One of the gas Jars A is filled with hydrogen gas and is covered by a lid.
The open mouth of another gas jar B is held upside down over gas Jar A and the lid Is then slowly removed. Now a burning taper is introduced Inside the gas jar B.
Observation: A “pop” sound Is heard. The taper extinguishes but the gas burns with a bluish flame.
Inference: This confirms that the gas in gas Jar B is hydrogen. Hydrogen was initially in the gas Jar A.
When the gas jar filled with air is Inverted over the gas jar filled with hydrogen, hydrogen is lighter than air, moves upwards and is collected in gas jar B by downward displacement of air.
Chapter 3 Common Gases Important Questions
Question 14. Discuss briefly the reaction of hydrogen with the following non-metals: chlorine, nitrogen and sulphur.
Answer:
Preparation of Oxygen from Sodium Peroxide at room temperature
Oxygen is produced easily when water is added to solid sodium peroxide. No heating is required for this process.
⇒ \(2 \mathrm{Na}_2 \mathrm{O}_2+2 \mathrm{H}_2 \mathrm{O} \rightarrow 4 \mathrm{NaOH}+\mathrm{O}_2\)
Materials and Apparatus Required: Solid sodium peroxide (Na2O2), distilled water, a conical flask, a cork with two holes in it, a dropping funnel, a bent delivery tube, and a gas jar.
Experiment: Solid Na2O2 is taken in the conical flask and the mouth of the conical flask is fitted with a cork.
Through one of the holes, a dropping funnel is attached and through another hole, one end of the bent delivery tube is inserted.
The other end of the bent delivery tube is introduced into the gas jar filled with water. Now water is added
Observation: Oxygen gas is evolved. The gas dropwise to solid Na2O2 through the dropping is collected in the gas jar by downward funnel displacement of water.
Class 8 Science Gases Study Material
Question 15. What precautions should be taken during the laboratory preparation of oxygen from potassium chlorate?
Answer:
Laboratory Preparation Of Oxygen From Potassium Chlorate
Oxygen is usually prepared in the laboratory by heating carefully a mixture of potassium chlorate (KCIO3) and manganese dioxide (MnO2).
Four parts of solid potassium chlorate are intimately mixed with one part of solid manganese dioxide and taken in a hard glass test tube.
The test tube is fitted in such a way that it is tilted downwards. A delivery tube is fixed at the mouth of the test tube with the help of the bore of the cork.
The other end of the delivery tube is Potassium introduced into the gas jar filled with water. chlorate + The test tube is then heated strongly by a dioxide Bunsen burner.
Potassium chlorate melts and decomposes, evolving oxygen. The gas is collected in the gas jar by the downward displacement of water.
⇒ \(2 \mathrm{KClO}_3+\left[\mathrm{MnO}_2\right] \stackrel{\text { heat }(\Delta)}{\longrightarrow} 2 \mathrm{KCl}+3 \mathrm{O}_2+\left[\mathrm{MnO}_2\right]\)
The gas is collected by the downward displacement of water, because,
The solubility of oxygen in water is low
Oxygen is almost as heavy as air, so it cannot be collected by the downward displacement of air
In this reaction, MnO2 acts as a catalyst. If KCIO3 is heated alone, oxygen is produced at a temperature higher than 610°C.
In presence of a little amount of MnO2, KCIO3 decomposes at about 250°C to produce oxygen.
Actually, when KCIO3 is heated alone, it melts at 357°C and rapidly gives off oxygen at 380°C. But the mass becomes pasty as the reaction proceeds due to the formation of potassium perchlorate (KCIO4 )whose melting point is 610°C.
⇒ \(4 \mathrm{KClO}_3 \rightarrow 3 \mathrm{KClO}_4+\mathrm{KCl}\)
When heated above 610°C, it decomposes to produce oxygen and a residue of KCI is left.
⇒ \(\mathrm{KClO}_4 \rightarrow \mathrm{KCl}+2 \mathrm{O}_2\)
In presence of a little amount of MnO2, KCIO3 smoothly decomposes at about 250°C to produce oxygen without the formation of KCIO4 in the intermediate stage.
Thus MnO2 accelerates the reaction and acts as a true catalyst. Moreover, both KCIO3 and KCIO4 are explosive substances. Any probable danger of explosion in the act of heating them to high temperatures is avoided by performing the reaction at lower temperatures with the help of a catalyst.
Precaution
- KCl03 and MnO2 should be mixed intimately
- MnO2 must not be contaminated with charcoal or antimony sulphide
- The hard glass test tube must be tilted downwards
- Heating should be done slowly and should continue from the front to the back side of the test tube.
WBBSE Class 8 Environment and Science Notes
Question 16. A soft white metal A reacts with water to form a compound B and a colourless gas C. When C is passed over heated copper oxide, water and a red-brown coloured element D are formed. Identify A, B, C, and D and write the reactions.
Answer:
Given:
A soft white metal A reacts with water to form a compound B and a colourless gas C.
When C is passed over heated copper oxide, water and a red-brown coloured element D are formed.
A is sodium. Sodium reacts with water to form sodium oxide (Na2O:B) and hydrogen (H2:C) 2Na + H2O = Na2O + H2O , When hydrogen is passed over heated cupric oxide (CuO), water and metallic copper (Cu :D) are formed.
⇒ \(\mathrm{CuO}+\mathrm{H}_2=\mathrm{Cu}+\mathrm{H}_2 \mathrm{O} \text {. }\)