Class 9 Science Note

Some Gases


Symbol: H

Molecular formula: H2

Valency: 1

Atomic number: 1.

Atomic weight: 1.008

Molecular weight: 2


Hydrogen consists of only one proton and one electron, it is considered as the simplest element.



Laboratory preparation of hydrogen gas


In the laboratory, hydrogen can be prepared by the reaction of zinc with dilute hydrochloric acid or sulphuric acid.

Zn + 2HCl          Ø                     ZnCl2 + H2

Zn + H2SO4     Ø                   ZnSO4 + H2






First of all, apparatus is fitted as shown in the figure. Few pieces of zinc in taken in Woulfe's bottle and dil. hydrochloric acid is poured through thistle funnel. The bottom of the thistle funnel should be dipped with HCl otherwise the gas will go away from the funnel. The apparatus used in the experiment should be air tight. The reaction takes place between zinc and hydrogen gas is evolved which gets collected through the delivery tube in the gas jar by the upward displacement of water.




Physical properties

a. It is odorless, colorless and tasteless.

b. It is the lightest gas found in the air.

c. It does not show any reaction on the litmus paper.

d. It is slightly soluble in water.



Chemical properties

When hydrogen is passed through metals like copper, sodium potassium   hydride is formed.

Ca +H2      Ø     CaH2


When the hydrogen gas is passed through vegetable oil (unsaturated compounds)at  required condition ,vegetable ghee(saturated compound ) is formed .this process is called  hydrogenation.

Example: Vegetable oil + H2      □(→┴(Ni/200) vegetable ghee


When metal oxide is treated with hydrogen, then metal and water is formed.

PbO + H2    Ø   Pb + H2O


 When hydrogen is mixed in air, it burns with a pop sound and forms water.But pure hydrogen gas burns with light flame when mixed with oxygen to form water.

2H2 + O2      Ø                2H2O





In the industrial sector, hydrogen is produced by the reaction between hydrocarbon and water vapor.

For industrial purpose, hydrogen gas is prepared by reacting methane with water vapour at very high temperature in the presence of nickel powder, which acts as catalyst in a reaction.

CH4 + H2O    CO + 3H2



a. It is used as a reducing agent in industries and laboratory.

b. It is used to fill in the balloons as it is the lightest gas but it is mixed with helium as hydrogen is highly inflammable.

c. It is used to prepare chemical fertilizers.







Symbol: O

Molecular formula: O2

Valency: 2

Atomic number: 8

Atomic weight: 16

Molecular weight: 32


Laboratory preparation of oxygen gas



For the manufacture of oxygen gas in the laboratory (by heating), potassium chlorate or potassium permanganate is heated in a herd test tube vigorously.

Small amount of potassium permanganate or potassium chlorate is taken in a hard test tube and heated vigorously. A burning matchstick is taken to the test tube and observed. We can see that at very high temperature of (360 – 370) o C oxygen is evolved. This is high temperature to maintain so manganese dioxide is taken on a different test tube along with potassium chlorate and is heated. Manganese dioxide acts as catalyst and this helps in formation of oxygen at (240-250) o C. The oxygen produced is collected in the gas jar by downward displacement of water.


KClO3                    Ø              2KCl + 3O2




Oxygen can be prepared in laboratory without the application of heat from hydrogen peroxide. Little amount of manganese dioxide is taken in a conical flask and hydrogen peroxide is slowly dropped through dropping funnel. The oxygen gas produced is collected in the gas jar through downward displacement of water.




2H2O2         Ø            2H2O + O2



By the electrolysis of water, oxygen can be formed. When electricity is passed on acidified water, it dissociates into hydrogen and hydroxyl ions. From hydroxyl ions, we get oxygen from anode.

H2O                Ø           H+ + OH-

OH-   + e-        Ø            OH

4OH                Ø           2H2O + O2



Physical properties

It is a neutral gas doesn’t have any effect on the indicators

It is colorless odorless and tasteless gas.

It is heavier than air.

It is slightly soluble in water.


Catalysts are the chemical reagent which does not take part in the chemical reaction but increases or decreases the rate of reaction.  Example: MnO2 acts as catalyst during the preparation of oxygen.


Chemical properties

Some metals Mg, Ca combine with oxygen to produce oxides

2Mg   + O2à  2MgO


When oxygen combines with organic compound, carbon dioxide and water is formed.

CH4 + 2O2à  CO2   +2H2O


When carbon is burned in an excess air carbon dioxide is formed.

C +O2à CO2




a. Living organisms use oxygen during respiration.

b. Liquid oxygen is used as fuel in missile and rocket.

c. Mountaineers carry oxygen cylinders and masks with them so that they can breathe easily at high altitude






Symbol: N

Molecular formula: N2

Valency: 3, 5

Atomic number: 7

Atomic weight: 14

Molecular weight: 28



 Laboratory preparation of nitrogen gas


Nitrogen gas is prepared in the laboratory by the mixture of sodium nitrate and ammonium chloride is heated. This results in the production of nitrogen which is collected in the gas jar by the downward displacement of water.

NaNO2 + NH4Cl          Ø   NaCl + 2H2O + N2



Nitrogen gas can be manufactured by liquefying air. Air is liquefied at a high temperature in order to produce nitrogen in a large amount. Oxygen boils at -1830C whereas nitrogen boils at 1960C. Hence nitrogen gas gets separated when the temperature of liquid air reac-196oC. By this process, nitrogen can be obtained along with nitrogen.



 Physical properties

It is colorless, odourless and tasteless.

It is slightly lighter than air.

It is not soluble in water.



Chemical properties


In the industrial sector, ammonia is manufactured at the Haber's synthesis process. In this process, nitrogen and hydrogen are heated in the ratio of 1:3 in the presence of iron as catalyst and molybdenum as promoter at temperature of about 500oC, under high pressure of about 200 - 900 atmospheric pressure. 
N2 + 3H2 ↔ 2NH3


Magnesium burns with nitrogen to form nitrites.

Mg + N2                                 →           Mg3N2


At very high temperature, nitrogen and oxygen combines with to form a nitric oxide.

N2 + O2 —3000c→ 2NO



It is used as fuel tank in aero plane.

It is used as chemical fertilizers.

It is used in electric bulbs.

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