Chapter 16 Some Gases
Class 10 Science Notes
Table of Contents (Chapter 16 Some Gases)
1. Summary: Some Gases
1.1 Important Terms & Definitions
Dry Ice
Definition: Solid carbon dioxide ($CO_2$) is known as dry ice. It sublimates (turns directly from solid to gas) at $-78^\circ C$ without leaving a liquid residue.
Use: Used as a refrigerant because it produces a cooling effect during sublimation.
Lime Water
Definition: A saturated solution of calcium hydroxide [$Ca(OH)_2$].
Test for CO₂: Turns milky when carbon dioxide gas is passed through it due to formation of insoluble calcium carbonate.
Haber’s Process
Definition: The industrial method of manufacturing ammonia gas ($NH_3$) by heating a mixture of nitrogen and hydrogen in a 1:3 ratio.
Conditions: High temperature ($500^\circ C$), high pressure ($200-900$ atm), with iron catalyst and molybdenum promoter.
Promoter
Definition: A substance (like Molybdenum in Haber’s process) that increases the efficiency of a catalyst.
Liquor Ammonia
Definition: A concentrated solution of ammonia gas in water. It is basic in nature.
Urea
Definition: An organic compound [$(NH_2)_2CO$] used as a chemical fertilizer.
Production: Produced by heating ammonia and carbon dioxide at high temperature and pressure.
1.2 Important Principles (Laboratory Preparation)
A. Carbon Dioxide ($CO_2$)
Principle: When calcium carbonate (limestone/marble chips) reacts with dilute hydrochloric acid, carbon dioxide gas is produced.
Chemical Equation: $$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$$
Collection: Collected by upward displacement of air (heavier than air).
B. Ammonia ($NH_3$)
Principle: When an ammonium salt (like Ammonium Chloride) is heated with a strong base (like Calcium Hydroxide), ammonia gas is liberated.
Chemical Equation: $$2NH_4Cl(s) + Ca(OH)_2(s) \xrightarrow{\Delta} CaCl_2(s) + 2H_2O(l) + 2NH_3(g)$$
Collection: Collected by downward displacement of air (lighter than air).
Drying Agent: Lime tower (Calcium Oxide) – acidic drying agents cannot be used as ammonia is basic.
1.3 Important Formulas & Chemical Equations
Carbon Dioxide ($CO_2$)
Laboratory Preparation:
$$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)$$
Test with Lime Water:
$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) \text{ (White precipitate)} + H_2O(l)$$
Excess CO₂ (Milkiness disappears):
$$CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq) \text{ (Soluble)}$$
Burning Magnesium in CO₂:
$$2Mg(s) + CO_2(g) \rightarrow 2MgO(s) + C(s) \text{ (Black spot)}$$
Reaction with Ammonia (Urea formation):
$$2NH_3(g) + CO_2(g) \xrightarrow[Pressure]{1500^\circ C} NH_2-CO-NH_2(s) + H_2O(l)$$
Ammonia ($NH_3$)
Laboratory Preparation:
$$2NH_4Cl(s) + Ca(OH)_2(s) \xrightarrow{\Delta} CaCl_2(s) + 2H_2O(l) + 2NH_3(g)$$
Haber’s Process:
$$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) + \text{Heat}$$
Reaction with Water:
$$NH_3(g) + H_2O(l) \rightarrow NH_4OH(aq)$$
Reaction with Acid:
$$NH_3(g) + HCl(aq) \rightarrow NH_4Cl(aq) \text{ (White fumes)}$$
$$2NH_3(g) + H_2SO_4(aq) \rightarrow (NH_4)_2SO_4(aq)$$
1.4 Quick Review (Short & Sweet)
Part 1: Carbon Dioxide ($CO_2$)
• Origin: Discovered by Van Helmont. Found in air (0.03%), produced by burning fuels, respiration, and decay.
• Preparation: Marble chips + dilute acid (no heating required).
• Properties: Colorless, odorless, heavier than air, acidic (turns wet blue litmus red).
• Key Test: Turns lime water milky.
• Uses: Photosynthesis, fire extinguishers, carbonated drinks, dry ice for cooling.
Part 2: Ammonia ($NH_3$)
• Origin: Compound of Nitrogen and Hydrogen.
• Preparation: Lab: Ammonium Chloride + Slaked Lime (inclined test tube). Industry: Haber’s Process.
• Properties: Pungent smell, lighter than air, highly soluble in water, basic (turns wet red litmus blue).
• Key Test: Forms white fumes with HCl.
• Uses: Fertilizers, nitric acid, blueprints, cleansing agent, refrigerant.
2.1 Choose the correct options for the following questions:
(a) What products are formed on heating of limestone to a high temperature using coal?
(b) Which of the following statements is true?
(c) Which compounds are formed when carbon dioxide gas is passed through lime water for some time?
(d) In what ratio should ammonium chloride and calcium hydroxide be mixed for the laboratory preparation of ammonia gas?
2.2 Give reason:
(a) Carbon dioxide can be collected in an open glass jar.
Reason: Carbon dioxide ($CO_2$) is approximately 1.5 times heavier (denser) than air. Because of its high density, it sinks to the bottom of the container and stays there, displacing the lighter air upwards. Therefore, it does not escape easily from an open upright jar.
(b) The bottle of liquid ammonia should be placed in cold water or ice for some time before opening its lid.
Reason: Liquid ammonia (“Liquor Ammonia Fortis”) has a very high vapor pressure at room temperature because ammonia gas is highly volatile. If opened directly, the gas may rush out with great force (spurting) and can cause accidents or injury to the eyes. Cooling the bottle in ice or cold water reduces the vapor pressure and solubility of the gas, making it safe to open.
2.3 Answer the following questions:
(a) Describe the laboratory preparation of carbon dioxide with a labelled diagram.
Answer:
Principle: In the laboratory, Carbon dioxide gas is prepared by the action of dilute hydrochloric acid ($HCl$) on marble chips or limestone (Calcium Carbonate, $CaCO_3$) at room temperature.
Chemical Equation:
$$CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g) \uparrow$$
Procedure:
1. Clean marble chips ($CaCO_3$) are taken in a Woulfe’s bottle.
2. The bottle is fitted with a thistle funnel and a delivery tube using corks. The lower end of the thistle funnel must be dipped under the acid, while the delivery tube should remain above the liquid level.
3. Dilute Hydrochloric acid is poured through the thistle funnel until the marble chips are submerged.
4. The reaction starts immediately, releasing Carbon dioxide gas.
5. The gas is collected in a gas jar by the upward displacement of air.
Fig: Laboratory preparation of carbon dioxide gas
(b) Study the given figure and answer the following questions:
Fig: Laboratory setup for carbon dioxide preparation
i. Which gas is being collected in the gas jar?
Carbon dioxide ($CO_2$) gas is being collected.
ii. Write a balanced chemical reaction for preparation of this gas.
$$CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2 \uparrow$$
iii. Which litmus paper is used to identify the gas?
Wet blue litmus paper is used. $CO_2$ is acidic and turns wet blue litmus paper into red.
iv. Why is this gas collected in the gas jar kept straight upright?
The gas jar is kept straight upright because Carbon dioxide is heavier than air. It is collected by the upward displacement of air, where the heavy gas settles down filling the jar from the bottom.
(c) Write any three properties of carbon dioxide gas.
Answer:
1. Physical: It is a colorless and odorless gas.
2. Density: It is heavier than air.
3. Chemical: It is acidic in nature and turns wet blue litmus paper red. It turns lime water milky.
(d) Write any four uses of carbon dioxide gas.
Answer:
1. It is used in fire extinguishers as it does not support combustion and is heavier than air.
2. It is used by green plants for photosynthesis to manufacture food.
3. It is used in the manufacture of carbonated soft drinks (soda water, coke, etc.).
4. Solid carbon dioxide (Dry Ice) is used as a refrigerant.
(e) Describe the laboratory preparation of ammonia gas with a labelled diagram.
Answer:
Principle: Ammonia gas is prepared in the laboratory by heating a mixture of Ammonium Chloride ($NH_4Cl$) and Calcium Hydroxide (Slaked Lime, $Ca(OH)_2$) in a ratio of 2:1.
Chemical Equation:
$$2NH_4Cl(s) + Ca(OH)_2(s) \xrightarrow{\Delta} CaCl_2(s) + 2H_2O(l) + 2NH_3(g) \uparrow$$
Procedure:
1. A mixture of Ammonium Chloride and Calcium Hydroxide is taken in a hard glass test tube.
2. The test tube is clamped to a stand in a slightly inclined position (mouth downwards).
3. On heating the mixture, ammonia gas is evolved.
4. The gas is passed through a “Lime Tower” containing Calcium Oxide ($CaO$) to dry the gas (remove moisture).
5. The dry ammonia gas is collected in an inverted gas jar by the downward displacement of air.
Fig: Laboratory preparation of ammonia gas
(f) Study the given figure and answer the following questions:
Fig: Laboratory setup for ammonia preparation
i. Which gas is being collected in the gas jar?
Ammonia ($NH_3$) gas is being collected.
ii. Write the balanced chemical equation for the preparation of this gas.
$$2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3 \uparrow$$
iii. Which litmus is used to identify this gas?
Wet red litmus paper. Ammonia is basic in nature and turns wet red litmus paper blue.
iv. Why is the hard glass test tube slightly inclined?
The test tube is slightly inclined (mouth downwards) to prevent the water formed during the reaction from trickling back into the hot bottom of the test tube, which would cause the glass to crack.
v. What is the use of the lime tower?
The lime tower is filled with lumps of quicklime ($CaO$). It is used to dry the moist ammonia gas. Other drying agents like $H_2SO_4$ cannot be used because they react with ammonia (which is basic).
(g) Write any four uses of ammonia gas.
Answer:
1. It is used in the manufacture of chemical fertilizers like Urea, Ammonium sulphate, etc.
2. It is used in the manufacture of Nitric acid by Ostwald’s process.
3. It is used as a cooling agent (refrigerant) in ice factories and cold storages.
4. It is used to prepare “blueprints” for maps and architectural plans.
(h) What happens in the following processes? Write with a balanced chemical reaction.
i) Carbon dioxide is passed through lime water for some time.
Observation: The lime water turns milky (white precipitate forms).
Reaction:
$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$$
(Lime water) + (Carbon dioxide) $\rightarrow$ (Calcium carbonate – White ppt) + Water
ii) Carbon dioxide is passed through lime water for a long time.
Observation: The milkiness disappears and the solution becomes clear (colorless) due to the formation of soluble Calcium bicarbonate.
Reaction:
$$CaCO_3(s) + H_2O(l) + CO_2(g) \rightarrow Ca(HCO_3)_2(aq)$$
(Calcium carbonate) + Water + (Excess $CO_2$) $\rightarrow$ (Calcium bicarbonate – Soluble)
iii) A burning magnesium is inserted into the jar full of carbon dioxide gas.
Observation: The magnesium ribbon continues to burn with a dazzling white light, and black particles of carbon are deposited on the walls of the jar.
Reaction:
$$2Mg(s) + CO_2(g) \rightarrow 2MgO(s) + C(s)$$
(Magnesium) + (Carbon dioxide) $\rightarrow$ (Magnesium oxide) + (Carbon)
iv) The mixture of ammonium chloride and calcium hydroxide is heated.
Observation: Ammonia gas is evolved, which has a pungent smell.
Reaction:
$$2NH_4Cl(s) + Ca(OH)_2(s) \xrightarrow{\Delta} CaCl_2(s) + 2H_2O(l) + 2NH_3(g)$$
(Ammonium Chloride) + (Calcium Hydroxide) $\rightarrow$ (Calcium Chloride) + Water + Ammonia
v) Ammonia is mixed with water.
Observation: Ammonia dissolves readily to form Ammonium Hydroxide (alkaline solution).
Reaction:
$$NH_3(g) + H_2O(l) \rightarrow NH_4OH(aq)$$
(Ammonia) + Water $\rightarrow$ (Ammonium Hydroxide)
vi) Ammonia reacts with hydrochloric acid.
Observation: Dense white fumes of Ammonium Chloride are formed.
Reaction:
$$NH_3(g) + HCl(g) \rightarrow NH_4Cl(s)$$
(Ammonia) + (Hydrogen Chloride) $\rightarrow$ (Ammonium Chloride – White fumes)