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Q.1. A water insoluble substance â—X on reacting with dilute H2SO4 released a colourless and odourless gas accompanied by brisk effervescence. When the gas was passed through water, the solution obtained turned blue litmus red. On bubbling the gas through lime water, it initially became milky and milkyness disappeared when the gas was passed in excess. Identify the substance X. Write its chemical equations of the reactions involved

. Q.2. Ahmad took a magnesium ribbon (cleaned) and burned it on a flame. The white powder formed was taken in a test tube and water was added to it. He then tested the solution formed with red and blue litmus paper. What change was seen? Why?

Q.3. Give one example of a combination reaction in which an element combines with a compound to give you a new compound.

Q.4. Marble statues often slowly get corroded when kept in open for a long time. Assign a suitable explanation.

Q.5. Mohan took pure water for the electrolytic decomposition of water but did not see any bubbles near the electrodes. Explain why? Q. 6 Rancidity is a process used for spoiling of cooked food materials like vegetables, etc. When kept for long time in open. How can you prevent such process to proceed? Give an example.

Q. 7 A substance â—Xdisplaces â—Y from its solution in water. It is called displacement reaction. What other chemical name can be given to such type of reactions? Explain, giving an example?

Q. 8 A grey coloured metal â—Z (Atomic weight=65) is used in making dry cell. It reacts with dil. HCl to liberate a gas. What is the gas evolved? Calculate the minimum amount of â—Z required to produce 100 litres of gas? Q. 9 Why is respiration considered an exothermic reaction? Explain. Q. 10 Why are decomposition reactions called opposite of combination reactions? Write equations for these reactions.

Q. 11 A shiny brown colored element â—X on heating in air becomes a black coloured compound. Name the element â—X black the coloured compound formed. Also write the equation

Q. 12 In one of the industrial process used for manufacture of sodium hydroxide, a gas â—X is formed as by product. The gas â—X reacts with lime water to give a compound â—Y which is used as a bleaching agent in chemical industry. Identify â—Xand â—Ygiving the chemical equation of the reaction.

Q 13 Dry hydrogen chloride gas does not turn blue litmus, whereas hydrochloric acid does. Why?

Q. 14. Acid when react with metals release hydrogen gas but there is one acid which when reacts with metals does not release hydrogen except for two metals. Prove this statement. Q. 15. Name the properties responsible for the following uses of baling powder. (i) Baking industry (ii) As an antacid (ii) As soda-acid fire extinguisher.

Q. 16 What is meant by water of crystallization of a substance? What is its importance?

Q. 17 What effect does an increase in concentration of â—H ions in a solution have on the pH of a solution

Q. 18 Fresh milk has a pH Of 6. When it changes to curd, will its pH value increase or decrease? Why?

Q. 19 How does the flow of acid rain water into a river make the survival of aquatic life in a river difficult?

20 Two solutions A and B have pH values of 5 and 8 respectively. Which solution will be basic in nature?

Q. 21 Why does an aqueous solution of acid conduct electricity?Q. 22 How is alkali different from a base?

Asked by Anindya Roy(parent) , on 13/7/14

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1. The substance X can be the carbonates of commonly known metals such as calcium carbonate or CaCO3 which is insoluble in water.

CaCO3(s) + H2SO4 (aq)------------- CaSO4(s) + H2O(l) + CO2(g)

THus your colourless and odourless gas is carbondioxide, which when dissolved in water, forms carbonic acid, which thus turns blue litmus red.

CO2(g) + H2O(l)⇌ H2CO3(aq)

When carbondioxide is passed through lime water, it turns it milky due to the formation of insoluble calcium carbonate.

Ca(OH)2(aq) + CO2(g) ------ CaCO3(s) + H2O(l)

However, when excess of carbon dioxide is passed through this solution, the milkyness disappears. This is due the formation of calcium bicarbonate, which is soluble in water. Thus, a clear solution is obtained.

CaCO3 (s)+ H2O(l) + CO2(g)→ Ca(HCO3)2 (aq)

Posted by Shubham Johri(student)on 4/6/14

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2. When Magnesium ribbon is burnt in oxygen, it produces Magnesium Oxide or MgO which when dissolved with water yields aqeous magnesium hydroxide or Mg(OH)2. Since magnesium oxide is a basic(metallic) oxide thus magnesium hydroxide is also basic. When this base is tested with red litmus paper, it will turn it into blue as all bases do. With blue litmus however, no change is observed as it is already a base.

Posted by Shubham Johri(student)on 4/6/14


3. A very easy example is of exothermic reaction of combustion of propane or any other substance, which is a combination reaction.

C3H8(g) + 5O2(g) ------ 3CO2(g) + 4H2O(l) + heat

Posted by Shubham Johri(student)on 4/6/14


Marble statues and buildings slowly get corroded when exposed to air. This is notably because of two pollutants of air, sulphur dioxide and nitrogen oxide. These may be released by industries, by burning coal which does contain small amounts of nitrogen and sulphur, or by vehicular exhausts. These gases, especially sulphur dioxide are corrosive and suffocating, very reactive in air and rapidly convert to higher oxides. Ultimately, in the form of sulphur trioxide and nitrogen dioxide do these gases combine with the water present in clouds to form sulphuric acid and nitric acid. These acids mix withwater and fall as acid rain. They react with the marble or calcium carbonate to form soluble calcium compounds which get washed away by water and so erode the marble. This is called marble cancer, and is presently happening to the Taj Mahal.

Posted by Shubham Johri(student)on 4/6/14


Pure water is a bad conductor of electricity. For making it electrically conductive, we need to add some ionic electrolyte i. e. acids or bases which can dissocciate or split or ionise into their respective ions to make it conductiive. eg. hydrochloric acid or hydrogen chloride dissociates into hydronium cation (H3O+) and chloride anion(Cl-) in water. Ions are charged particles and conduct electricity to facilitate electrolysis of water. Otherwise electrolysis won't happen and no gas bubbles will be seen.

Posted by Shubham Johri(student)on 4/6/14


Rancidity can be prevented by adding anti-oxidants, vacuum packing of food products like tinned fish, replacing air in chips packets by nitrogen, refrigeration of the food-stuff which slow down rancid reactions,or packaging of food items in airtight containers also helps prevent rancidity.

Posted by Shubham Johri(student)on 4/6/14

This displacement reaction can be called single diplacement or single replacement reactions.

eg. Fe(s) + CuSO4 (aq) ---- FeSO4(aq) + Cu(s)

Double displacements can be called precipitate reactions as they lead to the formation of insoluble precipitates.

eg.(NH4)2S(aq) + FeSO4(aq) --- FeS(s) + (NH4)2SO4(aq)

Posted by Shubham Johri(student)on 4/6/14


ans 13.Hydrogen Chloride gas will turn blue litmus paper red only if the litmus paper is wet with some pure water. This is because when the gas comes into contact with the water on the paper, it becomes acidic, turning the litmus paper red.If the litmus paper is dry, it won't. This is beause acid shows acidic property(i. e. high concentartion of H+ ions) only in the presence of water i. e. it only disocciates iinto H+ ions in water. Since dry HCl does not contain water it dos not show acidic property, thus blue litmus doesn't turn red.

Posted by Shubham Johri(student)on 4/6/14


X is the metal zinc and the gas evolved is hydrogen.

Zn(s) + 2HCl(aq) -- ZnCl2(aq) + H2(g)

Therefore, 1 vulume of zinc will react with 2 volumes of HCl to form 1 volumes of both zinc chloride and hydrogen gas.

Therefore, in terms of litres, 100litres zinc + 200 litres of HCl ---- 100 litres of zinc chloride + 100 litres of Hydrogen gas.

Thus the required minimal amount is 100 litres of liquid zinc.

Posted by Shubham Johri(student)on 4/6/14


During respiration glucose combines with oxygen in presence of water to form carbondioxide and water and releasing heat energy that energizes us to do work. Therefore respiration is exothermic.

C6H12O6(aq) + 6O2(aq)+ 6H2O(l)---- 6CO2(aq) + 12H2O(l) + Energy

Posted by Shubham Johri(student)on 4/6/14


before ans 13 all ansers are in order from the start and after answer 13 it is from Q 8

10. In decomposition reaction one or more substances simply break down into simpler substances or constituents in the presence of some form of energy. i. e. A ---energy-- B + C

in combination reaction two or more substances combine or react to form a single product under specific conditions.

i. e. A + B --- C

Posted by Shubham Johri(student)on 4/6/14


ans 11. X is copper and black coloured compound is CuO

2Cu(s) + O2(g) --heat-- 2CuO(s)

Posted by Shubham Johri(student)on 4/6/14


12. sodium hydroxide can be produced by the chloralkali process as follows-

he most common chloralkali process involves the electrolysis ofaqueoussodium chloride(abrine) in amembrane cell.

Saturated brine is passed into the first chamber of the cell where thechlorideions areoxidisedat theanode, losing electrons to becomechlorinegas (Ain figure):

2Cl →Cl2+ 2e

At thecathode, positivehydrogen ionspulled from water molecules arereducedby the electrons provided by the electrolytic current, to hydrogen gas, releasinghydroxideions into the solution (Cin figure):

2H2O+ 2e → H2+ 2OH

The ion-permeableion exchange membraneat the center of the cell allows thesodiumions (Na+) to pass to the second chamber where they react with the hydroxide ions to producecaustic soda(NaOH) (Bin figure). The overall reaction for the electrolysis of brine is thus:

2NaCl + 2H2O→Cl2+H2+ 2NaOHThe gases thus formed are chlorine and hydrogen. Since hydrogen doesn't react with lime water, the gas X is chlorine gas, which combines with lime water or calcium hydroxide as follows-

2Cl2+ 2Ca(OH)2→Ca(OCl)2+CaCl2+ 2H2O

The bleaching agent used in bleaching industry, or the substance Y isCa(OCl)2 or calcium hypochlorite also known as chloride of lime.

Posted by Shubham Johri(student)on 4/6/14