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?
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)
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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.
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.
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
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.
20. B is basic
21. Ionic acids dissocciate into their respective ions in water. ions are charged particles and hence conduct electricity. acids thus conduct electricity only in solution in water r aqeous state, not in pure form. they are thus used as electrolytes
for eg HCl dissocciates into hydronium cation H3O+and chloride anion Cl- in water
22. Alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Alkalis are best known for being bases that dissolve in water. (Most common form is hydroxide.) A base is most commonly thought of as an aqueous substance that can accept H+ ions. A soluble base is also often referred to as an alkali if hydroxide ions (OH) are involved. "All alkali are bases but not all bases are alkali"
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above was ans 14.
15. baking powder- baking industry- releases carbondioxide on heating which makes doughs of cakes and bakery items rise, making them fluffy, aerated and soft.
antacid- being a base, it can be taken in aqeous form to neutralise the excess HCl in stomach to cure acidity and relieve pain releasing heat and water
as fire extinguisher- as said above, in form of dry chemical, it releases carbondioxide on heating which surrounds the fire like a cool blanket that cuts off oxygen supply to keep the fire burning and so helps in dousing fires.
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16. Water of crystallisation/ hydration of a substance means the water molecules of a substance that are attached to the molecules of the substance itself and gives it a crystalline form. it is the water inside crystals.In some contexts, water of crystallization is the total weight of water in a substance at a given temperature and is mostly present in a definite (stoichiometric) ratio. Classically, "water of crystallization" refers to water that is found in the crystalline framework of ametal complexor asalt, which is not directly bonded to the metal cation.
it is important as it is necessary for giving a compound a definite shape in form of crystals. for eg ferrous sulphate heptahydrate or FeSO4. 7H2O, copper sulphate pentahydrate or CuSO4. 5H2O, sodium carbonate decahydrate or Na2CO3. 10 H2O. if a substance doesn't have ths water, it is amorphous i. e. has a powdery form.Asaltwith associated water of crystallization is known as ahydrate. it makes compounds hydrous, those which dont have it are called anyhdrous.necessary for the maintenance of crystalline properties but capable of being removed by sufficient heat, this water is very important.
17. An increase in concentration of H+ ions means the acid is stronger, which means the ph decreases as the decrease in ph of a substance means increase of acid character and strenght i. e. increase in h+ ion concentration.
18. fresh mil has ph of 6 meaning it is weakly acidic. when it turns to curd, it becomes more acidic and sour in taste due to the lactic acid and hence acidic charactr increases and hence ph decreases
All acids contain hydrogen. Therefore metals more reactive than hydrogen which are placed higher than H in reactivity series can displace hydrogen from acids, releasing hydrogen gas. There is no such acid that liberates hydrogen with only 2 metals as all metals above H in reactivity series can liberate H2 from acids, which is more than 2.
19. if acid rain flows into water bodies, they may suffer chronic acidification and deadly drop in ph.Acid rain causes a cascade of effects that harm or kill individual fish, reduce fish population numbers, completely eliminate fish species from a waterbody, and decrease biodiversity. As acid rain flows through soils in a watershed, aluminum is released from soils into the lakes and streams located in that watershed. So, as pH in a lake or stream decreases, aluminum levels increase. Both low pH and increased aluminum levels are directly toxic to fish. In addition, low pH and increased aluminum levels cause chronic stress that may not kill individual fish, but leads to lower body weight and smaller size and makes fish less able to compete for food and habitat.
Some types of plants and animals are able to tolerate acidic waters. Others, however, are acid-sensitive and will be lost as the pH declines. Generally, the young of most species are more sensitive to environmental conditions than adults. At pH 5, most fish eggs cannot hatch. At lower pH levels, some adult fish die. Some acid lakes have no fish. The chart below shows that not all fish, shellfish, or the insects that they eat can tolerate the same amount of acid; for example, frogs can tolerate water that is more acidic (i.e., has a lower pH) than trout.
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
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.
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.
ans 11. X is copper and black coloured compound is CuO
2Cu(s) + O2(g) --heat-- 2CuO(s)
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.
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)
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
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.
Smoke iscloud of tiny particles: a mass of tiny particles in the air that rises up from something burning but smog isthick dirty fog: a mixture of fog and smoke or other airborne pollutants such as exhaust fumes.