Chemical reactions and equations activity based question 10th

Chemical reactions and equations Activity Based Question 10th Chemistry Q. A student added a few pieces of zinc into dil.H₂SO₄ taken in a test tube. Suddenly, evolution of gas occurs. What gas is evolved? Answer: Hydrogen gas Q. Why do copper sulphate crystals change colour from blue to white on heating? Answer: Due to lose of water of crystallization which can also be called as dehydration Q. What happens when a small piece of iron is kept in copper sulphate solution? Answer: Simple displacement reaction results in the Formation of a brown coating of copper on the surface of iron nail. The green colour of the solution is due to the displaced Fe2⁺ ions from the Iron nail Q. While performing an experiment, a student adds sodium sulphate solution to a colourless solution. A white precipitate is immediately formed and is insoluble in dil. HCl. The colourless solution is …………………………… and the precipitate formed is…………………… Answer: Barium chloride reacts with sodium sulphate to form a white precipitate of barium sulphate[BaCl₂ and BaSO₄] Q. When NaOH is added to zinc sulphate solution, a precipitate is formed which is soluble in excess of NaOH. The precipitate obtained is……………………. & the colour of the precipitate is………………. Answer: A white gelatinous precipitate of zinc hydroxide is formed Zn(OH)₂ & white gelatinous Q. On heating Pb(NO₃)₂, what is observed during the reaction?

Answer: Lead nitrate decomposes on strong heating with a crackling sound. It produces NO₂ (g), O₂ (g) and PbO (s) Q. Take dilute sulphuric acid in a test tube and put a few zinc granules into the test tube. What will you observe?

Answer: Since Zn and dil H₂SO₄ react with each other liberating H₂ gas, the size of zinc granules keep on decreasing Q. When magnesium combines with oxygen, it produces magnesium oxide that appears to be like: Answer:int: Mg combines with O₂ to form a white ash of MgO Q. 100 ml of a saturated copper sulphate solution was taken in a beaker. A small amount of iron filings was added to the beaker and left undisturbed overnight. The mixture was then filtered to obtain the residue. The residue was found to be what colour? Answer: In this reaction copper particles are obtained which appears as reddish brown residue. Q. When you place an iron nail in copper sulphate solution, the reddish brown coating formed on the nail is _______. Answer: The reddish brown coating is a soft and dull coating of copper Q. What is observed when dilute sulphuric acid is added to zinc granules?

Answer: Zn reacts with dil. H₂SO₄ to liberate H₂ gas and bubbles start coming from the surface of the Zinc granules.

Q. Why is magnesium ribbon held with a pair of tongs and not with bare hands, when it is burnt in air? Answer: Magnesium metal is a good conductor of heat, so it gets very hot when it is held in the flame. ASSIGNMENT : FIRST TERM (2012-2013) X SUBJECT : CHEMISTRY CHAPTER : CHEMICAL REACTIONS AND EQUATIONS Max Marks: 55 A. Very short answer questions: (1 mark each) (1x 14= 14 marks) 1. What are reactants ? 2. What are products ? 3. What is the term used for symbolic representation of a chemical reaction ? 4. What does the + sign between the reactants indicate ? 5. How is the salt solution in water indicated in a chemical reaction / 6. Aluminium burns in chlorine to form aluminum chloride. Write a balanced equation for this reaction ? 7. Balance the following equation and add the state symbols :

Zn(s) + 2HCl(aq) → ZnCl2 + H2

8. Name the following reaction.

AB + CD → AD + BC

9. Aluminium metal displaces iron from ferric oxide (Fe2O2) giving aluminium oxide and iron. Which is more reactive, aluminium or iron ?

10. Identify from the following reaction : H2S + Cl2 → 2HCl + S

a. Component oxidized b. Oxidizing agent 11. Give one example of a redox reaction which is also a combination reaction. 12. Which atom is reduced on burning an element in air. 13. When SO2 gas is passed through saturated solution of H2S, the following reaction occurs

SO2 + 2H2S → 2 H2O + 3S

In this reaction, which substance is oxidized and which one is reduced. 14. In the following reaction between lead sulphide and hydrogen peroxide

PbS(s) + 4H2O(aq) → PbSO4(s) + 4H2O(l)

a. Which substance is reduced ? b. Which substance is oxidized ? B. Short answer questions: (2 marks each) (2x4=8) 1. Write a chemical equation for each of the following reactions : a. Zinc metal reacts with aqueous solution of hydrochloric acid to form zinc chloride solution and hydrogen gas. b. When solid mercury (II) oxide is heated, liquid mercury and oxygen gas are produced.

2. H2 + O → H2O is a balanced chemical equation. It is considered to be incorrect. Why? 3.

Write balance chemical equations to represent the following statements : a. Carbon (coke) burns in air to form carbon dioxide gas. b. A piece of sodium metal on putting in water forms caustic soda and hydrogen gas. 3. Write the balanced chemical equations for the following chemical reactions : a. Aqueous solutions of sulphuric acid and sodium hydroxide react to form aqueous sodium sulphate and water. b. Phosphorus burns in chlorine gas to form phosphorus penta chloride. 4. Magnesium is more reactive than copper or iron but iron is more reactive than copper. a. A solution of magnesium sulphate is put into a copper pot. b. A solution of copper sulphate is put into a iron pot. c. A solution of magnesium sulphate is put into a iron pot. Which of the above pots will get damaged? Why? What is this reaction called?

C. Short answer questions: (3 marks each)(3x 4= 12) 1. What are the three essentials of a chemical equation ? 2. Explain the term reduction with examples. 3.A colourless lead salt, when heated, produces a yellow residue and brown fumes. a. Name the lead salt. b. Name the brown fumes. c. Write a chemical equation of the reaction involved. 4. When a green iron salt is heated strongly, its colour finally changes to black and colour of burning sulphur is given out. a. Name the iron salt. b. Name the type of reaction that takes place during the heating of iron salt. c. Write a chemical equation for the reaction involved. D. Balance the following skeleton equation by hit and trial method : (2x6=12)

a. C2H4 + O2 → CO2 +H2O

b. C2H2 + O2 → CO2 +H2O

c. CH4 + O2 → CO2 +H2O

d. Al2(SO4)3 + NaOH → Al(OH)3 + Na2SO4

e. Cu2S + O2 → Cu2O +S2O

f. AlN + H2O → Al(OH)3 + NH3

E Define: (1x9=9) 1. Rancidity 2. Combination reactions 3. Decomposition reactions 4. Double displacement reaction 5. Displacement reaction 6. Redox reaction 7. Corrosion 8. Oxidising agent 9. Reducing agent

Questions/ Answers 1. What happens when magnesium ribbon burns in air? Ans. When magnesium ribbon burns in air, it combines with the oxygen to form magnesium oxide.

2Mg(s) + O2(g)→ 2MgO(s)

2. Name the gas evolved when zinc reacts with dil. HCl. Ans. Hydrogen gas is evolved. 3. What is a chemical equation? Ans. A chemical equation is a symbolic notation that uses formulae instead of words to represent a chemical equation. 4. On what chemical law, balancing of chemical equation is based? Ans. Balancing of a chemical equation is based on the law of conservation of mass. 5. Represent decomposition of ferrous sulphate with the help of balanced chemical equation.

Ans. 2FeSO4(s)→ Fe2O3(s) + SO2(g) + SO3(g)

6. When carbon dioxide is passed through lime water, it turns milky, why? Ans. Lime water (calcium hydroxide) combines with carbon dioxide to form a suspension of

calcium carbonate which makes lime water milky. Ca(OH)2 + CO2→ CaCO3 + H2O

7. A zinc rod is left for nearly 20 minutes in a copper sulphate solution. What change would you observe in zinc rod? Ans. Zinc rod will change into zinc sulphate.

8. What type of reaction is this: Na2SO4 + BaCl2 → BaSO4 + 2NaCl

Ans. It is a double displacement reaction.

9. Identify the compound oxidized in the following reaction. H2S (g) + Cl2 → S(s) + 2HCl (g)

Ans. H2S is oxidized. 10. What is rust? Ans. Rust is mainly hydrated iron (III) oxide, Fe2O3.xH2O. 11. How does the food become rancid? Ans. Food becomes rancid when fat and oils present in the food are oxidized. 2-marks Questions/ Answers

Q.1. An iron knife kept dipped in a blue copper sulphate solution turns the blue solution light green. Why? Ans. As we know iron is more reactive than copper. So, it displaces Cu from CuSO4 solution and forms ferrous sulphate which is of Light Green Colour.

CuSO4 (aq) + Fe (s) → FeSO4(aq) + Cu(s)

Blue colour light green colour Q.2. A copper coin is kept in a solution of silver nitrate for some time. What will happen to the coin and the colour of the solution? Ans. We know that copper is more reactive than silver, so it will displace silver from its salt solution:

Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)

So the solution will turn blue due to the formation of copper nitrate. Q.3. What do you understand by precipitation reaction? Explain with suitable examples. Ans. Precipitate. When two reactants react and product formed remains insoluble and settles as a solid it is called a precipitate. Such reactions in which precipitate is formed are called precipitation reactions. For example,(i) when aqueous solution of sodium sulphate is mixed with the aqueous solution or barium chloride, barium sulphate comes in the form of white precipitate

Na2SO4(aq) + BaCl2(aq) → BaSO4(↓) + 2NaCl(aq)

(ii) When aqueous solution of sodium chloride is mixed with the aqueous solution of silver nitrate, silver chloride comes in the form of white precipitate. Q.4. What is lime-water test for the detection of carbon dioxide? Ans. When carbon dioxide gas is passed through lime water ,it turns milky due to the formation of milky suspension (precipitate) of calcium carbonate. Carbon dioxide is produced by the action of dilute HCl on sodium carbonate.

Na2CO3(s) + 2HCl(aq) → 2NaCl + H2O(l) + CO2

Carbon dioxide gas produced in this reaction is passed through lime water that changes to milky colour due to the formation of calcium carbonate.

Ca(OH)2(aq) + CO2(g) → Caco3 + CO2(g)

3 Marks Questions/ Answers Q.5. What is corrosion? State the conditions necessary for rusting of iron. How rusting is harmful? Ans: Corrosion. The process of eating away of the metal by the action of atmospheric reagents changing the metal into its compound is called corrosion. Rusting of Iron. When iron and iron objects are exposed to atmosphere, they are attacked by air and moisture (water) of the atmosphere and a brown and orange coloured layer is formed on the surface. It is called rust which is mainly hydrated iron (iii) oxide Fe2O3.xH2O. Harmful Effect of Rusting. Hydrated iron (iii) oxide is brittle substance and moves away from the surface thus the object is damaged. The objects get holes, cavities and rough surface. Conditions necessary for rusting : (i) Open surfaces of the metal. (ii) Presence of air (Oxygen). (iii) Presence of moisture (water). Q.6. What is rancidity? Write the common method to prevent it. Ans. When food item are kept unprotected for some time, they give some unpleasant smell and

taste and become rancid. This process is called rancidity. Actually, the micro organisms oxidise the fat and oils present in them. So oxidation of food items need to be prevented to protect them. Common methods to Prevent Rancidity of Food item: (i) Keeping the food at low temperature (ii) Keeping food item in air tight containers (iii) By filling nitrogen in the food storage bags. 5 Marks Questions/ Answers 1. a. Why cannot a chemical change be normally reversed? b. why is it always essential to balance a chemical equation? c. what happens when CO2 gas is passed through lime water and why does it disappear on passing excess CO2? d. Can rusting of iron takes place in distilled water? Ans: a. In a chemical change some bonds are broken and some bonds are formed. The products are quite different from the reactants. Therefore it normally can’t be reversed. b. A chemical equation has to be balanced to satisfy the law of conservation of mass. c. On passing CO2 gas through lime water, it turns milky due to formation of insoluble calcium carbonate which dissolves on passing excess CO2 due to formation of soluble calcium bicarbonate.

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

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

d. No, rusting of iron cannot take place in distilled water because it neither contains dissolved oxygen nor CO2 both are essential for rusting of iron. Hots Questions Q 1 . The marble statues often slowly get corroded when kept in open for a long time .Assign a suitable explanation Ans-SO2,NO2 gases are released into the atmosphere from various sources. These dissolve in rain water to give acid which corrodes marble statues

2SO2+O2 --------------→ 2SO3

H2O+SO3 --------------→ H2SO4

2NO2+H2O 2HNO3 CaCO3+H2SO4--------------→ CaSO 4 +H2O+CO2

CaCO3+2HNO3 --------------→Ca(NO3)2+H2O+CO2

Q 2. You are given the following materials (1) marble chips (2)dilute hydrochloric acid (3)Zinc granules ,identify the type of reaction when marble chips and Zinc granules are added separately to acid taken in two test tubes Ans-(i) marble chips react with dilute hydrochloric acid to form calcium chloride and carbon dioxide .it is a double displacement reaction

CaCO3+2HCl → CaCl2 + H2O +CO2

(ii)Zinc granules react with dilute hydrochloric acid to give hydrogen gas. it is a displacement reaction

Zn(s)+2HCl → ZnCl2(aq)+H2(g)

Q3. The gases hydrogen & chlorine do not react with each other even if kept together for a long time . However , in the presence of sunlight , they readily combine . What does actually happen ? Ans-In Chemical reactions , energy is needed to break the bonds present in the reacting molecules so that they may combine to form the products . In this reaction , sunlight is the source of energy in the form of photons . The energy made available by sunlight helps in breaking the bonds & this leads to chemical reaction between hydrogen & chlorine.

H2 (g) + Cl2(g) sunlight→ 2HCl (g)

Q 4. A,B&C are three elements which undergo chemical reactions in the following way A2O3 + 2B®B2O3 + 2A 3CSO4 + 2B -® B2(SO4)3 + 3C 3CO+

2A→A2O3 +3C

Answer the following (A) Which element is most reactive ? (B) Which element is least reactive ? Ans: a) The most reactive element is ‘B’. It has displaced both ‘A’ and ‘c’ from their compounds. b) The least reactive element is ‘C’ as it has been displaced by both ‘A’ and ‘B’ . Q. A water insoluble substance ‗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 turn blue litmus red . On bubbling the gas through lime water , it initially became milky and the milkiness disappeared when the gas was passed in excess . Identify the substance ‗X‘ and write the chemical equations of the reaction involved ANS: the water insoluble substance ‘X’ is most probably metal carbonate ( CaCO3 ). The chemical reaction that involved are given below

CaCO3 (s) + H2SO4 (aq) →CaSO4 ( aq) + H2O (aq) + CO2 (g)

Ca(OH)2 (s) + CO2 (g) → CaCO3 (s) ( milky) + H2O (l)

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

Extra score questions: Q 1. If you collect silver coins and copper coins. After some day a black coating on silver coins and a green coating on copper coins. Which chemical phenomenon is responsible for these coatings? Write the chemical name of black and green coatings Ans: Corrosion is responsible for this coating. Black coating is due to formation of Ag2S and green coating is due to formation of CuCO3.Cu (OH)2 Q.2. An aqueous solution of metal nitrate P reacts with sodium bromide solution to form yellow ppt of compound Q which is used in photography. Q on exposure to sunlight undergoes decomposition reaction to form metal present in P along with reddish brown gas. Identify P&Q . Write the chemical reaction & type of chemical reaction.

Ans: P = Ag NO3 , Q = AgBr 2 A gBr (s)→ 2Ag(s) + B r2(g) Photochemical

decomposition Q.3. A substance X used for coating iron articles is added to a blue solution of a reddish brown metal Y, the color of the solution gets discharged Identify X and Y & also the type of reaction. Ans. 6 X= Fe, Y = Cu, Displacement reaction. Q.4. A student burnt a metal A found in the form of ribbon. The ribbon burnt with a dazzling flame & a white powder B is formed which is basic in nature. Identify A &B. Write the balanced chemical equation. Ans.11 X = Mg, Y = MgO, Mg + O2® 2 MgO Q.5. why magnesium ribbon cleaned before burning? Ans: The magnesium ribbon is always found with a white layer of MgO due attack of moist air. So, magnesium ribbon cleaned before burning. Q.6. During electrolysis of water gas collected in one test tube is double than other why? Ans: On electrolysis water decompose into hydrogen and oxygen in ratio 2:1 by volume so,H2 gas collected in one test tube is double than O2

Evaluation Assignment Very short answer questions (V.S.A) : X Chemical reactions and Chemical Equation:FA 1. Arrange the following metals in the decreasing under of reactivity or electro positive character. K Zn Na Fe Pb Cu 2. Which of them will be a reducing agent a metal or non metal?

3. Balance the chemical equation Na(s) + H2O → NaOH + H2

4. Select the combination reaction from following

(i)KClO3→ KCl + 3 O2 (ii) S + O2→ SO2

5. Select the decomposition reaction from following

(i) 2H2O + O2 → 2H2 O

(ii) CaCO3 ------Heat------® CaO + CO2 6. Select the double displacement reaction (i) BaCl2 + H2SO4 ---® Ba SO4 + H2

(ii) 2KBv + Cl2 → K Cl2 + Bv2

7. Fill in the following about reactivity series of metal (a) the …………………. Metal replaces the ………………. Metal from its salt solution. 8. Which of the following is not the necessary condition for rusting (i) Presence of oxygen (ii) Presence of catalyst (iii) Presence of moisture 9. Which of the following is required for prevention of corrosion? (i) coating of less electropositive metal (ii) coating of move electropositive metal 10. Choose the correct about rancidity (i) Rancidity is a reduction process (ii) Rancidity is an oxidation process Answers 1. K > Na > Zn > Fe > Pb > Cu 2. Non metal

3. 2 Na(s) +2 H2O → 2NaOH + 2H2

4. S + O2→ SO2

5. CaCO3 ------Heat® CaO + CO2

6. BaCl2 + H2SO4→ Ba SO4 + H2

7. Strong, Weak 8. Presence of catalyst 9. Coating with move electropositive metal 10. Rancidity is an oxidation process See some questions: Short Answer type Question (2 marks) 1. (i) What happens when aqueous solution of barium chloride is added to aqueous sodium sulphate solution? (ii) When silver colored metal (A) is added to dilute sulphuric acid, solution (B) is formed which is colorless solution and a gas (c) is evolved which is colorless name ‘A’, ‘B’ and ‘C’ and write the equation? (iii) An ‘A’ blue colored crystalline salt on heating decomposes to ‘B’ white substance. W hen in blue salt solution, a metal (Zinc) is added, its gives rise to colorless solution ‘C’ wth leaving behind ‘D’ reddish brown metal. 2. Name the compounds ‘A’, ‘B’, ‘C’ and ‘D’ (iv) A reaction mixture of chromium Oxide is heated with aluminum powder on strong heating 3. a. Write the reaction? b. Give one application of this reaction?

(v) Define Rusting? 4. Give two methods of protection of metal trans rusting. 5. (i) Rusting is a slow electrochemical process in which hydrated oxide of metal in formed from metal when it comes in with air moisture (ii) By Painting, by galvanization, by tinning 6. (i) Explains the term ‘corrosion’? (ii) Write the chemical reaction of corrosion of Iron? (iii) Name two metals which corrodes easily? (iv) Aluminum corrodes in moist air but is widely used is making utensils. (v) what is galvanization and why do we use Zn for it?\ 7. (i) What is rancidity? Where does it happen (ii) what are anti oxidants? Why are they added fat and oil containing food to prevent from rancidity? (iii) why do most items puffed with nitrogen. Solution: 1. White precipitate of BaSO4 is obtained. Reaction will be as Na2SO4 + BACl2 ----------------BaSO4 + 2NaCl 2. A is Zinc, B is ZnSO4 solutions C is H2gas and reactions will be Zn (s) + H2SO4 (d) ------------Zn SO4 (aq) + H2 (g) 3. A, B, C, and D are respectively- CuSO4, 5 H2O, CuSO4, ZnSO4 (aq), Cu (s) 4. (i)Cr2O3 + 2Al ------Al2O3 + 2 Cr (ii) This reaction is used in termite welding 5. (i) The process of slow destruction of metals when exposed to air, moisture and pollutant gases , in called as corrosion. (ii) Fe + 3H2 + 2H2O ------------ Fe2O3 x H2O (iii) Formation of green coating on copper and formation of thin layer of oxide on aluminum are example of corrosion. (iv) Aluminum forms oxide layer on exposure to air, this oxide layer further checks corrosion. (v) Galvanization is a process in which Zinc layer is applied on Iron. It is used to protect iron from rusting. 6. (i) The oxidative deterioration of oils and fats leading to stinking smell or stale taste is known as rancidity. (ii) Anti Oxidants are those chemicals which an addition to fat and oil checks it from rancidityan addition of anti oxidants fatty food is protected from stale. (iii) Nitrogen is filled in potato chips & other food to create anaerobic (air free atmosphere) which avoids rancidity.

X Formative assessment Chemical reactions and equations Q1. The given equation represents the reaction of sodium metal with water. Sodium + Water Sodium hydroxide + Hydrogen Which of the following chemical equations represents a complete balanced chemical equation for the given word equation?

A. 2Na(s) + H2O → 2NaOH(aq) + H2(g)

B. Na(s) + 2H2O → NaOH(aq) +2H2(g)

C. 2Na(s) + 2H3O → 2NaOH(aq) + 2H2(g)

D. 2Na(s) + 2H2O → 2NaOH(aq) + H2(g)

Q2. Identify the chemical equation which represents a complete balanced equation for the

reaction of barium chloride with sodium sulphate to produce barium sulphate and sodium chloride.

A. BaCl2(aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl(aq)v

B. BaCl2(aq) + Na2SO4 (aq) → 2BaSO4(s) + 2NaCl(aq)

C. 2BaCl2(aq) + Na2 → SO4 (aq) 2BaSO4(s) + NaCl(aq)

D. BaCl2(aq) + Na2 → SO4 (aq) BaSO4(s) + NaCl(aq)

Q3. Ferric oxide reacts with aluminum to produce aluminum oxide and iron. The balanced chemical equation for the given reaction is

Fe2O3 + 2Al → Al2O3 + 2Fe

Which of the following substances is oxidized in the given reaction? A. Al2O3 B. Fe2O3 C. A1 D. Fe Q4. White silver chloride changes to grey in sunlight because of the formation of silver metal along with the evolution of chlorine gas. Identify the chemical equation that gives a correct representation of the process.

A. 2AgCl → 2Ag + Cl2

B. AgCl → Ag + Cl2

C. 2Ag + Cl2 → 2AgCl

D. Ag + Cl2 → 2AgCl

Q5. When copper oxide is treated with hydrogen gas copper is produced along with water. The

balanced chemical equation for the given reaction is CuO + H2 → Cu + H2O Which

substance is oxidized in the given reaction? A. H2 B. Cu C. H2O D. CuO Q6. The balanced chemical equation for the reaction of nitrogen with hydrogen to produce Ammonia gas is-

A. N2(g) + H2(g) → 2NH3(g)

B. 2N2(g) + H2(g) → 2NH3(g)

C. N2(g) + 3H2(g) → 2NH3(g)

D. 2N2(g) + H2(g) → 2NH3(g)

Q7. Identify the chemical equation that correctly represents production of magnesium nitrideby burning magnesium metal in a pure nitrogen atmosphere from the following–

A. Mg3N2(s)→ Mg(g) + N2(g) B. Mg + N2→ Mg3N2

C. 3Mg(g)N2(g) →Mg3N2(g) D. Mg3N2→ 3Mg + N2

Q8. Which of the following reactions represents a combination reaction?

A. CaO(s) + H2O(g)→ Ca(OH)2(aq) B. CaCO3(s)→ CaO(s) + CO2(g)

C. Zn + CuSO4(aq)→ ZnSO4(aq) + Cu(s) D. 2FeSO4(s)→ Fe2O3(s) +

SO2(g) + SO3(g) Q9. Which of the following reactions represents a double displacement reaction?

A. BaCl2(aq) + Na2SO4(aq)→ BaSO4(s) + 2NaCl(aq)

B. 2FeSO4(s)→ Fe2O3(s) + SO2(g) + SO3(g)

C. 2Pb(NO3)2(g)→ 2PbO(s) + 4NO2(g) + O2(g)

D. Zn(s) + 2AgNO3(aq)→Zn(NO3)2(aq) + 2Ag(s)

Q10. The reaction: Fe2O3(s) + 2Al(s)→Al2O3(s) + 2Fe(s) is an example of a–

A. combination reaction B. double displacement reaction

C. decomposition reaction D. displacement reaction Q10. What happens when dilute hydrochloric acid is added to iron fillings? A. Hydrogen gas and iron chloride are produced B. Chlorine gas and hydroxide are produced C. heat is absorbed, i.e. test tube becomes cold. D. Iron salt and water are produced Q. Write balanced chemical equations for the following word equations: A. Calcium hydroxide + Carbon dioxide Calcium carbonate + Water Skeletal equation:______________________________________ Balanced equation: ______________________________________ B. Zinc + Silver nitrate® Zinc nitrate + Silver Skeletal equation:______________________________________ Balanced equation:______________________________________

C. Aluminium + Copper chloride→ Aluminium chloride + Copper

Skeletal equation:______________________________________ Balanced equation:______________________________________ D. Potassium bromide (aq) + Barium iodide (aq ) Potassium iodide (aq) + Barium bromide(s) Skeletal equation:______________________________________ Balanced equation:______________________________________

E. Zinc carbonate(s)→ Zinc oxide(s) + Carbon dioxide(g)

Skeletal equation:______________________________________ Balanced equation:______________________________________

CLASS X SCIENCE

CHEMICAL REACTIONS AND EQUATIONS

1 – Mark questions answer

1. What happens when magnesium ribbon burns in air?

Ans. When magnesium ribbon burns in air, it combines with the oxygen to form magnesium oxide.

2Mg(s) + O2(g) 2MgO(s)

2. Name the gas evolved when zinc reacts with dil. HCl.

Ans. Hydrogen gas is evolved.

3. What is a chemical equation?

Ans. A chemical equation is a symbolic notation that uses formulae instead of words to represent a

chemical equation.

4. On what chemical law, balancing of chemical equation is based?

Ans. Balancing of a chemical equation is based on the law of conservation of mass.

5. Represent decomposition of ferrous sulphate with the help of balanced chemical equation.

Ans. 2FeSO4(s) Fe2O3(s) + SO2(g) + SO3(g)

6. When carbon dioxide is passed through lime water, it turns milky, why?

Ans. Lime water (calcium hydroxide) combines with carbon dioxide to form a suspension of calcium

carbonate which

makes lime water milky. Ca(OH)2(aq) + CO2(g) CaCO3( ) + H2O(l)

7. A zinc rod is left for nearly 20 minutes in a copper sulphate solution. What change would you

observe in zinc rod?

Ans. Zinc rod will change into zinc sulphate.

8. What type of reaction is this: Na2SO4 + BaCl2 BaSO4 +2NaCl

Ans. It is a double displacement reaction.

9. Identify the compound oxidized in the following reaction. H2S (g) + Cl2 S(s) + 2HCl (g)

Ans. H2S is oxidized.

10. What is rust?

Ans. Rust is mainly hydrated iron (III) oxide, Fe2O3.xH2O.

11. How does the food become rancid?

Ans. Food becomes rancid when fat and oils present in the food are oxidized.

2-marks Questions/ Answers

Q.1. An iron knife kept dipped in a blue copper sulphate solution turns the blue solution light green.

Why? [2]

Ans. As we know iron is more reactive than copper. So, it displaces Cu from CuSO4 solution and

forms ferrous

sulphate which is of Light Green Colour.

CuSO4 (aq)+Fe (s) → FeSO4(aq) + Cu(s)

Blue colour light green colour

Q.2. A copper coin is kept in a solution of silver nitrate for some time. What will happen to the coin

and the colour of the

solution? [2]

Ans: We know that copper is more reactive than silver,

so it will displace silver from its salt solution: Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq)+2Ag(s)

So the solution will turn blue due to the formation of copper nitrate.

Q.3. what do you understand by precipitation reaction? Explain with suitable examples. [2]

Ans. Precipitate. When two reactants react and product formed remains insoluble and settles as a

solid it is called a

precipitate. Such reactions in which precipitate is formed are called precipitation reactions. For

example,

(i) When aqueous solution of sodium sulphate is mixed with the aqueous solution or barium

chloride, barium sulphate

comes in the form of white precipitate

Na2SO4(aq)+BaCl2(aq) → BaSO4(↓)+2NaCl(aq)

(ii) When aqueous solution of sodium chloride is mixed with the aqueous solution of silver nitrate,

silver chloride comes

in the form of white precipitate.

Q.4. What is lime-water test for the detection of carbon dioxide? [2]

Ans. When carbon dioxide gas is passed through lime water, it turns milky due to the formation of

milky suspension

(precipitate) of calcium carbonate. Carbon dioxide is produced by the action of dilute HCl on sodium

carbonate.

Na2CO3(s)+2HCl(aq) →2NaCl+H2O(l)+CO2

Carbon dioxide gas produced in this reaction is passed through lime water it changes to milky colour

due to the

formation of calcium carbonate.

Ca(OH)2(aq)+CO2(g) → CacO3+ CO2(g)

3 Marks Questions/ Answers

Q.5. what is corrosion? State the conditions necessary for rusting of iron. How rusting is harmful? [3]

Ans: Corrosion. The process of eating away of the metal by the action of atmospheric reagents

changing the metal into

its compound is called corrosion.

Rusting of Iron. : When iron and iron objects are exposed to atmosphere, they are attacked by air

and moisture

(water) of the atmosphere and a brown and orange colored layer is formed on the surface. It is

called rust which is

mainly hydrated iron (iii) oxide Fe2O3.xH2O.

Harmful Effect of Rusting. : Hydrated iron (iii) oxide is brittle substance and moves away from the

surface thus the

object is damaged. The objects get holes, cavities and rough surface.

Conditions necessary for rusting:

(i) Open surfaces of the metal.

(ii) Presence of air (Oxygen).

(iii) Presence of moisture (water).

Q.6. what is rancidity? Write the common method to prevent it. [3]

Ans. When food item are kept unprotected for some time, they give some unpleasant smell and

taste and become

rancid. This process is called rancidity. Actually, the micro organisms oxidize the fat and oils present

in them. So,

oxidation of food items need to be prevented to protect them.

Common methods to Prevent Rancidity of Food item:

(i) Keeping the food at low temperature

(ii) Keeping food item in air tight containers

(iii) By filling nitrogen in the food storage bags.

Q.6. a. Why cannot a chemical change be normally reversed? b. why is it always essential to balance

a chemical

equation? c. what happens when CO2 gas is passed through lime water and why does it disappear

on passing excess

CO2? d. Can rusting of iron takes place in distilled water?

Ans: a. In a chemical change some bonds are broken and some bonds are formed. The products are

quite different

from the reactants. Therefore it normally can’t be reversed.

b. A chemical equation has to be balanced to satisfy the law of conservation of mass.

c. On passing CO2 gas through lime water, it turns milky due to formation of insoluble calcium

carbonate which

dissolves on passing excess CO2 due to formation of soluble calcium bicarbonate.

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

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

No, rusting of iron cannot take place in distilled water because it neither contains dissolved oxygen

nor CO2 both are

essential for rusting of iron.

HOTS QUESTIONS (Chemical Reactions and Equations)

Q. The marble statues often slowly get corroded when kept in open for a long time .Assign a suitable

explanation

Ans-SO2, NO2 gases are released into the atmosphere from various sources. These dissolve in rain

water to give acid

which corrodes marble statues

2SO2+O2 2SO3

H2O+SO3 H2SO4

2NO2+H2O 2HNO3

CaCO3+H2SO4→ CaSO4 +H2O+CO2

CaCO3+2HNO3 → Ca(NO3)2+H2O+CO2

Q. You are given the following materials (a) marble chips (b)dilute hydrochloric acid (c)Zinc granules

,identify the type

of reaction when marble chips and Zinc granules are added separately to acid taken in two test tubes

Ans-(a) marble chips react with dilute hydrochloric acid to form calcium chloride and carbon dioxide

.it is a double

displacement reaction

CaCO3+2HCl → CaCl2 + H2O +CO2

(b)Zinc granules react with dilute hydrochloric acid to give hydrogen gas. it is a displacement

reaction Zn(s)+2HCl →

ZnCl2(aq)+H2(g)

Q. The gases hydrogen & chlorine do not react with

each other even if kept together for a long time . However, in the presence of sunlight, they readily

combine. What

does actually happen? Ans-In Chemical reactions, energy is needed to break the bonds present in

the reacting

molecules so that they may combine to form the products. In this reaction, sunlight is the source of

energy in the form

of photons. The energy made available by sunlight helps in breaking the bonds & this leads to

chemical reaction

between hydrogen & chlorine. H2(g) + Cl2(g) sunlight→ 2HCl (g)

Q. A,B and C are three elements which undergo chemical reactions in the following way

A2O3 + 2B B2O3 + 2A

3CSO4 + 2B B2(SO4)3 + 3C

3CO+ 2A A2O3 +3C

Answer the following

(A) Which element is most reactive? (B) Which element is least reactive ?

Ans: a) The most reactive element is ‘B’. It has displaced both ‘A’ and ‘c’ from their compounds.

b) The least reactive element is ‘C’ as it has been displaced by both ‘A’ and ‘B’ .

Q. A water insoluble substance ‗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

turn blue litmus

red . On bubbling the gas through lime water, it initially became milky and the milkiness disappeared

when the gas was

passed in excess. Identify the substance ‗X‘ and write the chemical equations of the reaction

involved .

Ans: the water insoluble substance ‘X’ is most probably metal carbonate ( CaCO3 ). The chemical

reaction that

involved are given below

CaCO3 (s) + H2SO4 (aq) →CaSO4 ( aq) + H2O (aq) + CO2 (g)

Ca(OH)2 (s) + CO2 (g) → CaCO3 (s) + H2O (l) ( milky)

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

Q. 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?

Ans. Red litmus paper turned blue. Blue litmus paper remained blue.

This is because the magnesium ribbon on burning in air forms the white magnesium oxide. Which

dissolved in water, it

forms magnesium hydroxide, which is Basic in nature.

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

give you a new

compound.

Ans. O2 + 2SO2 2SO3

8NH3 + 3Cl2 6NH4Cl