CBSE CLASS 10 CHEMISTRY Metals and Non-Metals Solved Important questions

Metals and Non-Metals Solved Important questions

One mark questions with answers

Q1. Which one of the following elements symbolized as A and B is a metal : 11A23, 16B32?

Ans1.  A is a metal as it contains one electron in its outer most shell.

Q2. Name the constituents of baking powder.

Ans2. Tartaric acid and sodium bicarbonate.

Q3. Name the main constituents of common brass.

Ans3. Zn and Cu are the main constituents of common brass.

Q4. What are washing soda and baking soda chemically?

Ans 4. Chemical formula of washing soda is Na2CO3.10H2O (sodium carbonate) and  baking soda is NaHCO3 (sodium bicarbonate).

Q5. Why are stainless steel cooking pans often given a copper outer bottom?

Ans5. Stainless steel cooking pans are often given a copper outer bottom because it is good conductor of heat.

Q6. Write the equation for the chemical reaction that takes place when steam is passed over red hot silicon.

Ans6. When steam is passed over red hot silicon, silicon dioxide is formed and hydrogen gas liberated. Si (silicon) + 2H2O (steam) -------> SiO2 (silicon dioxide) + 2H2 (hydrogen)

Q7. Write the chemical equation to show the effect of heating on sodium carbonate crystals.

Ans7. When sodium carbonate crystals are heated water molecules are reduced. Na2CO3.10H2O ------->Na2CO3 + 10H2O.

Q8. Name one metal element and one non-metal element which are obtained on a large scale generally from sea water.

Ans8. Sodium and Chlorine.

Q9. An iron knife kept dipped in a blue copper sulphate solution changes to light green. Why?

Ans9. Iron displaces copper from copper sulphate solution and forms ferrous sulphate solution which is light green. Fe (iron) + CuSO4 (copper sulphate, green colour) --------> FeSO4 (ferrous sulphate, light green) + Cu (copper)

Q10. A given non-metal forms sulphuric acid on reacting with concentrated HNO3. Identify the non-metal.

Ans10. Sulphur is non-metal forms sulphuric acid on reacting with concentrated HNO3. S + 6HNO3 (conc. nitric acid) ------> H2SO4 + 6NO2 + 2H2O

Two mark questions with answers Q1. A trivalent metal X is manufactured by the process of electrolysis. It is the most abundant metal in the earth's crust. Identify the metal and state its two uses. Ans1. The metal is Aluminium (Al). Uses :  (i) Aluminium is used for making cooking utensils because it is good conductor of heat.  (ii) It is used for making silver paints. Q2. A white powder having an odour of chlorine is used to remove yellowness of white clothes in laundries. Name this powder. How is it prepared? Write the chemical equation for the reaction involved in its preparation. Ans2. White powder is bleaching powder (CaOCl2). It is prepared in the laboratory by passing the chlorine gas over dry slaked lime Ca(OH)2 (Calcium hydroxide 'slaked lime') + Cl2 (Chlorine)  -------à CaOCl2 Calcium oxychloride 'bleaching powder') + H2O (Water) Q3. Explain why zinc metal can displace copper from copper sulphate solution but copper cannot displace zinc from zinc sulphate solution. Ans3. Zinc is more reactive element than copper so zinc displaces copper from copper sulphate solution. Zn + CuSO4 (Copper sulphate) ------à  ZnSO4 + Cu Copper is less reactive than zinc so it can't displace the zinc sulphate solution. Q4. A non-metal belonging to group V in the periodic table has two major allotropic forms. Name its two forms. Identify the element. Write two distinctions between these two forms. Ans4. Non-metal is phosphorus (P). The allotropes of phosphorus are white phosphorus (yellow phosphorus) and red phosphorus. Distinctions : (i) White phosphorus is soft and red phosphorus is brittle powder. (ii) White phosphorus is poisonous while red phosphorus is non-poisonous. Q5. What are the products formed? When silicon reacts with hydrochloric acid? Write the necessary equation. Ans5. When silicon reacts with HCl it forms silicon tetrachloride and hydrogen gas is liberated. Si + 4HCl (Hydrochloric acid) ---àSiCl4 (Silicon tetrachloride) + 2H2 (Hydrogen gas). Q6. Describe the method of mining sulphur. Ans6. The sulphur is mined from underground by Frasch Process. This process is based that sulphur has low melting point (159ºC). A hole is bored to the sulphur beds and three concentric pipes are pushed to sulphur beds, through the outer most pipe super heated water is forced down which melts sulphur beds. Through the inner most pipe, air at high pressure is forced which pushes the molten sulphur through the middle pipe which is collected in pots.  Three mark questions with answers Q1. You are given a sample of an ore containing sulphide of copper along with a high percentage of unwanted materials. With the help of a diagram describe the process by which you would remove the unwanted materials from the given ore. Ans1. This ore is concentrated by Froth Flotation method, This method is used to concentrate the sulphide ores of copper and zinc. The crushed ore is put into a tank containing water and little pine oil. Compressed air is passed through the water. The froth that is produced because the pine oil rises to the surface and carries the ore particles with it. The gangue sinks to the bottom. Q2. An element has two allotropic forms A and B and belongs to group V of the periodic table. A is much darker in colour than B. Ignition temperature of A is 260ºC and that of B is 30ºC. What may A and B be? Which one of them reacts with (i) chlorine on heating but not in the cold, (ii) hot concentrated solution of Na OH producing a gas? Write the chemical equations for the reactions involved. Ans2. B is yellow phosphorus and A is red phosphorus. (i) Red phosphorus reacts and form phosphorus penta chloride. P4 (Red phosphorus) + 10Cl2 (Chlorine) --à 4PCl5 (Phosphorus penta chloride) (ii) White phosphorus reacts with hot concentrated solution of NaOH producing phosphine gas and form sodium hypophosphite P4 (White phosphorus) + 3NaOH (Sodium hydroxide) + 3H2O –--Heat--àPH3­ (Phosphine) + 3NaH2PO2 (Sodium hypophosphite). Q3. What is chemical name and formula of limestone? What happens when limestone is strongly heated? Write the chemical equation involved. How can the presence of excess of CO2 be tested in a class room from the products formed. Ans3. Lime stone is calcium carbonate (CaCO3) when limestone is strongly heated it decomposes and forms calcium oxide and carbondioxide. The presence of excess of CO2 in the class room is tested by  passing the air through lime water (CaO). It will change to milky due to the formation of a white precipitate of calcium carbonate. CaCO3 (Calcium carbonate or lime stone) CaO (Calcium oxide or lime water) + CO2 (Carbon dioxide) Q4. What is meant by allotropy? Name the two allotropes of phosphorus. Mention any three differences in their properties Ans4. Allotropy : "Existence of an element in two or more distinct forms which have different physical properties but same chemical properties is called allotropy and the forms are called allotropes." Two allotropes of phosphorus are white phosphorus and red phosphorus. Differences : Red phosphorus White phosphorus (i) It is stored in open atmosphere. (i) Its ignition temperature is very low 30ºC. So it is kept under water. (ii) It is non-poisonous. (ii) It is highly poisonous. (iii) Red phosphorus does not react with sodium hydroxide. (iii) White phosphorus reacts with sodium hydroxide.

NCERT solution Class X Science Acids, Bases and Salts

Class X Self Evaluation Tests For Science :Acids, Bases and Salts

Q.1: A solution turns red litmus blue; its pH is likely to be a) 1 b) 4 c) 5 d) 10 

Answer: (d)

Q.2: A solution reacts with crushed egg-cells to give a gas that turns lime water milky. The solution contains -

a) NaCl b) HCl c) LiCl d) KCl
Answer: (b)
Q.3: 10 ml of a solution of NaOH is found to be completely neutralized by 8 ml of a given solution of HCl. If we take 20 ml of the same solution of NaOH, the amount of HCL (the same solution as before) required to neutralize it will be - a) 4 ml b) 8 ml c) 12 ml d) 16 ml

Answer: (d)
Q.4: Which one of the following types of medicoines is used fr treating indigestion?
a) Antibiotic b) Analgesic c) Antacid d) Antiseptic

Answer: (c)
Q.6: Compounds such as alcohols and glucose also contain hydrogen but are not categorized as acids. Describe an activity to prove it.
Answer: The nails are fitted in a cork and are kept in a 100 ml beaker. Then these are connected to the two terminals of a 6 volt battery along with a bulb and a switch. Now some dilute HCl is poured in the beaker and current is allowed to pass through the circuit. The same experiment is carried out using glucose and alcohol solutions separately in place of HCl. 

Observations

The bulb glows in HCl solution only. The reason is that HCl solution dissociates into H+ and Cl– ions which help to conduct electricity while glucose and alcohol solutions do not dissociate into ions and so, do not conduct electricity. That is why we find the bulb glowing in HCl solution only.

Conclusions
The above activity proves that all acids contain hydrogen but not all compounds containing hydrogen are acids. Due to this reason compounds such as alcohols and glucose although contain hydrogen, but they are not categorized as acids.
Q.7: Why does distilled water not conduct electricity, whereas rain water does?
Answer: Distilled water which is a pure form of water, is neither acidic nor basic in nature. So distilled water does not dissociate into ions. Since, conduction of electricity requires free ions so, distilled water does not conduct electricity.

Rain water being an impure form of water, contains many ionic species. These ions are responsible for electrical conductivity of rain water.
Q.8: Why do acids not show acidic behavior in the absence of water?
Answer: In absence of water acids do not dissociate into ions and hence, they do not show acidic behaviour.
Q.9: Five solutions A, B, C, D, and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is:
a) Neutral? b) Strongly alkaline? c) Strongly acidic?  d) Weakly acidic? e) Weakly alkaline?
Arrange the pH in increasing order of hydrogen ion concentration.
Ans: (a) D (b) C (c) B (d) A (e) E.   pH in increasing order of H+ ions concentration:
C (11) < E (9) < D (7) < A (4) < B (1).

CBSE Acid base and salt Short type Questions with solution for 10

Chemistry Adda  VERY SHORT QUESTIONS ANSWER

Q.1. Name two natural indicators.

Ans. Litmus solution, beet-root extract and turmeric solution.

Q.2. Name the acid present in vinegar and lemon.

Ans. In vinegar acetic acid is present and in lemon it is citric acid.

 Q.3. What is an Alkali. 

Ans. The base that dissolves in water is called alkali.

Q.4. What is the colour of methyl orange in acidic solution.

Ans. The colour of methyl orange is pink in acidic solution.

Q.5. In a solution phenolphthalein turns pink. What is the nature or solution?

Ans. The solution is basic.

Q.6. Hydrogen ion concentration of an acid is 1×10-2 mol/L, what is its pH.

Ans. Its pH is 2.

Q.7. pH of one solution is 7, is it acidic basic or neutral.

Ans. Solution with pH no 7 is neutral.

Q.8.Which has a higher pH value, 1M HCl or 1M NaOH

solution. Ans. 1M NaOH solution has higher pH value.

Q.9. Name two strong and two weak acids.

Ans. Strong acid: Hydrochloric acid and Nitric acid. Weak acid: Acetic acid and carbonic acid.

 Q.10. Name the gas formed when sodium hydroxide reacts with zinc.

Ans. Hydrogen gas is formed when NaOH reacts with zinc.

Q.11. What is bleaching powder?

Ans. Bleaching powder is a mixture of CaOCl2 .Ca(OH)2 .CaCl2 .2H2O.

Q.12. Write the chemical formula for washing soda.

Ans. Chemical formula for washing soda is Na2CO3.10H2O.

Q.13. Write the chemical name of baking soda.

 Ans. Its chemical name is sodium hydrogen carbonate.

Q.14. Write one domestic use of sodium carbonate. 

Ans. It is used as a cleansing agent.

Q.15. What happens when gypsum is heated at 373 K.

Ans. When gypsum is heated at 373 K .it changes to plaster of paris. 

CaSO4. 2H2O → CaSo4. ½ H2O +1 ½ H2O 

Gypsum                              Plaster of Paris

Q.16. Doctors use paste of a white substance in water to maintain a 
fractured bone in its place. Identify the substance and write its chemical formula.

Ans. Name of the compound: Plaster of Paris.

ncert solution metal and non metal for class 10

NCERT  solution metal and non metal for class 10
Page 40
Question 1:Give an example of a metal which
(i) is a liquid at room temperature.(ii) can be easily cut with a knife.
(iii) is the best conductor of heat. (iv) is a poor conductor of heat.

Ans:(i) Metal that exists in liquid state at room temperature → Mercury
(ii) Metal that can be easily cut with a knife → Sodium
(iii) Metal that is the best conductor of heat → Silver
(iv) Metals that are poor conductors of heat → Mercury and lead

Question 2: Explain the meanings of malleable and ductile.

Ans: Malleable: Substances that can be beaten into thin sheets are called malleable. For example - most of the metals like gold are malleable.
Ductile: Substances that can be drawn into thin wires are called ductile. For example -most of the metals like copper and aluminium are ductile.

Page 46
Question 1: Why is sodium kept immersed in kerosene oil?

Answer 1:Sodium is a very reactive metal. It  explosively react  with air as well as water. Hence, it catches fire if kept in open at room temparature. Therefore, to prevent accidental fires and accidents, sodium is stored immersed in kerosene oil.

Q2:Write equations for the reactions of:  (i) iron with steam  (ii) calcium and potassium with water
Ans:     (i) 3 Fe (s) + 4 H2O (g) → Fe3O4 (aq)+ 4 H2( g)
           (ii) Ca(s) + 2 H2O(l) → Ca(OH)2 (aq) + H2 (g) + Heat
             2 K (s) + 2 H2O(l) → 2 KOH (aq) + H2 (g) + Heat

Q3: Samples of four metals A, B, C and D were taken and added to the following solution one by one. The results obtained have been tabulated as follows. Metal  Iron (II) sulphate   Copper (II) sulphate  Zinc sulphate  Silver  nitrate
A No reaction Displacement    
B Displacement   No reaction  
C No reaction No reaction No reaction Displacement
D No reaction No reaction No reaction No reaction

Use the table above to answer the following questions about metals A, B, C and D.
(i) Which is the most reactive metal?
(ii) What would you observe if B is added to a solution of copper (II) sulphate?
(iii) Arrange the metals A, B, C and D in the order of decreasing reactivity.

Ans: (i) B is the most reactive metal.
       (ii) If B is added to a solution of copper (II) sulphate, then it would displace copper.
       B + CuSO4 → Displacement
       (iii) The arrangement of the metals in the order of decreasing reactivity is:
           B > A > C > D
          A + FeSO4 → No reaction, A is less reactive than iron
          A + CuSO4 → Displacement,  A is more reactive than copper
          B + FeSO4 → Displacement,  B is more reactive than iron
         B + ZnSO4 → No reaction,  B is less reactive than zinc
        C + FeSO4 → No reaction,  C is less reactive than iron
        C + CuSO4 → No reaction, C is less reactive than copper
        C + ZnSO4 → No reaction, , C is less reactive than zinc
        C + AgNO3 → Displacement, C is more reactive than silver
        D + FeSO4 / CuSO4 / ZnSO4 / AgNO3 → No reaction, D is less reactive than iron, copper, zinc,      
        and silver

Q 4: Which gas is produced when dilute hydrochloric acid is added to a reactive metal? Write the chemical reaction when iron reacts with dilute H2SO4.


Ans: Hydrogen gas is evolved when dilute hydrochloric acid is added to a reactive metal. When iron reacts with dilute H2SO4, iron (II) sulphate with the evolution of hydrogen gas is formed.

Fe (s) + H2SO4 (aq) → FeSO4 (aq) + H2 (g)

Q 5: What would you observe when zinc is added to a solution of iron (II) sulphate? Write the chemical reaction that takes place.
Ans:  Zinc is more reactive than iron. Therefore, if zinc is added to a solution of iron (II) sulphate, then it would displace iron from the solution.
Zn (s) + FeSO4 (aq) → ZnSO4 (aq) + Fe (s)
Concept insight:  For answering this question recall the reaction between zinc is added to a solution of iron (II) sulphate.

NCERT SOLUTION 10th chemical Reaction and Equations

NCERT Science Textbook In-Text Questions 

Q.1: Why should magnesium ribbon be cleaned before burning in air ?
Ans: Magnesium ribbon is a very reactive metal. When stored it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer of magnesium oxide being a stable compound prevents further reaction of magnesium with oxygen. The magnesium is cleaned before burning in air to remove this layer so that the metal can be exposed to air properly. 
Q.2: Write the balanced equation for the following chemical reactions:
(i) Hydrogen + Chlorine --> Hydrogen chloride
(ii) Sodium + Water --> Sodium hydroxide + Hydrogen
Ans:
(i) H2 (g) + Cl2 (g) --> 2HCl (g)
(ii) 2Na (s) + 2H2O (liq) --> 2NaOH (aq) + H2 (g).
Q.3: Write a balanced chemical equation with state symbols for the following chemical reactions:
(i) Solution of barium chloride and sodium sulphate in water to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) react with hydrochloric acid solution (in water) to produce sodium chloride and water. 
Ans:           (i) BaCl2 (aq) + Na2SO4 (aq) --> BaSO4 (s) + 2NaCl (aq)
                     (ii) NaOH (aq) + HCl (aq) --> NaCl (aq) + H2O (liq)
Q.1: A reaction of a substance X is used for white washing.
(i) Name the substance X and write its formula.
(ii) Write the reaction of the substance X named in (i) above with water.
Ans:
(i) The substance X is lime which is used for white washing. Its formula is CaO.
(ii) CaO + H2O --> Ca(OH)2 + Heat
Q.2: Why is the amount of gas collected in one of the test tubes in Activity 1.7, double of the amount collected in the other ? Name this gas. 
Ans: water contains two parts of hydrogen and one part oxygen. Therefore, during the electrolysis of water the amount of hydrogen gas collected in one of the test tubes is double than that of the oxygen produced and collected in the other test tube. 
(Page 13)
Q.1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it ? 
Ans: Iron is more reactive than copper. So, when an iron nail is dipped in a copper sulphate solution, iron displaces copper from its solution to form iron sulphate, which is green in color.
Fe (s) + CuSO4 (aq) --> FeSO4 (aq) + Cu (s) 
Hence, the blue color of copper sulphate solution changes into green color because of this displacement reaction. 
Q.2: Give an example of the double displacement reaction other than the one given in Activity 1.10.
Ans: When lead (II) nitrate is mixed with potassium iodide, potassium nitrate and lead iodide are formed.
Pb(NO3)2 + 2KI --> 2KNO3 + PbI2
Na2CO3 (aq) + CaCl2 (aq) --> CaCO3 (s) + 2NaCl (aq)
Here sodium carbonate and calcium chloride exchange ions to form two new compounds.
Q.3: Identify the substances that are oxidized and the substances that are reduced in the following reactions:
(i) 4Na(s) + O2(g) --> 2Na2O(s)
(ii) CuO(s) + H2(g) --> Cu(s) + H2O(liq)
Ans:
(i) Sodium (Na) is oxidized to Na2O.
(ii) CuO (Copper oxide) is reduced to Cu, while H2 gas is oxidized to H2O
Solutions of Science Textbook Exercise Questions
Q.1: Which of the statements about the reaction below are incorrect?
2PbO + C ---> 2Pb + CO2­
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidized.
(c) Carbon is getting oxidized.
(d) Lead oxide is getting reduced.
(i) a and b (ii) a and c (iii) a, b and c (iv) all.
Ans: (i) a and b.
Q.2: Fe2O3 + 2Al ---> Al2O3 + 2Fe
The above reaction is an example of a
(a) combination reaction     (b) double displacement reaction
(c) decomposition reaction (d) displacement reaction.Ans: (d)
Q.3: What happens when dilute HCl is added to iron filings ? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced. Ans: (a)
Q.4: What is a balanced chemical equation ? Why should chemical equation be balanced ?
Ans: An equation having an equal number of atoms of each element on both the sides is called a balanced chemical equation.
According to this law, mass can neither be created nor destroyed. In other words, the total mass of reactants should be equal to the total mass of the products in a chemical equation. Therefore, the total number atoms of each element participating in the reaction must be equal on both sides of the chemical equation i.e. before and after reaction. Hence, it is for the above reason that a chemical equation should be balanced.
Q.5: Translate the following statements into balanced chemical equations.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

(d) Potassium metal reacts with water to give potassium hydroxide andhydrogen gas.
Ans:
(a) N2 + 3H2 ---> 2NH3
(b) 2H2S + 3O2 ---> 2H2O + 2SO2
(c) 3BaCl2 + Al2(SO4)3 ---> 3BaSO4 + 2AlCl3
(d) 2K + 2H2O ---> 2KOH + H2
Q.6: Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 ---> Ca(NO3)2 + H2O
(b) NaOH + H2SO4 ---> Na2SO4 + H2O
(c) NaCl + AgNO3 ---> AgCl + NaNO3
(d) BaCl2 + H2SO4 ---> BaSO4 + HCl
Ans:
(a) 2HNO3 + Ca(OH)2 ---> Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 ---> Na2SO4 + 2H2O
(c) NaCl + AgNO3 ---> AgCl + NaNO3
(d) BaCl2 + H2SO4 ---> BaSO4 + 2HCl
Q.7: Write balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide ---> Calcium carbonate + Water
(b) Zinc + Silver Nitrate ---> Zinc nitrate + Silver
(c) Aluminium + Copper chloride ---> Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate --->. Barium sulphate + Potassium chloride
Ans:
(a) Ca(OH)2 + CO2 ---> CaCO3 + H2O
(b) Zn + 2AgNO3 ---> Zn(NO3)2 + 2Ag
(c) 2Al + 3CuCl2 ---> 2AlCl3 + 3Cu
(d) BaCl2 + K2SO4 ---> BaSO4 + 2KCl
Q.8: Write the balanced chemical equations for the following and identify the type of reaction in each case.
(a) Potassium bromide + Barium iodide ---> Potassium iodide + Barium bromide
(b) Zinc carbonate ---> Zinc oxide + Carbon dioxide
(c) Hydrogen + Chlorine ---> Hydrogen chloride
(d) Magnesium + Hydrochloric acid ---> Magnesium chloride + Hydrogen
Ans:
(a) 2KBr + BaI2 ---> 2KI + BaBr2
(b) ZnCO3 ---> ZnO + CO2
(c) H2 + Cl2 ---> 2HCl
(d) Mg + 2HCl ---> MgCl2 + H2
Q.9: What does one mean by exothermic and endothermic reactions? Give examples.
Ans:Exothermic reactions: Those reactions in which energy is released in the form of heat are called exothermic reactions. Examples -
(1) All combustion reactions e.g.  CH4 + 2O2 ---> CO2 + 2H2O + Heat
(2) Thermite reactions e.g.           2Al + Fe2O3 ---> 2Fe + Al2O3 + Heat
Combinations are generally exothermic in nature. The decomposition of organic matters into compost is an example of exothermic reaction.
Endothermic reactions: Those reactions in which energy is absorbed are called endothermic reactions. Examples CaCO3 ------> Cao + O2

also, the reaction of photosynthesis -

6CO2 + 6H20  (SUNLIGHT AND CHLOROPHYL)--->C6H12O6+6O2

Q.10: Why is respiration considered an exothermic reaction? Explain.
Ans: For the sustenance of life we require energy. We obtain this energy from our food. In the process of digestion, food molecules get converted into simpler substances such as, glucose. Then glucose combine with oxygen and provide energy to our body. The whole process is known as respiration. As because, energy is released during the process of respiration, so it is considered an exothermic reaction. For example,
C6H12O6 + 6O2 ---> 6CO2 + 6H2O + energy
Q.11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Ans: Decomposition reactions are those reactions in which a compound is broken into two or more new compounds.

                   Heat

CaCO3 -----------------> Cao + O2

Combination reactions are those reactions in which two substances are combined to form a new substance.          

CaO + CO₂ ---> CaCO₃ + energy

In above examples, both the reactions are same but show opposite directions, so decomposition reaction is called the opposite of combination reaction.

Q.12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.  

Ans:

(a) Decomposition by heat or thermal energy

                   Heat

CaCO3 -----------------> Cao + O2

(b) Decomposition by light or photolysis

2AgCl ---> 2Ag + Cl₂ (in presence of light)

(c) Decomposition by electricity or electrolysis

               electricity

2H2O------------------>2H2 +O2

Q.13: What is the difference between the displacement and double displacement reactions? Write equations for these reactions.  

Ans: In displacement reaction, one element from its salt is displaced by a more reactive element e.g. in following example, Cu is displaced by Zn from CuSO₄ because Zn is more reactive.

CuSO₄ + Zn ---> ZnSO₄ + Cu  

In double displacement reaction, exchange of ions takes place between two reactants to form new products.

Na₂SO₄ + BaCl₂ ---> BaSO₄ + 2NaCl

Q.14: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.   

Ans: When copper is mixed in silver nitrate solution, it displaces the silver because copper is more reactive than silver.

2AgNO₃ + Cu ---> Cu(NO₃)₂ + 2Ag

Q.15: What do you mean by a precipitation reaction? Explain by giving examples.

Ans: The reactions in which a precipitate is formed are called precipitation reactions. Examples:

1. Na₂SO₄ + BaCl₂ ---> BaSO₄ + 2NaCl

In this reaction, BaSO4 is obtained as precipitate.

2. Na₂CO₃ + CaCl₂ ---> CaCO₃ + 2NaCl

Here calcium carbonate is obtained as precipitate.

Q.16: Explain the following in terms of gain or loss of oxygen with two examples each.

          (a).Oxidation

          (b).Reduction

Ans:

(a) Oxidation: The reactions in which gain of oxygen takes place are called oxidation. Examples -

1. 2Cu + O₂ ---> 2CuO

2. 2H₂ + O₂ ---> 2H₂O

(b) Reduction: The reactions in which loss of oxygen takes place are called reduction. Examples -  

1. ZnO + C ---> Zn + CO

2. CuO + H2 ---> Cu + H₂O

Q.17: A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Ans: The shinny brown colored element ‘X’ is copper. When it is heated in air, it becomes black due to the deposit of copper oxide. 2Cu + O2 ----------------heat--> 2CuO

Q.18: Why do we apply paint on iron articles ?

Ans: We apply paint on iron articles to prevent them from corrosion. Paint prevents iron from coming in contact with air and moisture. Thus, painting prevents rusting of iron articles.  

Q.19: Oil and fat containing food items are flushed with nitrogen. Why ?

Ans: Nitrogen being an inert gas does not react easily with the oil and fat present in the food substances. But oxygen reacts easily with the food substances and makes them rancid.  It is because of this reason that food items containing oil and fat are flushed with nitrogen gas before their packingto remove oxygen present in the pack.

Q.20: Explain the following terms with an example each:

(a) Corrosion (b) Rancidity

Ans:

(a) Corrosion: Corrosion is a process in which materials deteriorate as a result of a chemical reaction with moisture and other chemicals present in its surroundings.  Generally, metals get corroded more easily. For example, iron in the presence of moisture, reacts with oxygen  (air) to form hydrated iron oxide also called Rust.

4Fe + 3O₂ +nH₂O ---> 2Fe₂O₃.nH₂O

(b) Rancidity: It is a process of oxidation in which fats and oils present in food items are oxidized that can be easily noticed by the change in their taste and smell.

Rancidity can be prevented by:

1. Storing food in refrigerators or in air-tight containers.

2. Flushing oil and fat containing food items with nitrogen before their packing.

3. By adding anti-oxidant