Q&A Exam class 10 Science

1 and 2 mark questions:

Q1. If an object is placed in front of a concave mirror of 20 cm focal length, what is the position of the object when it will give a real and diminished image?

Q2. An object is placed in front of a concave mirror such that its image also formed at the same place at 10 cm in front of the mirror. What is the focal length of the mirror?

Q3. We wish to obtain an erect image of an object using a concave mirror of focal length 15 cm. 

a. What should be the range of distance of the object from the mirror?

b. What is the nature of the image?

Q4. An object is placed in front of a concave mirror such that its image also formed at the same place at 10 cm in front of the mirror. What is the focal length of the mirror?

Q5. At what distance from a concave mirror of focal length 20 cm, should an object 1.5 cm long be placed in order to get an erect image 4.5 cm tall?

Q6. Between which two points of concave mirror should an object be placed to obtain a magnification of m = - 2.

Q7. Why does a concave mirror cannot be used as a rear view mirror in vehicles?

Q8. Two spherical mirrors A and B produce images with linear magnifications + 1.5 and + 0.5 respectively. Identify which one of them is a a convex mirror and why?

Q9. State two effects caused by the refraction of light?

Q10. The refractive index of water with respect to air is 4/3. What is the refractive index of air with respect to water?

Q11. The refractive indices of kerosene, turpentine and water are 1.44, 1.47 and 1.33 respectively. In which of these materials does light travel fastest?

Q12. A virtual, erect and magnified image of an object is to be obtained with a convex lens. For this purpose, where should the object be placed?

Q13. If an object is placed at the centre of curvature in front of a concave lens, what kind of image it will produce?

Q14. A spherical lens has a power of, - 2.5 D. What is the focal length and nature of the lens?

Q15. Explain the spectrum of white light. 

Q16. While passing through a prism, which colour deviates least ?

Q17. Why do the stars always appear to be twinkling?

Q18. What is tyndall effect?

Q19. Why does the sky always appear to be blue?

Q20. Name any two effects of atmospheric refraction.

4 mark questions:

Q21. If the magnification of a mirror is +4,

a. What type of mirror is this?

b. If the focal length of the mirror is 5 cm, what is the the object distance?                 1 + 3

Q22. Draw a diagram to show the refraction of light through a glass prism. On this diagram, mark (i) incident ray, (ii) emergent ray, and (iii) angle of deviation.                             2½ + 1½

Q23.  A virtual image at a distance of 20 cm in front of the lens is produced when an object is placed 100 cm from the lens. Calculate (i) focal length of the lens and (ii) magnification produced. (iii) Also calculate the power of the lens.                                                2 + 1 + 1

Q24. An object is placed at a distance of 6 cm from a convex lens of focal length 4 cm. If the the height of the object is 5 cm, find

(i) position, (ii) nature and (iii) height of the image.                                           2 + 1 + 1

Q25. An object is placed at a distance of 20 cm from a convex mirror of radius of curvature 60 cm. If the the height of the object is 3 cm, find (i) position, (ii) nature and (iii) height of the image.                                          2 + 1 + 1

Q26. A real image of 6 cm height is produced when an object of 2 cm height is placed at a distance of 16 cm from a concave mirror. 

(i) what is the focal length of the mirror?

(ii) find the position of the image?         2 + 2 

LECTURE 3 : TYPES OF CHEMICAL REACTIONS (CLASS X)

TYPES OF CHEMICAL REACTIONS

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There are many types of Chemical reaction. Some of the most important types of Chemical Reaction are:

1. Combination Reaction

2. Decomposition Reaction

3. Displacement Reaction

4. Double Displacement Reaction

5. Oxidation and Reduction Reaction

A. COMBINATION REACTION

Those reactions in which two or more substances combine to form a single substance, are called combination reaction.

Examples: 

1. Burning of Magnesium in air.

2. Formation of water from hydrogen and oxygen.

3. Burning of carbon to form carbon dioxide.

4. Reaction between hydrogen and chlorine to form hydrochloric acid. 

5. Sodium metal burns in chlorine to form sodium chloride.

6. When iron powder is heated with sulphur, iron sulphide is formed. 

7. Calcium oxide reacts vigorously with water to form calcium hydroxide.  

8. Ammonia reacts with hydrogen chloride to form ammonium chloride. 

9. Carbon monoxide reacts with oxygen to form carbon dioxide. 

10. Sulphur dioxide reacts with oxygen to produce sulphur trioxide. 

B. DECOMPOSITION REACTIONS

Those reactions in which a compound splits up into two or more simpler substances are known as decompostion reactions. It always needs energy either in form of heat, light or electricity to carry out a decompostion reaction. 

When a decompostion reaction is carried out by heating, it is called "thermal decomposition". 

Examples. 

1. When calcium carbonate (CaCO₃) is heated, it decomposes to give calcium oxide (CaO) and carbon dioxide (CO₂). (Thermal decomposition)

2. When Potassium chlorate (KClO₃)

is heated in the presence of manganese dioxide catalyst, it decomposes to give potassium chloride (KCl) and oxygen (O₂).  (Thermal decomposition)

3. When ferrous sulphate (green) (FeSO₄) is heated strongly, it decomposes to form ferric oxide (brown) (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃).  (Thermal decomposition)

4. When colourless lead nitrate Pb(NO₃)₂ is heated strongly, it breaks down to form simpler substances like yellow lead monoxide PbO, Nitrogen dioxide NO₂ and oxygen O₂.  (Thermal decomposition)

5. When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas. (Decomposition using electricity)

6. When electric current is passed through molten sodium chloride, it decomposes to give sodium metal and chlorine gas.  (Decomposition using electricity)

7. When electric current is passed through molten aluminium oxide Al₂O₃, it decomposes to give aluminium metal and oxygen gas.  (Decomposition using electricity)

8. When Silver Chloride is exposed to light, it decomposes to form silver metal and chlorine gas. (Decomposition using light energy)

9. When silver bromide (pale yellow) (AgBr) is exposed to light, it decomposes into silver metal (greyish white) and Bromine gas. (Decomposition using light energy). 

Uses of Decomposition Reactions

The decomposition reactions using electricity are used to extract several metals like Aluminium, Sodium from their chlorides or oxides. 

Like when fused or molten metal chlorides or oxides is decomposed by passing electricity, metals are produced in the Cathode (negative electrode). 

Decomposition Reactions in our body

The digestion of food in our body is an example of decomposition reaction. Like complex carbohydrates (starch) decomposes into simple sugar and proteins into amino acids. 

DISPLACEMENT REACTIONS 

Those reactions in which one element takes place of another element in a compound, are known as displacement reactions. 

A more reactive elements always displaces a less reactive element from its compound. 

Metal Reactivity Series

(How to remember reactivity series)

1.  Please.      Potassium

2.  Stop.          Sodium

3.  Calling.      Calcium

4.  Me.             Magnesium

5.  A.                Aluminium

6.  Careless.   Carbon

7.  Zebra.         Zinc

8.  Instead.      Iron

9.  Try.              Tin

10. Learning.   Lead

11. How.          Hydrogen

12. Cow           Copper

13. Saves.       Silver

14. Goat.         Gold

1. As Zinc is more reactive than Copper, When a strip of Zinc metal is placed in Copper sulphate solution, then Zinc displaces Copper and Zinc Sulphate solution and Copper are obtained. 

CuSO₄ (aq) (blue solution) + Zn (s) (silvery white) → ZnSO₄ (aq) (colourless solution) + Cu (s) (red brown)

2. When a piece of Magnesium metal is placed in copper sulphate solution and Copper metal are formed. 

CuSO₄ (aq) (blue solution) + Mg (s) → MgSO₄ (aq) (colourless solution) + Cu (s) (red brown)

As Magnesium is more reactive than Copper.

3. When a piece of Iron metal is placed in Copper Sulphate solution, then Iron (II) Sulphate solution and Copper metal are formed. 

CuSO₄ (aq) (blue solution) + Fe (s) → FeSO₄ (aq) (greenish solution) +  Cu (s) (red brown)

As Iron is more reactive than Copper.

4. When a strip of lead metal is placed in a solution of copper chloride, then lead chloride solution and copper metal are formed. 

CuCl₂ (aq) (green solution) +  Pb (s) → PbCl₂ (aq) (Lead Chloride) (colourless solution) + Cu (s) (red brown)

As Lead (Pb) is more reactive than Copper (Cu)

5. When a copper strip is placed in a solution of silver nitrate, then copper nitrate solution and silver metal are formed. 

2AgNO₃ (aq) (colourless solution) + Cu (s) (red brown) → Cu(NO₃)₂ (aq) (blue solution) + 2Ag (s) (Greyish white)

As Copper (Cu) is more reactive than Silver (Ag)

6. Iron metal reacts with dilute hydrochloric acid to form iron(II) chloride and hydrogen gas. 

Fe (s) + 2HCl (aq) → FeCl₂ (aq) + H₂ (g)

As Iron is more reactive than Hydrogen.

7. Magnesium metal reacts with dilute hydrochloric acid to form magnesium chloride and hydrogen gas. 

Mg (s) + 2HCl (aq) → MgCl₂ (aq) + H₂ (g)

As Magnesium is more reactive than Hydrogen.

8. Sodium metal reacts with water to form sodium hydroxide solution and hydrogen gas. 

2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g)

As Sodium is more reactive than Hydrogen. 

9. Chlorine gas reacts with potassium iodide solution to form potassium chloride and iodine. 

Cl₂ (g) + 2KI (aq) → 2KCl (aq) + I₂ (s)

Chlorine displaces iodine from potassium iodide. 

10. When copper oxide is heated with magnesium powder, then magnesium oxide and copper are formed. 

CuO (s) + Mg (s) → MgO (s) + Cu (s)

Magnesium displaces Copper from from its oxide. 

12. When iron (III) oxide is heated with Aluminium powder, then aluminium oxide and iron metal is formed. The excessive heat produced during the reaction makes the iron to be melted. 

Fe₂O₃ (s) + 2Al (s) → Al₂O₃ (s) + 2Fe (l) (molten iron)