Given 
(a) Position of candle = 12.0 cm 
Position of convex lens = 50.0 cm 
Position of the screen = 88.0 cm 
Therefore u = 50.0 - 12.0 = 38.0 cm 
v = 88.0-50.0 = 38.0 cm 
Therefore u = v 
Hence both lie at 2F, 
therefore F = 38/2 = 19 cm. 
(b) u = 50.0-31.0 = -19 cm 
f = +19 cm, v = ?
Using \(\frac { 1 }{ f } =\frac { 1 }{ u } +\frac { 1 }{ v } \)
We have \(\frac { 1 }{ 19 } =\frac { -1 }{ 19 } +\frac { 1 }{ v } \)
or v=infinity.
(c) If he further shifts the candle towards the lens the image will become virtual and erect. 
(d) The ray diagram is as shown

Given, image distance, v = + 50 cm
Magnification, m = -1  [\(\because\)image is inverted]
\(m=\frac{v}{u}\Rightarrow u=\frac{v}{m}=\frac{50}{-1}=-50 \quad cm\)
So, needle is placed at 50 cm in front to the lens.
By lens formula, \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\Rightarrow \frac{1}{f}=\frac{1}{50}-\frac{1}{(-50)}\)
\(\frac{1}{f}=\frac{1}{25}\Rightarrow f=25\quad cm = 0.25 \quad m\)
\(\therefore\) Power, \(P=\frac{1}{f}=\frac{1}{0.25}=4 D\)

\(\underset{Ethanoic\ acid}{CH_3COOH} + \underset {Sodium\ hydrogen\\ Carbonate}{NaHCO_3} \rightarrow \underset{sodium\ ethanoate}{CH_3COONa}+ \underset{Cabon\ dioxide}{CO_2} + \underset{Water}{H_2O}\)
Activity to show that ethanoic acid reacts with metal hydrogen carbonate to liberate carbon dioxide.
Procedure:
i) Take two test tubes and label them as A and B.
ii) Now, take few grams of sodium carbonate in test tube A.
iii) Take few grams of sodium hydrogen carbonate in test tube B.
iv) Now, add ethanoic acid to both the test tubes.
v) Note your observation.
vi) Pass the gas liberated in test tube A and B through lime water or calcium hydroxide solution.
vii) Record your observation.
Observation
a) A brisk effervescence is formed.
b) The lime water turns milky in both the test tubes.

When carboxylic acids react with sodium carbonate or sodium bicarbonates, it liberates carbon dioxide gas which turns lime water milky.
\(\underset{Calcium\ hydroxide\\ (Lime\ wate)}{Ca(OH)_2(aq)}+\underset{Carbon\ dioxide}{CO_2(g)}\rightarrow \underset{Calcium\ carbonate\\ (White \ ppt)}{CaCO_3(s)\downarrow}+\underset{Water}{H_2O(l)}\)

(a) Magnetic field is the region around a magnet in which a magnetic material experiences a force because of that magnet.
(b) The direction of magnetic field at a point is determined by using a magnetic compass. The direction along which the needle of a magnetic compass gets aligned indicates the direction of the magnetic field.
(c) A straight thick conducting wire is inserted normally through centre of a plane paper. Now, electric current is passed through the wire. When a small magnetic compass is placed on the paper, the magnetic needle of the compass gets aligned along a particular direction. The positions of the North Pole and South Pole are marked on the paper. Now, the magnetic needle is shifted to a new position such that its South Pole occupies the position previously occupied by its North Pole. Again, the positions of the North Pole and South Pole are marked. These steps are repeated over and over again and the marked points are joined to obtain the direction of the magnetic field generated by the current-carrying straight wire.
(d) The direction of magnetic field at the centre of a current of a current carrying loop is normal to the plane formed by the loop.

Since the metal X react with cold water, it must be high in the reactivity series i.e., sodium or potassium.
X is alkali metal, Na or K.
Since the metal Y reacts with hot water, it must be middle in the reactivity series i.e., magnesium or calcium.
Y is alkaline earth metal, Mg or Ca.
Since the metal X reacts with steam only, it must be low in the reactivity series i.e., iron Z is Fe.
Increasing reactivity series : Na > Mg > Fe.

(a) A white precipitate of barium sulphite is formed when barium chloride is added to the solution of sodium sulphite.
\(\underset{Barium\\ Chloide}{BaCl_2} +\underset{Sodium\\ sulphite}{ Na2S03} \rightarrow \underset{Baru\ sulphate\\ (white\ ppt)}{ BaS03\downarrow} +\underset{Sodium\\ chloride}{2NaCI}\)
(b) This precipitation reaction is also a double displacement reaction.
(c) Barium chloride, suphur dioxide and water are formed when dilute hydrochloric acid is added to this solution of barium sulphite. Since barium chloride is a soluble substance, thus white precipitate of barium sulphite disappears.
\(\underset { Barium\ sulphate\\ (white\ ppt) }{ BaCl_{ 2 } } +\underset { Hydrochloric\\ acid }{ 2HCl(aq) } \rightarrow \underset { Barium\\ chloride }{ BaC{ l }_{ 2 }(aq) } +\underset { Sulphur\\ dioxde }{ { SO }_{ 2 } } +\underset { Water }{ { H }_{ 2 }O } \)

Sodium hydroxide (NaOH) is a commonly used base and is hygroscopic, that is, it absorbs moisture from the atmosphere and becomes sticky.  The acidic oxides react with base to give salt and water. The reaction between NaOH and \(CO_2\) can be given as \(2NaOH+CO_2\rightarrow Na_2CO_3+H_2O\)

Exothermic reaction.

(I) CuO(s) + H₂(g) \(\rightarrow \) Cu(s) + H₂O(g)
(ii) Redox reaction.

In the dry state, HCI gas does not give any H ⁺ ions. It does not behave as an acid. That is why the colour of litmus paper does not change.

Hydrogen gas is formed. Bring a burning matchstick near to it, H₂ will burn explosively with 'pop' sound.

Between pole and focus, behind the convex mirror.

Sir Issac Newton

Those hydrocarbons in which valency of carbon is satisfied by single bonds only are called saturated hydrocarbons.

The strength of magnetic field varies directly as the magnitude of the electric current. So, in case of an increase in current the magnetic field would increase.

a)Hydrogen is not used as a common fuel because when it is lighted, it burns with an explosion and it is very difficult to store and transport hydrogen.
b) Fuel cells are being developed that can store energy produced by burning hydrogen. Development of materials that can absorb hydrogen and then release it at a steady rate.

Power (P) = Potential difference (V) \(\times\) Current (I)

A solution of ammonia in water results in the formation of ammonium hydroxide (NH₄OH).  Ammonium hydroxide is basic in nature and turns red litmus blue.

(a) Group 17 belongs to halogen family and thus, A is non-metal.
(b) C is less reactive than A because the reactivity of non-metal decreases from top to bottom.
(c) Since the size decreaes across a period on moving from left to right, the size of C is smaller than B because B and C belong to the same period.
(d) A forms anion because C is non-metals for anion (negative ion).

Environmental consequences of using fossil are air pollution, greenhouse effect, acid rain, global warming. To minimize the pollution caused by various source of energy.
1. We should use smokeless appliances
2. Technology should be constantly upgraded
3. Use of energy should be judicious
4. Proper and safe disposal of nuclear waste.