Write down the changes that will be seen in each instance and explain the reason behind it. (a) nbsp; 50ml water is added to 50ml solution of copper sulphate. (b) Two drops of the indicator phenlphthalein were added to 10ml nbsp;solution of sodium hydroxide. (c) Two or three filings of copper were added to 10ml dilute nitric acid and stirred. (d) A litmus paper was dropped into 2ml dilute HCl. Then 2ml concentrated NaOH was added to it and stirred. (e) Magnesium oxide was added to dilute HCl and magnesium oxide was a added to dilute NaOH. (f) Zinc oxide was added to dilute HCl and zinc oxide was added to dilute NaOH. (g) Dilute HCl was added to limestone. (h) Pieces of blue vitriol were heated in a test tube. On cooling, water was added to it. (i) Dilute H 2 SO 4 was taken in an electrolytic cell and electric current was passed through it.

There are actually two equations for the reaction of copper with nitric acid take place. It depends on whether the nitric acid is concentrated or not. If it is concentrated, then the ratio is 1:4 for copper to nitric acid. If it is dilute then the ratio is 3:8.

C u + 4 H N O_{3} (d i l .) \to C u (N O_{3})_{2} + 2 N O_{2} + 2 H_{2} O 3 C u + 8 H N O_{3} (c o n c .) \to 3 C u (N O_{3})_{2} + 2 N O + 4 H_{2} O

Concentrated Nitric acid is act as strong oxidising agent so it makes sense that a higher oxidation state of nitrogen (IV) oxide is formed .

d. When litmus paper is dipped into 2 mL of dilute HCl solution , then blue litmus paper is turned into red colour and there is no effect on red litmus paper. Again, if the same litmus paper is dipped into 2 mL of concentrated NaOH solution, then red litmus paper turns into blue colour but there is no effect on blue litmus paper.This is due to the respective properties of blue and red litmus paper with acid and base.

e. Magnesium oxide is a base. when base reacts with an acid, it forms salt and water.This reaction is known as neutralisation reaction.

$M g O (s) + 2 H C l (l) = M g C l_{2} (a q) + H_{2} O (l)$

MgO doesnot react with NaOH.As NaOH is a base and bases react only with oxides of non-metal to form salt and water because oxides of non-metals are said to be acidic in nature so neutralization reaction take place. But MgO is a oxide of metal so, reaction is not possible.

f. Zinc oxide is added to dilute HCl, then neutralization reaction takes place to form salt and water.
The reaction is as follows:

$Z n O (a q) + 2 H C l (a q) \to Z n C l_{2} (a q) + H_{2} O (l)$

Zinc oxide reacts with sodium hydroxide to produce zincate sodium and water. This reaction takes place at a temperature of 500-600°C.it is exothermic reaction.
$ZnO + 2 NaOH \to {Na}_{2} {ZnO}_{2} + H_{2} O$

g. Limestone is calcium carbonate. When limestone is added to a 10% solution of dilute HCl, then brisk effervescence of CO₂ is released due to the reaction of acid with carbonate of metals.

$2 H C l + C a C O_{3} \to C a C l_{2} + C O_{2} + H_{2} 0$

h. When pieces of blue vitriol are heated in a test tube, then crystal structure of blue vitriol broke down to forn a colourless powder and water come out. This water is called water of crystallization. when water is added to the same test tube, then white powder turned into blue colour again.This is due to reversible reaction take place from anhydrous salt to hydrous salt and vice-versa.
$C U S O_{4} . 5 H_{2} O \leftrightarrow C U S O_{4} + 5 H_{2} O$

i. When a dilute solution of sulfuric acid is electrolysed, gases are produced at both the anode and the cathode electrode.

Hydrogen gas forms at the cathode, oxygen gas forms at the anode.

The gas produced at the cathode burns with a 'pop' sound, when a sample is lit with a lighted splint. This shows that the gas is hydrogen.

The gas produced at the anode relights a glowing splint dipped into a sample of the gas. This shows that the gas is oxygen.

The gases are produced when ions move towards the electrodes.

At the cathode:

2H⁺ +2e^- → H₂

At the anode:

4OH^- - 4e^- → 2H₂O + O₂