A cloth strip dipped in onion juice is used for testing a liquid 'X'. The liquid 'X' changes its odour. Which type of an indicator is onion juice? The liquid 'X' turns blue litmus red. List the observations the liquid 'X' will show on reacting with the following:

(a) Zinc granules

(b) Solid sodium carbonate

Write the chemical equations for the reactions involved.

OR

Define water of crystallisation. Give the chemical formula for two compounds as examples. How can it be proved that the water of crystallisation makes a difference in the state and colour of the compounds?

PART 1

1. Type of Indicator:

Onion juice is an olfactory indicator. Olfactory indicators are substances whose odour changes depending on whether they are in an acidic or basic medium.

1. Nature of Liquid 'X':

Since liquid 'X' turns blue litmus paper red, it is acidic in nature.

1. Observations and Chemical Equations:

(a) Reaction with Zinc granules:

• Observation: Bubbles of hydrogen gas are evolved on the surface of the zinc granules. When these gas bubbles are passed through a soap solution, gas-filled soap bubbles are formed. Bringing a burning candle near these bubbles causes the gas to burn with a characteristic 'pop' sound.
• Chemical Equation:

$$Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$$

(b) Reaction with Solid Sodium Carbonate:

• Observation: Brisk effervescence is observed due to the evolution of carbon dioxide gas. When this gas is passed through lime water (calcium hydroxide solution), the lime water turns milky due to the formation of a white precipitate of calcium carbonate ($CaCO_3$).
• Chemical Equation:

$$Na_2CO_3(s) + 2HCl(aq) \rightarrow 2NaCl(aq) + H_2O(l) + CO_2(g)$$

Reaction with lime water:

$$Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)$$

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OR

1. Definition of Water of Crystallisation:

Water of crystallisation is the fixed number of water molecules chemically combined in one formula unit of a salt.

1. Examples (Chemical Formulas):

• Hydrated Copper Sulphate: $CuSO_4 \cdot 5H_2O$
• Washing Soda (Hydrated Sodium Carbonate): $Na_2CO_3 \cdot 10H_2O$

1. Experimental Proof of Difference in State and Colour:

• Experiment: Take a few crystals of hydrated copper sulphate ($CuSO_4 \cdot 5H_2O$), which are blue in colour and crystalline in shape, in a clean, dry boiling tube and heat it over a burner.
• Observations:
• On heating, the blue colour of the crystals disappears, and they turn into a white, amorphous powder (anhydrous copper sulphate, $CuSO_4$).
• Droplets of water are observed on the cooler inner walls of the boiling tube.
• If you add $2$ to $3$ drops of water back onto the white sample, the blue colour is restored, and it regains its crystalline nature.
• Chemical Equation:

$$CuSO_4 \cdot 5H_2O(s) \xrightarrow{\text{Heat}} CuSO_4(s) + 5H_2O(g)$$

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