Water

1. Hoffmann's Voltameter consists of three interconnected vertical glass tubes with platinum electrodes at the base of the two outer limbs.
   2. The apparatus is filled with acidified water (water containing a few drops of dilute $\text{H}_2\text{SO}_4$ to provide conducting ions).
   3. When DC electric current is passed through the electrodes:
   • At the Cathode (Negative Electrode): Hydrogen gas ($\text{H}_2$) evolves.
   • At the Anode (Positive Electrode): Oxygen gas ($\text{O}_2$) evolves.
   4. The volume of gas collected in the cathode limb is observed to be EXACTLY TWICE the volume of gas collected in the anode limb:
   

$$\mathbf{V_{\text{Hydrogen}} : V_{\text{Oxygen}} = 2 : 1}$$

Equation: $2\text{H}_2\text{O} \xrightarrow{\text{electrolysis}} 2\text{H}_2 \uparrow + \text{O}_2 \uparrow$.

• Temporary Hardness: Caused by dissolved bicarbonates of calcium and magnesium: $\text{Ca(HCO}_3)_2$ and $\text{Mg(HCO}_3)_2$. It can be easily removed by simple boiling.
• Permanent Hardness: Caused by dissolved chlorides and sulphates of calcium and magnesium: $\text{CaCl}_2, \text{CaSO}_4, \text{MgCl}_2, \text{MgSO}_4$. It cannot be removed by boiling.
• Removal of Temporary Hardness by Boiling:
Heating decomposes soluble calcium bicarbonate into insoluble calcium carbonate ($\text{CaCO}_3$), which precipitates out and is filtered off:

$$\mathbf{\text{Ca(HCO}_3)_2\text{ (aq)} \xrightarrow{\Delta} \text{CaCO}_3 \downarrow \text{ (white ppt)} + \text{H}_2\text{O (l)} + \text{CO}_2\text{ (g)} \uparrow}$$

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• Efflorescence:
The phenomenon whereby a hydrated crystalline salt loses its water of crystallisation when exposed to dry air, crumbling into an amorphous powder.
Examples:
1. Washing soda: $\text{Na}_2\text{CO}_3 \cdot 10\text{H}_2\text{O} \to \text{Na}_2\text{CO}_3 \cdot \text{H}_2\text{O} + 9\text{H}_2\text{O}$
2. Glauber's salt: $\text{Na}_2\text{SO}_4 \cdot 10\text{H}_2\text{O} \to \text{Na}_2\text{SO}_4 + 10\text{H}_2\text{O}$
• Deliquescence:
The phenomenon whereby certain water-soluble substances absorb moisture from the atmosphere, dissolve in that absorbed water, and turn into a liquid solution.
Examples:
1. Solid Sodium Hydroxide ($\text{NaOH}$ pellets)
2. Anhydrous Calcium Chloride ($\text{CaCl}_2$)
3. Ferric chloride ($\text{FeCl}_3$).

• Water of Crystallisation: The fixed number of water molecules chemically combined in a definite stoichiometric ratio to a salt formula unit, forming its regular crystalline shape and color.
• Heating Blue Vitriol ($\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$):
1. When heated strongly, the salt loses all its water of crystallisation:

$$\mathbf{\text{CuSO}_4 \cdot 5\text{H}_2\text{O [blue]} \xrightarrow{\Delta} \text{CuSO}_4\text{ [white anhydrous]} + 5\text{H}_2\text{O (vapor)}}$$

2. The blue crystalline solid crumbles into an amorphous white powder of anhydrous copper sulphate.
3. Steam condenses into water droplets on the cool upper walls of the test tube.
4. Adding a drop of water back turns the powder bright blue again!

• Solubility: The maximum mass of solute in grams that can dissolve in $100\text{ grams}$ of solvent at a specific temperature to produce a saturated solution.
• Calculation:

$$\text{Solubility } S = \frac{\text{Mass of Solute}}{\text{Mass of Solvent}} \times 100$$

$$S = \frac{24\text{ g}}{80\text{ g}} \times 100 = \frac{3}{10} \times 100 = \mathbf{30\text{ grams}}$$

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The solubility of the salt at $25^{\circ}\text{C}$ is $30\text{ grams per 100 g of water}$.

• Permutit (Zeolite): An artificial hydrated sodium aluminum silicate (formula $\text{Na}_2\text{Z}$ or $\text{Na}_2\text{Al}_2\text{Si}_2\text{O}_8 \cdot x\text{H}_2\text{O}$).
• Hard water containing $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions is passed through a cylinder packed with a thick bed of permutit.
• An Ion-Exchange Reaction takes place: the zeolite replaces the hardness-causing calcium and magnesium ions with soluble sodium ions:

$$\mathbf{\text{Na}_2\text{Z} + \text{Ca}^{2+} \to \text{CaZ} + 2\text{Na}^+}$$

$$\mathbf{\text{Na}_2\text{Z} + \text{Mg}^{2+} \to \text{MgZ} + 2\text{Na}^+}$$

• The water emerging from the base is completely soft.
• The exhausted column is easily regenerated by backwashing with concentrated brine solution ($10\%\text{ NaCl}$).

• Fresh natural water contains small quantities of dissolved gases (Oxygen and Carbon Dioxide) as well as beneficial dissolved mineral salts, which impart a pleasant, brisk, refreshing taste to water.
• The solubility of gases in water decreases dramatically with rising temperature.
• When water is boiled, all the dissolved oxygen and carbon dioxide gases escape into the atmosphere.
• The absence of these dissolved gases makes boiled water taste flat and unappetizing.

• Saturated Solution: A solution that cannot dissolve any more solute at a given specific temperature in the presence of excess undissolved solute.
• Cooling a Hot Saturated Solution:
The solubility of potassium nitrate ($\text{KNO}_3$) decreases sharply as temperature decreases.
As the hot solution cools, the excess dissolved solute that can no longer remain in solution precipitates out of the liquid in the form of beautiful, pure needle-like crystals of potassium nitrate.