Comprehensive Discussion on Water
1. What is Density in the Context of Water?
Q: What is density?
A: Density is defined as the mass per unit volume of a substance, usually expressed in grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³).
Q: What is the standard density of water?
A: The density of pure water at 4°C is 1 g/cm³ (1000 kg/m³).
Q: How does temperature affect the density of water?
A: Water exhibits an anomalous expansion. As it cools to 4°C, it contracts and becomes denser. Below 4°C, it expands, decreasing in density, which is why ice floats on water.
2. What is the Boiling Point of Water?
Q: What is the standard boiling point of water?
A: At sea level, pure water boils at 100°C under atmospheric pressure of 760 mm Hg (1 atmosphere).
Q: How does altitude affect the boiling point of water?
A: At higher altitudes, atmospheric pressure is lower, causing water to boil at temperatures lower than 100°C. For example, on Mount Everest, water boils at around 70°C.
Q: Why are pressure cookers useful in the hills?
A: Pressure cookers increase the pressure inside the cooker, raising the boiling point of water and allowing food to cook properly at higher altitudes.
Q: How do impurities affect the boiling point of water?
A: Dissolved impurities raise the boiling point of water, a phenomenon known as boiling point elevation.
3. What is the Freezing Point of Water?
Q: What is the freezing point of pure water?
A: Pure water freezes at 0°C under normal atmospheric pressure.
Q: How does pressure affect the freezing point of water?
A: Increasing the pressure decreases the freezing point of water because it makes it more difficult for water molecules to form a solid structure.
Q: How do impurities affect the freezing point of water?
A: Dissolved impurities lower the freezing point of water, a phenomenon known as freezing point depression.
4. What is Latent Heat of Fusion?
Q: What is the latent heat of fusion of ice?
A: The latent heat of fusion of ice is 336 joules per gram (J/g) or 80 calories per gram (cal/g). This is the amount of heat energy required to change 1 gram of ice at 0°C into 1 gram of water at 0°C without a change in temperature.
Q: What happens during freezing and melting?
A: When ice melts, it absorbs 336 J/g of heat energy. Conversely, when water freezes, it releases the same amount of heat energy.
5. What is Latent Heat of Vaporization?
Q: What is the latent heat of vaporization of water?
A: The latent heat of vaporization of water is 2268 joules per gram (J/g) or 540 calories per gram (cal/g). This is the amount of heat energy required to change 1 gram of water at 100°C into 1 gram of steam at 100°C without a change in temperature.
Q: What happens during vaporization and condensation?
A: When water vaporizes, it absorbs 2268 J/g of heat energy. When steam condenses, it releases the same amount of heat energy.
6. What is Specific Heat Capacity?
Q: What is the specific heat capacity of water?
A: The specific heat capacity of water is 4.18 joules per gram per degree Celsius (J/g°C). This is the amount of heat energy required to raise the temperature of 1 gram of water by 1°C.
Q: Why is the high specific heat capacity of water important?
A: The high specific heat capacity of water helps regulate temperature in the environment and within living organisms by absorbing and releasing large amounts of heat energy.
7. What are the Characteristics of True Solutions?
Q: What are the characteristics of true solutions?
- Homogeneous mixture
- Solute particles are very small (about \(10^{-10}\) meters)
- Clear and transparent
- Does not scatter light
- Components cannot be separated by filtration
- Solute particles do not settle down
8. Effect of Cooling on Solutions
Q: How does cooling affect saturated solutions?
A: Cooling a saturated solution typically causes excess solute to precipitate out as the solubility decreases.
Q: How does cooling affect unsaturated solutions?
A: Cooling an unsaturated solution usually does not result in precipitation since it can still dissolve more solute.
Q: How does cooling affect supersaturated solutions?
A: Cooling a supersaturated solution can lead to rapid crystallization if the solution is disturbed.
9. Concentration of Solutions
Q: What is mass percent?
A: Mass percent is the mass of the solute divided by the total mass of the solution, multiplied by 100. Formula:
\[
\text{Mass Percent} = \left( \frac{\text{Mass of Solute}}{\text{Mass of Solution}} \right) \times 100
\]
Example: If 10 grams of salt is dissolved in 90 grams of water, the mass percent is:
\[
\text{Mass Percent} = \left( \frac{10 \text{ g}}{10 \text{ g} + 90 \text{ g}} \right) \times 100 = 10\%
\]
Q: What is volume percent?
A: Volume percent is the volume of the solute divided by the total volume of the solution, multiplied by 100. Formula:
\[
\text{Volume Percent} = \left( \frac{\text{Volume of Solute}}{\text{Volume of Solution}} \right) \times 100
\]
Example: If 50 milliliters of ethanol is added to 450 milliliters of water, the volume percent is:
\[
\text{Volume Percent} = \left( \frac{50 \text{ mL}}{50 \text{ mL} + 450 \text{ mL}} \right) \times 100 = 10\%
\]
10. Solubility and Solubility Curves
Q: What is solubility?
A: Solubility is the maximum amount of solute that can dissolve in a given amount of solvent at a specific temperature to form a saturated solution.
Q: How is solubility determined?
A: Solubility is determined by preparing a saturated solution at the desired temperature, filtering the solution, and measuring the amount of dissolved solute.
11. Examples and Calculations
Example: Calculate the solubility of KCl.
Given: 12 grams of saturated KCl solution at 20°C leaves 3 grams of solid residue after evaporation.
Solution: \[ \text{Mass of Solvent} = 12 \text{ g (solution)} – 3 \text{ g (solute)} = 9 \text{ g} \] \[ \text{Solubility} = \left( \frac{3 \text{ g (solute)}}{9 \text{ g (solvent)}} \right) \times 100 = 33.33 \text{ g/100 g solvent} \]
Example: Calculate the weight of NaNO₃ required to prepare 60 grams of pure crystals.
Given: Solubility of NaNO₃ is 140 g/100 g water at 70°C and 100 g/100 g water at 25°C.
Solution: \[ \text{Mass of NaNO₃ in saturated solution at 70°C} = \left( \frac{60 \text{ g crystals}}{140 \text{ g NaNO₃}} \right) \times 240 \text{ g solution} = 102.86 \text{ g solution} \] Mass of water in 102.86 g solution: \[ \text{Mass of water} = 102.86 \text{ g} – 60 \text{ g} = 42.86 \text{ g} \] Mass of NaNO₃ remaining dissolved at 25°C: \[ \text{Mass of dissolved NaNO₃} = 42.86 \text{ g} \] Amount of NaNO₃ crystallized out: \[ 60 \text{ g} – 42.86 \text{ g} = 17.14 \text{ g NaNO₃} \]
12. Effect of Pressure and Temperature on Solubility of Gases
Q: How does temperature affect the solubility of gases?
A: The solubility of gases in liquids typically decreases with increasing temperature because gases are more likely to escape from the solution.
Q: How does pressure affect the solubility of gases?
A: The solubility of gases in liquids increases with increasing pressure, as described by Henry’s Law, which states that the amount of dissolved gas is proportional to the pressure of the gas above the liquid.
13. Crystallization
Q: What is crystallization?
A: Crystallization is the process by which a solute precipitates from a solution as a crystalline solid.
Q: How is crystallization performed?
A: Crystallization is performed by preparing a supersaturated solution, cooling it slowly, and collecting the formed crystals.
Q: What are the applications of crystallization?
A: Crystallization is used in chemical purification, production of pharmaceuticals, and formation of pure solid compounds.