Short Answer Questions — Chapter 9: Solutions
Solute: The component present in smaller quantity that is dissolved. (e.g., salt in salt water)
Solvent: The component present in larger quantity that does the dissolving. (e.g., water in salt water)
Types of solutions by state:
| Type | Example |
|---|---|
| Solid in liquid | Salt in water, sugar in water |
| Gas in liquid | CO₂ in water (aerated drinks) |
| Liquid in liquid | Alcohol in water |
| Gas in gas | Air (N₂, O₂, other gases) |
| Solid in solid | Alloys (brass = Cu + Zn) |
Solubility: The maximum amount of solute that can dissolve in a given amount of solvent at a particular temperature to form a saturated solution.
Usually expressed as: grams of solute per 100 g of solvent.
Factors affecting solubility:
Concentration of a solution is the amount of solute present in a given amount of solution or solvent.
Mass percentage (% w/w):
Mass % = (Mass of solute / Mass of solution) × 100
Where: Mass of solution = Mass of solute + Mass of solvent
Example: 10 g of NaCl dissolved in 90 g of water:
- Mass of solution = 10 + 90 = 100 g
- Mass % = (10/100) × 100 = 10%
Interpretation: A 10% NaCl solution contains 10 g of NaCl in every 100 g of solution.
Molarity (M) is the number of moles of solute dissolved per litre (1000 mL) of solution.
M = n / V = (mass of solute / molar mass) / Volume in litres
Unit: mol/L (also written as M)
Example: Find the molarity of a solution containing 4 g of NaOH in 500 mL of solution. (Molar mass of NaOH = 40 g/mol)
n(NaOH) = 4/40 = 0.1 mol V = 500 mL = 0.5 L
M = n/V = 0.1/0.5 = 0.2 mol/L = 0.2 M
Note: Molarity changes with temperature (because volume changes). Molality (mol/kg of solvent) is temperature-independent.
Osmosis: The spontaneous flow of solvent molecules from a region of lower solute concentration (dilute solution) to a region of higher solute concentration (concentrated solution) through a semi-permeable membrane.
The membrane allows solvent molecules to pass but not solute molecules.
Osmotic pressure: The minimum pressure that must be applied on the concentrated solution to stop osmosis.
Importance and applications:
1. Plant water absorption: Roots absorb water from soil by osmosis (soil solution is more dilute than cell sap).
2. Cell function: Animal and plant cells maintain shape and function via osmosis:
- In hypotonic solution: cell swells (water enters)
- In hypertonic solution: cell shrinks/plasmolysis (water leaves)
- In isotonic solution: no net movement
3. Desalination: Reverse osmosis (applying pressure > osmotic pressure) purifies sea water.
4. Preservation: Salting of food (high salt concentration draws water out of bacteria by osmosis — bacteria die).
5. Medical: IV fluids must be isotonic with blood plasma.
| Property | Electrolytes | Non-electrolytes |
|---|---|---|
| Definition | Conduct electricity when dissolved in water | Do not conduct electricity when dissolved |
| Dissociation | Ionise (split into ions) in solution | Dissolve as molecules, no ions formed |
| Strong examples | NaCl, HCl, H₂SO₄, NaOH | Glucose (C₆H₁₂O₆), urea, sugar |
| Weak examples | CH₃COOH (acetic acid), NH₄OH | Ethanol (partial) |
| Reason for conduction | Free ions carry charge | No free ions to carry charge |
Types of electrolytes:
- Strong electrolytes: Completely ionise (e.g., NaCl, HCl, NaOH)
- Weak electrolytes: Partially ionise (e.g., acetic acid, carbonic acid)
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