GCSE Biology - How to Solve: Osmosis and Percentage Change in Mass

How to Solve: Osmosis and Percentage Change in Mass

Complete Guide For GCSE/A-Level Physics, Chemistry, Biology (AQA, Edexcel, OCR, WJEC)

Introduction

"Master osmosis and percentage change in mass, and you’ll nail 6-mark questions on cell transport, plant physiology, and even kidney dialysis—worth up to 10% of your GCSE Biology paper. Let’s break it down so you never lose marks again."

WHAT YOU NEED TO KNOW FIRST

1. Diffusion: Movement of particles from high to low concentration.
2. Osmosis: Diffusion of water across a partially permeable membrane from a dilute to a concentrated solution.
3. Percentage change formula: (Change ÷ Original) × 100.

KEY TERMS & FORMULAS

Key Terms

• Osmosis: Net movement of water molecules from a region of higher water potential (dilute solution) to lower water potential (concentrated solution) through a partially permeable membrane.
• Water potential (Ψ): Measure of the tendency of water to move out of a solution (pure water = 0 kPa; more negative = more concentrated).
• Partially permeable membrane: Allows small molecules (e.g., water) to pass but blocks larger solutes (e.g., sugar).
• Hypotonic solution: Lower solute concentration than the cell (water moves into the cell).
• Hypertonic solution: Higher solute concentration than the cell (water moves out of the cell).
• Isotonic solution: Equal solute concentration (no net water movement).

Formulas

1. Percentage change in mass
   MEMORISE THIS
   [
   \text{Percentage change} = \left( \frac{\text{Final mass} - \text{Initial mass}}{\text{Initial mass}} \right) \times 100
   ]
2. Final mass: Mass after osmosis (g).
3. Initial mass: Mass before osmosis (g).
4. Positive value: Mass increased (water entered).

5. Negative value: Mass decreased (water left).

6. Rate of osmosis (if time is given)
   [
   \text{Rate} = \frac{\text{Change in mass (g)}}{\text{Time (minutes)}}
   ]

7. Given on exam sheet (check your spec).

STEP-BY-STEP METHOD

Step 1: Identify the direction of water movement

• Compare the water potential of the solution and the cell/tissue.
• If the solution is hypotonic (lower solute concentration than the cell), water moves into the cell → mass increases.
• If the solution is hypertonic (higher solute concentration than the cell), water moves out of the cell → mass decreases.
• If isotonic, no net movement → mass stays the same.

Step 2: Record initial and final masses

• Write down:
• Initial mass (g) (before placing in solution).
• Final mass (g) (after osmosis, e.g., after 30 minutes).

Step 3: Calculate the change in mass

[ \text{Change in mass} = \text{Final mass} - \text{Initial mass} ] - If positive, the object gained water. - If negative, the object lost water.

Step 4: Calculate percentage change

[ \text{Percentage change} = \left( \frac{\text{Change in mass}}{\text{Initial mass}} \right) \times 100 ] - Include the sign (+ or –) to show direction of change.

Step 5: Interpret the result

• Positive % change: Hypotonic solution (water entered).
• Negative % change: Hypertonic solution (water left).
• 0% change: Isotonic solution (no net movement).

Step 6: (If asked) Calculate rate of osmosis

[ \text{Rate} = \frac{\text{Change in mass (g)}}{\text{Time (minutes)}} ] - Units: g/min.

WORKED EXAMPLES

Example 1 – Basic

Question: A potato cylinder has an initial mass of 5.0 g. After 30 minutes in distilled water, its mass is 5.8 g. Calculate the percentage change in mass.

Solution: 1. Direction of water movement: Distilled water is hypotonic → water enters the potato → mass increases. 2. Initial mass = 5.0 g
Final mass = 5.8 g 3. Change in mass = 5.8 g – 5.0 g = +0.8 g 4. Percentage change = (0.8 ÷ 5.0) × 100 = +16% 5. Interpretation: The potato gained 16% of its mass due to water entering.

What we did and why: - Used the percentage change formula to quantify osmosis. - The positive sign confirms water moved into the potato.

Example 2 – Medium

Question: A piece of carrot has an initial mass of 8.2 g. After 20 minutes in a 10% salt solution, its mass is 7.1 g. a) Calculate the percentage change in mass. b) Explain the direction of water movement.

Solution: 1. Direction of water movement: 10% salt solution is hypertonic → water leaves the carrot → mass decreases. 2. Initial mass = 8.2 g
Final mass = 7.1 g 3. Change in mass = 7.1 g – 8.2 g = –1.1 g 4. Percentage change = (–1.1 ÷ 8.2) × 100 = –13.4% 5. Interpretation:
- The carrot lost 13.4% of its mass.
- Water moved out of the carrot into the salt solution.

What we did and why: - Calculated negative percentage change to show water loss. - Linked the result to the hypertonic solution causing osmosis out of the cell.

Example 3 – Exam-Style

Question (6 marks): A student investigates osmosis using potato cylinders. The table shows their results:

a) Calculate the percentage change in mass for each concentration. (3 marks) b) Explain why the mass changes in each case. (3 marks)

Solution: a) Calculations: 1. 0.0 mol/dm³ (distilled water):
- Change in mass = 7.2 g – 6.5 g = +0.7 g
- Percentage change = (0.7 ÷ 6.5) × 100 = +10.8%

1. 0.2 mol/dm³:
2. Change in mass = 6.3 g – 6.5 g = –0.2 g

3. Percentage change = (–0.2 ÷ 6.5) × 100 = –3.1%

4. 0.4 mol/dm³:

5. Change in mass = 5.8 g – 6.5 g = –0.7 g
6. Percentage change = (–0.7 ÷ 6.5) × 100 = –10.8%

b) Explanation: - 0.0 mol/dm³ (distilled water): Hypotonic solution → water moves into the potato → mass increases (+10.8%). - 0.2 mol/dm³: Slightly hypertonic → small net water loss → mass decreases (–3.1%). - 0.4 mol/dm³: Hypertonic → large net water loss → mass decreases (–10.8%).

What we did and why: - Calculated three percentage changes to show the trend. - Explained each result using osmosis principles (hypotonic/hypertonic).

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

"Right, listen up—this is your last-minute osmosis cheat sheet. Osmosis is water moving from high to low water potential through a membrane. To find percentage change in mass: (final – initial) ÷ initial × 100. Positive? Water went in. Negative? Water went out. Zero? Isotonic. If they ask for rate, divide change by time. Watch out for signs, units, and whether the solution is hypotonic or hypertonic. Now go smash that exam!"