IB myp 4-5 Chemistry – Practice Questions- All Topics
Topic :Pure and impure substances–states and properties of matter
Topic :Pure and impure substances- Weightage : 21 %
All Questions for Topic : types of mixtures [solutions, oils, alloys, emulsions],separation techniques including: filtration, distillation [including
crude oil],chromatography
Question (7 marks)
A student knows that water freezes at 0°C but notices that other liquids freeze at different temperatures when put into a freezer. When chemicals are dissolved in water, the freezing point of the solution becomes lower than pure water. The student investigated this by testing different solutions.
Experimental Setup:
- Tested salt water, sugar water, carbonated water, tap water, and pure water
- Used identical containers
- Placed in freezer at -15°C
- Recorded freezing temperatures
Question a (1 mark)
State the gas that is added to drinks to make them carbonated:
▶️Answer/Explanation
Answer: CO2 (carbon dioxide)
Explanation: Carbonation involves dissolving CO2 gas under pressure, which forms carbonic acid (H2CO3) in water.
Question b (2 marks)
The student placed salt water, sugar water, carbonated water, tap water and pure water into a freezer to investigate their freezing points.
State the variables in this experiment:
Independent variable:
Dependent variable:
▶️Answer/Explanation
Answer:
Independent variable: Type of solute (salt, sugar, CO2, etc.)
Dependent variable: Freezing point/temperature (°C)
Key Point: The student was testing how different solutes affect the colligative property of freezing point depression.
Question c (2 marks)
Suggest two control variables for this investigation:
1.
2.
▶️Answer/Explanation
Possible Answers:
- Volume/mass of water (e.g., 50 mL each)
- Solute concentration (e.g., 1 molal for all)
- Container type/size/material
- Freezer temperature setting
- Cooling time duration
Purpose: These controls ensure observed differences are solely due to solute type.
Question d (2 marks)
Formulate a research question for this investigation:
▶️Answer/Explanation
Model Answer: “How does the type of solute (salt, sugar, or carbon dioxide) affect the freezing point depression of aqueous solutions?”
Characteristics of Good Research Questions:
- Specifies independent variable (solute type)
- Identifies dependent variable (freezing point depression)
- Defines system (aqueous solutions)
- Is measurable and testable
Freezing Point Depression Theory
ΔTf = iKfm
- ΔTf: Freezing point lowering (°C)
- i: Van’t Hoff factor (particles per formula unit)
- NaCl → i≈1.8 (dissociates into Na+ + Cl–)
- Sugar → i=1 (doesn’t dissociate)
- Kf: Cryoscopic constant (1.86°C·kg/mol for water)
- m: Molality (moles solute/kg solvent)