Chemistry Notes — Planning Solubility Investigation & Paper Chromatography (Grade VII)
2.3 Planning a solubility Investigation
Variables to keep constant when investigating the effect of temperature on solubility
i. Mass of solute added
iii. Volume of solvent
iv. Type of thermometer
Reason: Temperature is the independent variable; to compare solubility at different temperatures, other factors that could influence solubility (amount of solute, amount of solvent, and how temperature is measured) must be kept the same.
b. Purpose of a control in a solubility investigation
ii. Provide a comparison for the experimental set-up
Rationale: Controls establish a baseline to see what changes when the independent variable (temperature) is altered, helping isolate the effect of temperature on solubility.
c. In paper chromatography, the solvent front is
ii. The furthest point reached by the solvent on the paper
Explanation: The solvent front marks how far the mobile phase has travelled; pigments separate along the way based on their interactions with the stationary phase.
d. Which factor will affect the Rf value of a substance in paper chromatography?
iv. All the above
Notes: Rf can be influenced by the distance traveled by the pigment (numerator), the distance travelled by the solvent front (denominator), and factors such as the type of paper and temperature that affect diffusion and solvent movement.
2.4 Paper Chromatography
e. Safety precautions are most important when heating a solution to determine solubility
ii. Wear safety goggles
Rationale: Eye protection is essential when handling chemicals due to splashes, fumes, or broken glass risks.
2. Practical questions
A State one reason why a pencil is used to draw the baseline in chromatography
Reason: Pencil (graphite) does not dissolve in the solvent and does not run, ensuring the baseline remains intact and does not contaminate or obscure the chromatogram. It avoids smearing or chemical reaction with the solvent.
B. Name one factor, apart from temperature, that can affect the solubility of a solid in water
Example: Particle size/surface area of the solute (smaller particles with greater surface area dissolve more readily); other acceptable options include stirring/agitation, pH of the solution, or presence of impurities.
C. Why should you repeat measurements in a solubility investigation?
To improve accuracy and reliability by identifying random errors, detecting anomalous results, and allowing calculation of a mean value.
3. A solubility investigation example
a. A student investigates the solubility of potassium nitrate at different temperatures. She adds excess potassium nitrate to water at 40 °C, stirs it, and measures the mass of solute dissolved.
(i) Identify two independent (controlled) variables that must be kept constant to make the test fair
Volume of water (solvent) used in each trial
Mass of excess solute added (or amount of solute used to create the saturated condition)
Rationale: These should be kept constant so that any observed differences in dissolved mass are due to temperature, not differing solvent amount or solute quantity.
(ii) Explain why adding excess solute is necessary in this experiment
To ensure the solution becomes saturated at the tested temperature, so the amount dissolved reflects the solubility limit at that temperature rather than being limited by an insufficient amount of solute.
4. Match the words to their meanings
a. solution → ii. a mixture formed when a solute dissolves in a solvent
b. saturated solution → v. a solution in which no more solute will dissolve
c. chromatogram → iv. separated colours as a result of the chromatograph
d. insoluble → i. a word to describe a substance that will not dissolve
e. solvent → iii. a liquid in which other substances will dissolve
f. Baseline → vii. A line drawn with a pencil where the sample is placed
g. solubility → viii. Amount of solute dissolved in 100 g of water
h. control variable → vi. The factor was kept the same for a fair test
Solubility (definition): the amount of solute that can dissolve in a given amount of solvent at a specific temperature, commonly expressed as