Module 1 Core Concepts

What is the difference between "Accuracy" and "Precision"?

• Accuracy: How close a measured value is to the true value.

• Precision: How close repeated measurements are to each other (the spread of the data). (Hint: You can be precise but not accurate, e.g., if your balance is zeroed incorrectly).

How do you calculate Percentage Error (Uncertainty) for a measurement?

\% \text{ error} = \left( \frac{\text{Uncertainty}}{\text{Measured Value}} \right) \times 100

• Note: If you make two readings to get a value—like a burette or temperature change—you multiply the uncertainty by 2).

What is a "Serial Dilution" and when would you use it?

• It is a stepwise dilution of a substance in solution. It is used to create a range of concentrations to determine an unknown concentration (e.g., finding the water potential of a potato or calibration curves for colorimetry)

In microscopy, what is the function of a Stage Micrometer and an Eyepiece Graticule?

• Eyepiece Graticule: A scale inside the eyepiece with arbitrary units. It stays constant.

• Stage Micrometer: A slide with a known scale (e.g., 1mm long). It is placed on the stage to calibrate the eyepiece graticule at a specific magnification.

What is the formula for Rf value in chromatography, and why is it used?

• R_f = \frac{\text{Distance moved by solute}}{\text{Distance moved by solvent}}

• It is used to identify biological molecules (amino acids, pigments) by comparing the calculated value to known standards.

Question 1 (Planning - AO3) A student wants to investigate the effect of temperature on the permeability of beetroot cell membranes. Plan a method for this investigation. Include variables and control measures.

• Independent Variable: Temperature (at least 5 different temperatures, e.g., 10, 20, 30, 40, 50°C).

• Dependent Variable: Absorbance of the solution (measured using a colorimeter / blue filter).

• Control Variables:

  • Surface Area: Use a cork borer and cut beetroot discs to the same length (e.g., 1cm).

  • Volume of water: Use the same volume (e.g., 10cm³) in each test tube.

  • Time: Leave beetroot in water for the same time (e.g., 30 mins).

  • Wash beetroot: Rinse discs with distilled water before starting to remove pigment released during cutting.

• Method: Place test tubes in water baths at set temperatures. Add beetroot. Remove beetroot after set time. Shake solution. Measure absorbance.

Question 2 (Maths - Uncertainty) A student measures 25.0 cm³ of water using a measuring cylinder with an uncertainty of $\pm 0.5$ cm³. Calculate the percentage uncertainty.

• \% \text{ Uncertainty} = \left( \frac{0.5}{25.0} \right) \times 100 = \mathbf{2.0\%}

Question 3 (Evaluation) Explain why using a pH meter is better than using universal indicator paper for measuring the rate of an enzyme reaction affected by pH.

• Quantitative vs Qualitative: A pH meter gives a numerical value (quantitative), whereas paper relies on subjective colour matching (qualitative).

• Precision: A pH meter can measure to 1 or 2 decimal places (higher resolution), whereas paper usually only gives whole numbers.

• Continuous monitoring: You can record pH changes in real-time with a meter/data logger.

Question 5 (Microscopy) Describe how you would calibrate an eyepiece graticule using a stage micrometer.

• Place the stage micrometer on the microscope stage and focus.

• Align the scale of the eyepiece graticule with the scale of the stage micrometer.

• Count the number of eyepiece divisions that correspond to a known measurement on the stage micrometer (e.g., "10 eyepiece units = 25 stage units").

• Calculate the size of one eyepiece unit:\frac{\text{Known Stage Distance}}{\text{Number of Eyepiece Units}}