3.3 Quantitative results

What is the difference between qualitative and quantitative results?

• Qualitative: descriptive (e.g. colour change)

• Quantitative: numerical measurements (e.g. concentration, rate)

Why are quantitative results better than qualitative results?

They are more precise, allow calculations, and make it easier to compare and analyse data.

How can the hydrolysis of starch be measured quantitatively?

Using a colorimeter to measure absorbance of the starch-iodine complex.

What does a colorimeter measure?

Optical density (absorbance or % transmission) of a solution.

How does absorbance relate to starch concentration?

Higher starch concentration → darker colour → higher absorbance.

What is a calibration graph?

A graph used to convert colorimeter readings into actual concentrations.

How is a calibration graph constructed?

• Prepare known concentrations

• Measure absorbance

• Plot concentration (x-axis) vs absorbance (y-axis)

How is a calibration graph used?

Use absorbance value → find corresponding concentration from the graph.

Why is a calibration graph necessary?

Because absorbance itself does not directly give concentration.

Why does the rate of starch hydrolysis decrease over time?

Substrate concentration decreases → fewer enzyme-substrate collisions.

When is the rate of reaction highest?

At the beginning, when substrate concentration is highest.

What happens to the rate at the end of the reaction?

It becomes zero because all substrate is used up.

How can rate be calculated using quantitative data

change in concentration ÷ time

What is meant by the “overall rate” of reaction?

The average rate over the entire reaction time.

Why is the overall rate not constant?

Because substrate concentration decreases during the reaction.

What are the key steps in using a colorimeter for starch hydrolysis?

• Prepare starch solutions

• Add iodine

• Measure absorbance

• Use calibration curve

What should be plotted on a graph when analysing enzyme activity?

• X-axis: independent variable (e.g. time)

• Y-axis: dependent variable (e.g. concentration)

What are the rules for drawing graphs in exams?

• Use pencil

• Label axes with units

• Use suitable scale (1, 2, 5, 10)

• Plot clearly (X or O)

• Use at least half the grid

Why should you not use multiples of 3 for graph scales?

They make plotting and reading values more difficult.

Why must axes be labelled with units?

To clearly show what is being measured and ensure scientific accuracy.

What is the independent variable in enzyme investigations?

The variable you change (e.g. time, concentration).

What is the dependent variable in these experiments?

The variable measured (e.g. absorbance or starch concentration).

Why must equal volumes be used in colorimeter experiments?

To ensure fair comparison and accurate absorbance readings.

What precautions should be taken when using a colorimeter?

• Use clean cuvettes

• Wipe cuvettes before use

• Use same wavelength/filter

• Zero the machine

• Avoid bubbles

How are dilutions prepared from a stock solution?

By mixing measured volumes of stock solution with distilled water to reduce concentration.

Why are multiple concentrations used in calibration?

To create an accurate and reliable calibration curve.

What trend is expected when plotting starch concentration vs time?

A decreasing curve (starch concentration falls over time).

Why is a line of best fit used on graphs?

To show the overall trend and reduce the effect of random errors.