krit.club logo

Scientific Inquiry and Skills - Designing Controlled Experiments

Grade 8IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

•

The Independent Variable (IV) is the factor that the investigator deliberately changes to observe its effect. In a graph, the IV is always plotted on the xx-axis.

•

The Dependent Variable (DV) is the factor that is measured or observed as it responds to changes in the IV. It is plotted on the yy-axis.

•

Controlled Variables are factors that must be kept constant throughout the experiment to ensure a 'fair test,' meaning any change in the DV is caused only by the IV.

•

A Hypothesis is a testable prediction, typically structured as: 'If the [Independent Variable] is increased, then the [Dependent Variable] will [increase/decrease] because [scientific reasoning].'

•

The Control Group is a setup that does not receive the experimental treatment (the IV), used as a baseline for comparison to ensure the results are valid.

•

Quantitative Data refers to numerical measurements (e.g., 15.5 cm15.5 \text{ cm}, 37.0∘C37.0 ^\circ C), while Qualitative Data refers to descriptive observations (e.g., 'the solution turned blue').

•

Reliability is improved by conducting multiple trials (at least 3-5) and calculating an average (barx\\bar{x}) to minimize the impact of random errors.

•

Accuracy describes how close a measurement is to the true value, while Precision describes how close repeated measurements are to each other.

📐Formulae

Average (Mean)=∑i=1nxin\text{Average (Mean)} = \frac{\sum_{i=1}^{n} x_i}{n}

Percentage Change=Final Value−Initial ValueInitial Value×100%\text{Percentage Change} = \frac{\text{Final Value} - \text{Initial Value}}{\text{Initial Value}} \times 100\%

Reaction Rate=ΔConcentrationΔt\text{Reaction Rate} = \frac{\Delta \text{Concentration}}{\Delta t}

Uncertainty Range=Max Value−Min Value2\text{Uncertainty Range} = \frac{\text{Max Value} - \text{Min Value}}{2}

💡Examples

Problem 1:

A student wants to investigate how the concentration of hydrochloric acid (HClHCl) affects the volume of hydrogen gas (H2H_2) produced when reacting with a constant mass of magnesium (MgMg) ribbon over 60 s60 \text{ s}. Identify the variables.

Solution:

Independent Variable: Concentration of HClHCl (e.g., 0.5 M,1.0 M,1.5 M0.5 \text{ M}, 1.0 \text{ M}, 1.5 \text{ M}). Dependent Variable: Volume of H2H_2 gas produced (cm3cm^3). Controlled Variables: Mass of MgMg (gg), surface area of MgMg, and temperature (TT) of the acid.

Explanation:

To ensure a controlled experiment, only the concentration of the reactant is varied. The volume of gas produced depends on the rate of collision between H+H^+ ions and the MgMg surface.

Problem 2:

In three trials, a student measures the time for a ball to fall: 1.22 s1.22 \text{ s}, 1.25 s1.25 \text{ s}, and 1.28 s1.28 \text{ s}. Calculate the mean time and the uncertainty.

Solution:

Mean: 1.22+1.25+1.283=1.25 s\frac{1.22 + 1.25 + 1.28}{3} = 1.25 \text{ s}. Uncertainty: 1.28−1.222=±0.03 s\frac{1.28 - 1.22}{2} = \pm 0.03 \text{ s}.

Explanation:

The mean provides a more reliable value for the dependent variable, while the uncertainty range indicates the precision of the measurements.