Chem: Key skills (graphs, conclusions and evaluations)

What is the difference between reliable results and valid results in an experiment?

Reliable results are reproducible and repeatable, giving the same data each time; valid results are both reliable and come from a fair test designed to answer the specific question.

What are independent, dependent, and control variables?

The independent variable is the one you change; the dependent variable is the one you measure; control variables are the factors kept the same to ensure a fair test.

What is the purpose of a control experiment?

A control experiment is kept under the exact same conditions as the main investigation, but without changing the independent variable, to show that any observed effect is due solely to the independent variable.

How can you improve the reliability of experimental measurements?

Repeat each measurement at least three times and calculate a mean (average) result.

What safety precautions should be taken when handling corrosive chemicals, open flames, or toxic gases?

Wear gloves and safety goggles for corrosive chemicals (like sulfuric acid); place Bunsen burners on heatproof mats; handle toxic/harmful gases inside a fume cupboard.

What is accuracy in experimental measurements, and how can it be improved when measuring gas volumes?

Accuracy is how close a measured result is to the true value; it can be improved by using precise collection methods (e.g., collecting gas in a gas syringe to measure volume rather than counting bubbles of varying sizes).

What is precision in experimental data?

Precision refers to how close repeated measurements are to one another and to their mean average (i.e., data points are not widely spread out).

What are anomalous results and how should they be treated when processing experimental data?

Anomalous results are data points that are totally different from the rest of the sample, usually caused by human error or faulty equipment setup; they should be identified and ignored when calculating the mean or range.

How do you calculate the mean and range for a set of repeated measurements?

To calculate the mean, add together all valid data values and divide by the total number of values; to calculate the range, subtract the smallest value from the largest value.

What types of data are displayed using bar charts versus line graphs?

Bar charts display categoric data (distinct categories) or discrete data (counted in whole chunks); line graphs display continuous numerical data (which can take any value within a range).

What key rules must be followed when drawing a bar chart?

Use a linear scale with equal values for each division, label both axes, include units, draw bars of equal width, leave gaps between bars for categoric or discrete data, and include a key if showing multiple data sets.

Which variables go on the x-axis and y-axis when plotting a line graph?

The independent variable goes on the x-axis (horizontal) and the dependent variable goes on the y-axis (vertical).

What are the key guidelines for plotting data points and drawing a line of best fit on a line graph?

Plot points with a sharp pencil as neat little crosses ($+$ or $\times$); draw a straight line or smooth curve of best fit that passes through or near as many points as possible while ignoring anomalous points.

What is the difference between direct proportion and a general positive correlation on a graph?

Direct proportion occurs when the line is straight and passes through the origin $(0,0)$, meaning one variable increases at the exact same rate as the other; positive correlation simply means as one variable increases, the other increases.

What are the three main types of correlation between two variables?

Positive correlation (as one variable increases, the other increases), inverse/negative correlation (as one variable increases, the other decreases), and no correlation (no relationship between variables).

Why does correlation between two variables not automatically prove cause?

Correlation can occur purely by chance, or because both variables are linked by a third unmeasured variable; cause can only be concluded if all other potential affecting variables are controlled.

How does the gradient of a line on a reaction graph relate to the rate of reaction?

The gradient (slope) represents the rate of reaction; a steeper gradient indicates a faster rate of reaction.

What is the formula for calculating the gradient of a straight line on a graph, and how are the units determined?

$\text{gradient} = \frac{\text{change in } y}{\text{change in } x}$; units are calculated by dividing the units of the y-axis by the units of the x-axis (e.g., $\text{cm}^3/\text{s}$).

How do you calculate the rate of reaction at a specific time on a curved graph?

Draw a tangent line touching the curve at that specific point, pick two points on the tangent line, and calculate its gradient using $\frac{\text{change in } y}{\text{change in } x}$.

What key steps should be included when planning an investigation or experiment?

1) Define the dependent variable (what is measured); 2) Define the independent variable (what is changed and how); 3) Describe the apparatus and method; 4) State control variables and how they will be kept constant; 5) State that repeats will be performed (at least three times); 6) State whether a control experiment is used.

What is a hypothesis in scientific experimentation?

A hypothesis is a possible explanation for an observation that can be tested by making predictions and gathering experimental evidence (data).

What key principle must be followed when drawing a conclusion from experimental data?

A conclusion must only describe the exact pattern or relationship shown by the data collected and must not extrapolate beyond those specific results.

How should experimental results be used to justify a conclusion?

By citing specific numerical data from the results to directly back up the observed trend or comparison.

How should a conclusion relate to the original hypothesis and prediction?

It should explicitly state whether the experimental data supports or refutes the original hypothesis and prediction.

What is an evaluation of a scientific investigation?

A critical analysis of the entire experimental process, examining the validity of the method, the quality of the results, and potential improvements.

What factors should be examined when commenting on the quality of experimental results during an evaluation?

Whether there was enough evidence to draw a valid conclusion, whether the data was reliable, valid, accurate, and precise, and whether any anomalous results occurred.

How can an experimental method be improved if results show a peak or optimum value?

By taking repeat measurements at narrower intervals around the estimated optimum value to obtain a more precise result.