ACT Science · Data Representation (~35% of Science)
Graph Patterns & Data Trends
Key ACT Rule at a Glance
ACT Science is an open-book visual reading test, not a science knowledge quiz. You do not need to memorize chemistry or biology formulas. To complete the section in 35 minutes (approximately 52 seconds per question), never read the introductory passage text first. Go straight to the question stem, locate the exact Figure or Table referenced, and trace the line or bar trend.
1. The Three Common Graph Relationships
Every trend question on ACT Science tests your ability to identify how the dependent variable ($y$-axis) reacts when the independent variable ($x$-axis) changes:
Direct Relationship
As the $x$-axis increases, the $y$-axis also increases (upward sloping curve or line).
Inverse Relationship
As the $x$-axis increases, the $y$-axis decreases (downward sloping curve or line).
Bell Curve / Optimum
The variable increases to a maximum peak, then decreases (e.g. enzyme activity vs temperature).
Plateau / Saturation
The curve rises rapidly and then flattens horizontally into an asymptote (constant value).
The ACT's favorite way to mislead students is placing two different vertical axes on the same graph: one on the left (e.g. Temperature in °C) and one on the right (e.g. Pressure in kPa). Always check which curve corresponds to which axis by looking at the legend or line style (solid vs dashed line).
2. Extrapolation vs. Interpolation
- Interpolation: Finding an estimated value between two known data points on the graph. Follow the curve smoothly between the plotted values.
- Extrapolation: Finding an estimated value beyond the measured range of the graph (e.g. estimating the value at 60°C when the graph ends at 50°C). Extend the existing trajectory in the same direction.
3. Worked ACT Science Practice Question
In an experiment testing gas solubility, as water temperature increases from 10°C to 50°C, the dissolved oxygen concentration drops steadily from 11.3 mg/L to 5.5 mg/L in a downward curve. Based on this pattern, what is the most likely dissolved oxygen concentration at 60°C?
View Step-by-Step Explanation
Correct Answer: C (Less than 5.5 mg/L)
The data demonstrates a clear inverse relationship: higher water temperatures result in lower dissolved oxygen concentrations. Since 60°C is hotter than 50°C, continuing the downward trajectory beyond the highest measured point (extrapolation) requires the concentration at 60°C to be lower than the value at 50°C, which was 5.5 mg/L. Therefore, Choice C is the only logically supported answer.