What is an independent variable (IV) in an experiment?
Answer: The variable that the experimenter deliberately changes or manipulates.
Explanation
It's usually listed across the top of a data table or plotted on the x-axis of a graph.
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ACT Science: Experiments, Data and Reasoning Vocabulary (High School)
Experimental Design BasicsReading Tables & GraphsUnits & Scientific NotationExperimental Design QuestionsConflicting ViewpointsCore Science Facts: DensityCore Science Facts: pHCore Science Facts: PhotosynthesisCore Science Facts: Phases of MatterCore Science Facts: GeneticsCore Science Facts: Newton's Laws
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Experimental Design Basics
What is a dependent variable (DV)?
Answer: The variable that is measured or observed; it responds to changes in the independent variable.
Explanation
Usually plotted on the y-axis; it's the 'outcome' or 'result' column in a data table.
What is a controlled variable (constant)?
Answer: A factor that is deliberately kept the same across all trials/groups so it cannot influence the results.
Explanation
Controlling variables ensures that any change in the DV can be attributed only to the IV.
What is a control group?
Answer: A group that does not receive the experimental treatment; it serves as a baseline for comparison.
Explanation
Comparing treated groups to the control group shows whether the treatment itself caused an effect, rather than some other factor (like time passing).
Why is a control group important in experimental design?
Answer: It isolates the effect of the independent variable by showing what happens without the treatment, ruling out other explanations.
Explanation
Without a control group, you can't tell whether the observed change was actually caused by the treatment.
Distinguish a hypothesis from a theory.
Answer: A hypothesis is a specific, testable prediction about one outcome; a theory is a broad, well-supported explanation of a natural phenomenon based on extensive evidence from many experiments.
Explanation
On the ACT, 'theory' does NOT mean 'guess' — it's a robust explanatory framework (e.g., cell theory), while a hypothesis can be confirmed or refuted by a single experiment.
What is a scientific law, and how does it differ from a theory?
Answer: A law is a descriptive statement (often mathematical) summarizing a consistent pattern in nature; it describes WHAT happens but not WHY, whereas a theory explains the underlying mechanism.
Explanation
Example: Newton's law of gravitation describes the force between masses, but doesn't explain why mass causes attraction.
Reading Tables & Graphs
How do you identify a 'trend' when reading a data table?
Answer: Track how the dependent variable changes as the independent variable increases: it may increase, decrease, stay constant, or show no clear pattern.
Explanation
Always check whether the trend is consistent across the whole table or changes partway through.
What is a direct (positive) relationship between two variables?
Answer: As one variable increases, the other variable also increases.
Explanation
Example: as study time increases, test score increases.
What is an inverse (negative) relationship between two variables?
Answer: As one variable increases, the other variable decreases.
Explanation
Example: as altitude increases, air pressure decreases.
What is interpolation?
Answer: Estimating a value that lies BETWEEN two data points that were actually measured (within the range of the collected data).
Explanation
Example: if data exist at x=2 and x=4, estimating the value at x=3 is interpolation.
What is extrapolation?
Answer: Estimating a value OUTSIDE the range of the data collected, by extending the observed trend.
Explanation
Example: if data only go up to x=8, estimating the value at x=12 is extrapolation.
Why is extrapolation considered riskier than interpolation?
Answer: Because it assumes the observed trend continues unchanged beyond the tested range, which may not actually be true.
Explanation
A relationship could level off, reverse, or change shape outside the measured range.
Data were collected for X = 0 to 8 showing a steady increasing trend. Is estimating Y at X=3 interpolation or extrapolation? What about at X=12?
Answer: X=3 is interpolation (within the measured range); X=12 is extrapolation (beyond the measured range).
Explanation
Always check the range of the x-axis data before estimating a value — this is a very common ACT Science trap question.
Units & Scientific Notation
Why must you carefully check units before comparing values from different tables or figures?
Answer: Different tables may report the same quantity in different units (e.g., grams vs. kilograms, °C vs. K), so values must be converted to the same units before comparing.
Explanation
ACT passages often use this trick to test careful reading rather than science knowledge.
How do you write a number in proper scientific notation?
Answer: Write it as a × 10^n, where 1 ≤ a < 10 and n is a positive or negative integer indicating how many places the decimal point moved.
Explanation
Example: 0.00045 = 4.5 × 10^-4; 45,000 = 4.5 × 10^4.
How can you quickly compare two numbers written in scientific notation?
Answer: First compare the exponents (a larger positive exponent means a larger number); if exponents are equal, compare the coefficients.
Explanation
Example: 3×10^5 < 2×10^6, even though 3 > 2, because the exponent dominates.
Experimental Design Questions
In a passage with multiple experiments, what should you check when comparing them?
Answer: Identify exactly what variable changed between experiments and what stayed the same — this tells you what each experiment is actually testing.
Explanation
Many questions ask you to compare results across experiments based on this single changed variable.
A question asks: 'Which experiment would test whether temperature affects reaction rate?' What must the correct experimental setup include?
Answer: A setup where temperature is the only variable changed between trials, all other factors (concentration, catalyst, etc.) held constant, while reaction rate is measured.
Explanation
This checks that you can recognize proper isolation of a single independent variable.
What is a confounding variable?
Answer: An uncontrolled variable that changes along with the independent variable, making it impossible to tell which one actually caused the change in the dependent variable.
Explanation
Good experimental design controls for confounding variables so results can be trusted.
Conflicting Viewpoints
What is the typical format of a 'Conflicting Viewpoints' passage on the ACT?
Answer: It presents 2–3 scientists or students who are given the same background information but propose different hypotheses or explanations for the same phenomenon.
Explanation
This is one of the three ACT Science passage types, usually with more text and fewer graphs than Data Representation or Research Summaries passages.
What is the best strategy for answering Conflicting Viewpoints questions?
Answer: Identify what the viewpoints agree on, where they diverge, and the specific evidence/reasoning each one relies on.
Explanation
Making a quick comparison chart of agreements/disagreements before answering questions saves time and avoids confusing viewpoints.
True or False: Conflicting Viewpoints questions may ask which viewpoint a new piece of data would support or weaken.
Answer: True.
Explanation
You must apply the new data to each scientist's specific claims/model to see which one it is consistent with.
Core Science Facts: Density
What is the formula for density, and what determines flotation?
Answer: Density = mass / volume (ρ = m/V). An object floats in a fluid if its density is less than the fluid's density, and sinks if greater.
Explanation
ρ = density, m = mass, V = volume; common units g/mL or g/cm³. Example: a block with mass 54 g and volume 20 mL has density 54/20 = 2.7 g/mL, so it would sink in water (density 1.0 g/mL).
If an object's density is greater than the surrounding liquid's density, what happens?
Answer: The object sinks.
Explanation
If density is less than the liquid's, it floats; if equal, it is neutrally buoyant (suspended).
Core Science Facts: pH
What does the pH scale measure, and what is its range?
Answer: It measures how acidic or basic (alkaline) a solution is, on a scale from 0 (most acidic) to 14 (most basic), with 7 being neutral.
Explanation
pH < 7 = acidic, pH = 7 = neutral, pH > 7 = basic.
What happens to pH as the concentration of H+ ions (acid) in a solution increases?
Answer: The pH decreases (solution becomes more acidic).
Explanation
The pH scale is logarithmic — each one-unit decrease in pH represents a 10-fold increase in H+ concentration.
Core Science Facts: Photosynthesis
Write the overall word (and chemical) equation for photosynthesis.
Answer: Carbon dioxide + water + light energy → glucose + oxygen; 6CO2 + 6H2O + light energy → C6H12O6 + 6O2.
Explanation
This occurs in the chloroplasts of plant cells; it converts light energy into chemical energy stored in glucose.
What is the role of chlorophyll in photosynthesis?
Answer: It absorbs light energy (mainly red and blue wavelengths, reflecting green light) to power the light-dependent reactions.
Explanation
This is why most plant leaves appear green — chlorophyll reflects green light rather than absorbing it.
Core Science Facts: Phases of Matter
Name the three common phases of matter and how particle arrangement/energy differs among them.
Answer: Solid: fixed shape and volume, particles tightly packed, low kinetic energy. Liquid: fixed volume but variable shape, particles close but able to move past each other. Gas: no fixed shape or volume, particles far apart with high kinetic energy.
Explanation
Plasma is a fourth state — an ionized gas at very high energy, common in stars and lightning.
What generally happens to particle motion and spacing as temperature increases?
Answer: Particle kinetic energy/motion increases and particles spread farther apart, which can drive phase changes (solid → liquid → gas).
Explanation
This explains melting, boiling, and thermal expansion.
Core Science Facts: Genetics
Distinguish genotype from phenotype.
Answer: Genotype is an organism's genetic makeup (allele combination, e.g., Aa); phenotype is the observable physical trait that results (e.g., brown eyes).
Explanation
Two organisms can have different genotypes (AA vs Aa) but the same phenotype if one allele is dominant.
What is the difference between a dominant and a recessive allele?
Answer: A dominant allele's trait is expressed if at least one copy is present (it masks the recessive allele); a recessive allele's trait is only expressed when two copies (homozygous recessive) are present.
Explanation
Example: if B (brown eyes) is dominant and b (blue eyes) is recessive, genotypes BB and Bb both give brown eyes, while only bb gives blue eyes.
What is a Punnett square used for?
Answer: To predict the probability of possible offspring genotypes and phenotypes from a cross between two parents with known alleles.
Explanation
Example: crossing Bb × Bb gives expected offspring ratio 1 BB : 2 Bb : 1 bb, or 3 brown : 1 blue phenotypically.
Core Science Facts: Newton's Laws
State Newton's First Law (Law of Inertia).
Answer: An object at rest stays at rest, and an object in motion stays in motion at constant velocity, unless acted on by a net external force.
Explanation
This is why passengers lurch forward when a car suddenly stops — their bodies continue moving due to inertia.
State Newton's Second Law and its formula.
Answer: Force equals mass times acceleration: F = m × a.
Explanation
F = net force (Newtons, N), m = mass (kg), a = acceleration (m/s²). Example: a 2 kg object accelerating at 3 m/s² experiences a net force of F = 2 × 3 = 6 N.
State Newton's Third Law.
Answer: For every action force, there is an equal and opposite reaction force.
Explanation
Example: when you push on a wall, the wall pushes back on you with equal force in the opposite direction.
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