ACT Science: Outside Knowledge Cheat Sheet

What You Need to Know

ACT Science is mostly a reading-and-graphs test, but a small slice of questions (often in Conflicting Viewpoints or as a standalone “outside information” question) expects you to bring basic high-school science facts.

Your goal: know the few core concepts that show up repeatedly so you can answer quickly without overthinking or importing extra assumptions.

Critical reminder: If the passage gives you the information, use it. Only lean on outside knowledge when the question clearly can’t be answered from the figures/tables/text.

The “outside knowledge” you actually need

Experimental design vocabulary (independent vs dependent variable, control, constants)
Core math/graph tools (slope, proportional reasoning, percent change)
A compact set of biology, chemistry, physics, and Earth/space facts
A few formulas that ACT expects you to recognize (density, speed, Ohm’s law, etc.)

Step-by-Step Breakdown

Use this when you suspect a question needs outside knowledge.

Classify the question
- If it asks for a definition (e.g., “What is pH?”), it’s outside knowledge.
- If it asks you to compute from a graph/table, it’s not outside knowledge.
Check the passage first (fast scan)
- Look for a provided definition, units, or a described relationship.
- If it’s there, don’t bring in outside facts.
Recall the smallest relevant fact
- Use a single principle (e.g., “pH lower means more acidic,” “slope is rate”).
- Avoid chaining multiple assumptions.
Eliminate aggressively
- ACT Science outside-knowledge answers are usually two obviously wrong, then a 50/50.
- Knock out choices that contradict a basic rule (e.g., “electrons are positively charged”).
Sanity-check with units and direction
- Does the answer match the units? Does it match “increase/decrease” logic?

Micro-examples of the process

If asked: “If temperature increases, what happens to average kinetic energy of particles?”
- Outside fact: average kinetic energy increases with temperature.
If asked: “Which variable is the dependent variable?”
- Outside fact: dependent is what you measure on the $y$ -axis (usually).

Key Formulas, Rules & Facts

Core math + graph rules (high-yield)

Rule / Formula	When to use	Notes
$m=\frac{\Delta y}{\Delta x}$	Slope from a graph	Slope = rate (how fast $y$ changes per $x$ )
$y=mx+b$	Linear trends	$b$ is $y$ -intercept (value when $x=0$ )
$\%\,\text{change}=\frac{\text{new}-\text{old}}{\text{old}}\times 100\%$	“Percent increase/decrease”	If old is baseline; watch sign
Direct proportion: $y\propto x$	“As $x$ increases, $y$ increases proportionally”	Straight line through origin
Inverse proportion: $y\propto \frac{1}{x}$	“As $x$ increases, $y$ decreases”	Hyperbola shape

Experimental design essentials

Term	Meaning	ACT trap to avoid
Independent variable	What you change/manipulate	Often on $x$ -axis, but read labels
Dependent variable	What you measure/observe	Often on $y$ -axis
Control group	Baseline for comparison	Not “controlled variable”
Constants	Variables kept the same	Don’t confuse with control group
Trial / replicate	Repeating to reduce random error	Replicates improve reliability
Accuracy vs precision	Close to true vs consistent	Precise can be wrong if biased
Correlation	Variables move together	Correlation $\ne$ causation

Unit + scientific notation basics

Fact	Use	Notes
Metric prefixes: kilo $10^3$ , centi $10^{-2}$ , milli $10^{-3}$ , micro $10^{-6}$ , nano $10^{-9}$	Convert units	Move decimal by powers of 10
$1\,dm^3 = 1000\,cm^3$	Volume conversions	Common lab volume relationship
Scientific notation format	Very large/small numbers	Example: $6.02 \times 10^{23}$
Density units often $g\,cm^{-3}$ or $kg\,m^{-3}$	Match units	Don’t mix systems mid-problem

Physics facts & formulas ACT loves

Formula / Rule	When to use	Notes
$v=\frac{d}{t}$	Speed/velocity	Velocity includes direction, but ACT often treats as speed
$a=\frac{\Delta v}{\Delta t}$	Acceleration	Slope of velocity-time graph
$F=ma$	Net force	If net force $=0$ , velocity constant
Weight near Earth: $W=mg$	Weight vs mass	$g\approx 9.8\,m/s^2$ (often $10\,m/s^2$ )
$P=\frac{F}{A}$	Pressure	Smaller area $\Rightarrow$ greater pressure
$\rho=\frac{m}{V}$	Density	Higher density objects sink in lower density fluid
$KE=\frac{1}{2}mv^2$	Kinetic energy	Depends on speed squared
$PE=mgh$	Gravitational potential energy	Increases with height
Ohm’s law: $V=IR$	Circuits	Increase $R$ lowers $I$ if $V$ fixed
Series resistors: $R_\text{eq}=R_1+R_2+\cdots$	Simple circuits	Total resistance increases
Parallel resistors: $\frac{1}{R_\text{eq}}=\frac{1}{R_1}+\frac{1}{R_2}+\cdots$	Simple circuits	Total resistance decreases

Waves & light (conceptual)

Wave speed relationship: $v=f\lambda$ (if needed conceptually)
- If $v$ constant: higher $f$ means smaller $\lambda$ .
Electromagnetic spectrum order (low $f$ to high $f$ ): radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
Refraction: light bends toward the normal entering a slower medium; away entering faster.

Chemistry facts ACT expects

Fact / Rule	Use	Notes
Atoms: protons $+$ , neutrons $0$ , electrons $-$	Structure questions	Neutral atom: protons = electrons
Ions form by gaining/losing electrons	Charges	Gain e $^-$ : negative ion; lose e $^-$ : positive ion
Periodic trends (general)	Compare elements	Across a period: atomic radius decreases, electronegativity increases
Conservation of mass	Reactions	Balance atoms, not just molecules
Acids vs bases	pH questions	Acid: more $H^+$ ; base: more $OH^-$
pH scale	Acidity	Lower pH = more acidic; higher pH = more basic
pH change meaning	Strength change	Each pH unit is a factor of $10$ in $[H^+]$
Solutions	Concentration intuition	More solute per volume $\Rightarrow$ more concentrated

High-yield pH fact

If pH drops by $2$ units, $[H^+]$ increases by $10^2=100$ times.

Biology facts ACT expects

Topic	Key outside knowledge	What ACT asks
Cell types	Prokaryotes lack nucleus; eukaryotes have nucleus	Identify cell type from features
Organelles	Mitochondria make ATP; chloroplasts do photosynthesis; ribosomes make proteins	“Where does X happen?”
DNA/RNA	DNA bases $A,T,C,G$ ; RNA uses $U$ instead of $T$	Base pairing logic
Protein synthesis	DNA $\rightarrow$ RNA $\rightarrow$ protein	Direction of information flow
Enzymes	Catalysts that lower activation energy	Rate changes with temperature/pH
Photosynthesis	Uses light to make sugars	Occurs in chloroplasts
Cellular respiration	Releases energy from glucose	Occurs in mitochondria
Natural selection	Traits that increase survival/reproduction become more common	Not “organisms choose to adapt”
Genetics	Dominant vs recessive alleles	Simple Punnett reasoning

Two equations that sometimes help (mostly conceptual)

Photosynthesis: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
Respiration: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$

Earth/space essentials

Seasons: caused mainly by Earth’s tilt, not distance from the Sun.
Moon phases: depend on Sun-Earth-Moon geometry (not Earth’s shadow except lunar eclipse).
Plate tectonics: earthquakes/volcanoes cluster near plate boundaries; seafloor spreading at mid-ocean ridges.
Rock types: igneous (cooled magma/lava), sedimentary (compacted layers), metamorphic (heat/pressure changed).

Examples & Applications

Example 1: Slope as rate (graph reasoning + outside knowledge)

A graph shows $\text{Temperature (}^\circ C\text{)}$ on the $y$ -axis versus $\text{Time (min)}$ on the $x$ -axis. Temperature rises from $20$ to $50$ over $6$ minutes.

Setup: $m=\frac{\Delta y}{\Delta x}=\frac{50-20}{6}=\frac{30}{6}=5$
Insight: Slope means the sample warms at $5\,^\circ C\,\text{min}^{-1}$ .

Example 2: Density to predict floating/sinking

An object has mass $30\,g$ and volume $10\,cm^3$ .

Setup: $\rho=\frac{m}{V}=\frac{30}{10}=3\,g\,cm^{-3}$
Insight: Water is about $1\,g\,cm^{-3}$ , so $3\,g\,cm^{-3}$ is denser than water $\Rightarrow$ it sinks.

Example 3: pH change as a power of 10

Solution A has pH $5$ and solution B has pH $3$ . How many times greater is $[H^+]$ in B?

Setup: Difference $=2$ pH units $\Rightarrow$ factor $10^2$
Answer: $[H^+]$ in B is $100$ times greater.

Example 4: Series vs parallel intuition (often conceptual)

Two identical resistors are added to a circuit.

Series: $R_\text{eq}=R+R=2R$ so current decreases if voltage is fixed (from $V=IR$ ).
Parallel: $\frac{1}{R_\text{eq}}=\frac{1}{R}+\frac{1}{R}=\frac{2}{R}$ so $R_\text{eq}=\frac{R}{2}$ and current increases if voltage is fixed.

Common Mistakes & Traps

Using outside knowledge when the passage already defines it
- What goes wrong: You override the experiment’s definitions (units, conditions, labels).
- Fix: Treat the passage like the “textbook” for that question.
Mixing up independent and dependent variables
- What goes wrong: You assume $x$ is always independent even when axes are swapped.
- Fix: Independent = what is changed; dependent = what responds/measured.
Forgetting slope is “per x-unit”
- What goes wrong: You compute $\Delta y$ but ignore $\Delta x$ .
- Fix: Always write $\frac{\Delta y}{\Delta x}$ with units.
Ignoring units and prefixes
- What goes wrong: You compare values without converting (milli vs micro, etc.).
- Fix: Convert everything to one scale before comparing.
Treating correlation as causation
- What goes wrong: You pick an answer claiming one variable causes another just because both change.
- Fix: Look for experimental control/manipulation before claiming causation.
pH direction errors
- What goes wrong: You think higher pH means more acidic.
- Fix: Lower pH = more acidic; each step is a factor of $10$ .
Mass vs weight confusion
- What goes wrong: You treat grams and newtons interchangeably.
- Fix: Mass in $kg$ ; weight is force: $W=mg$ in $N$ .
Series/parallel flip
- What goes wrong: You say adding resistors always increases resistance.
- Fix: Series increases $R$ ; parallel decreases $R$ .

Memory Aids & Quick Tricks

Trick / Mnemonic	What it helps you remember	When to use
IV DV: “I Vary, D is Data”	Independent vs dependent variable	Experiment questions
OIL RIG: Oxidation Is Loss, Reduction Is Gain (electrons)	Redox direction	Chemistry outside-knowledge
pH is backwards	Lower pH = higher $[H^+]$	Acids/bases
Direct vs inverse: “Together vs opposite”	$y\propto x$ vs $y\propto \frac{1}{x}$	Trend questions
EM spectrum: Radio, Micro, IR, Visible, UV, X, Gamma	Increasing frequency/energy	Light questions
SERIES = SUM	$R_\text{eq}$ adds in series	Circuit questions
Parallel is ‘less than the smallest’	$R_\text{eq}$ in parallel is below smallest branch	Quick elimination
Density: heavy-for-size sinks	Higher $\rho$ sinks in lower $\rho$ fluid	Buoyancy questions

Quick Review Checklist

You only use outside knowledge when the passage can’t answer it.
You can define: independent, dependent, control, constants, accuracy, precision.
You can compute: $m=\frac{\Delta y}{\Delta x}$ , $\%\,\text{change}$ , $\rho=\frac{m}{V}$ .
You remember: lower pH = more acidic; pH is powers of 10.
You know: electrons are negative; ions form by gaining/losing electrons.
You know: series resistors add; parallel resistors reduce total resistance.
You can apply: $v=\frac{d}{t}$ , $F=ma$ , $P=\frac{F}{A}$ , $KE=\frac{1}{2}mv^2$ , $PE=mgh$ .
You remember big Earth/space traps: tilt causes seasons, phases are geometry (not Earth’s shadow).

You don’t need more facts, you need faster recognition and cleaner elimination.