20 terms the ACT Science section expects you to already know when a passage turns to chemistry and matter — each with a definition, an example sentence, and a memory cue.
ACT Science does not test whether you can define these words — it tests whether you can read a figure fast enough, and an unfamiliar term is what stops you. The section gives you well under a minute per question, so the goal here is recognition, not recall: you want to meet these terms in a question stem and know immediately what they point to.
Read the definition, then the example, then the memory cue. Come back a few days later and cover the definitions — recalling a term cold is what fixes it, and rereading it is not.
The 20 terms
Alphabetical. Everything in the Chemistry & Matter guide.
01
amalgamateverb
to combine or unite to form one structure or organization.
The university will amalgamate three small departments into one interdisciplinary research center, making the advanced word understandable through its consequences and surrounding details.
Memory cueAmalgamate separate pieces by melting them into one unified structure.
02
amorphousadjective
Without a clearly defined shape or form; vague and unorganized
The committee's goals remained amorphous , lacking any concrete plan of action.
Memory cueAn amorphous blob has no clearly defined shape.
03
atomnoun
The smallest unit of an element that retains that element's chemical identity.
Each carbon atom contains six protons, although different carbon isotopes contain different numbers of neutrons.
Memory cueAn atom is one tiny identity card for an element, defined by its proton count.
04
brinenoun
Water saturated or strongly impregnated with salt
Life in the deep ocean brine pools has adapted to salinity levels that would kill most organisms.
Memory cueBrine is water so saturated with salt that crystals nearly form.
05
catalystnoun
A person or thing that precipitates an event or change
The economic downturn served as a catalyst for sweeping regulatory reform.
Memory cueA catalyst is the spark that starts change without becoming the final product.
06
coalesceverb
To come together and form one mass or whole; to unite
Several grassroots movements coalesced into a single powerful political party.
Memory cueSeparate droplets coalesce into one larger pool.
07
componentadjective / noun
A constituent part; one of the individual parts that make up a larger whole
Trust is a critical component of any successful negotiation between parties with a history of conflict.
Memory cueA component is one piece fitted into a larger working whole.
08
compoundadjective
made up of multiple parts; to make something worse
The compound interest on the loan added up quickly, giving observers a clearer basis for judging its extent or relation to the larger whole.
Memory cueA compound joins multiple blocks; as a verb, it adds another block to make a problem worse.
09
condenseverb
to make something more compact or concentrated; to shorten or compress.
Editors had to condense the lengthy report into a concise summary for the public meeting.
Memory cueCondense a long message by compressing it into a smaller, concentrated form.
10
distilledadjective
Purified by the process of evaporation and condensation
The author distilled decades of research into a slim volume accessible to any educated reader.
Memory cueDistilled liquid emerges purified after evaporation and condensation.
11
flotationnoun
The action of floating on or in a liquid; an experiment testing buoyancy
The engineers tested the raft's flotation capacity before allowing passengers aboard for the river crossing.
Memory cueFlotation tests whether an object can stay floating on liquid.
12
inertadjective
Lacking the power to move or act; having no effect
Noble gases are chemically inert , rarely reacting with other elements under normal conditions.
Memory cueAn inert object remains still because it lacks the strength to move.
13
moleculenoun
A group of two or more atoms held together by chemical bonds.
One water molecule contains two hydrogen atoms bonded together with a single oxygen atom.
Memory cueA molecule is an atom team whose members are fastened together by chemical bonds.
14
pigmentnoun
A substance used for coloring; the natural coloring matter of animal or plant tissue
The ancient Egyptians extracted blue pigment from lapis lazuli, a stone so rare it was valued more highly than gold.
Memory cuePigment supplies the coloring matter in paint, skin, leaves, or fabric.
15
reactverb
to respond or behave in a particular way in response to something
How did your parents react when you told them the news, setting in motion a chain of events that changed what happened afterward.
Memory cueA reaction arrow springs out immediately after a trigger presses the button.
16
solutenoun
The substance that becomes dissolved when a solution forms.
Sugar was the solute because it dissolved completely in the larger quantity of warm water.
Memory cueThe solute is the smaller substance sent into solution to be dissolved.
17
solutionnoun
A uniform mixture in which one or more substances are dissolved in another substance.
The clear salt solution remained uniform because dissolved ions were distributed throughout the water.
Memory cueIn a solution, dissolved particles spread so evenly that every sample has the same composition.
18
solventnoun
The substance that dissolves a solute and usually makes up most of a solution.
Water served as the solvent, surrounding and separating the ions from the added salt crystals.
Memory cueThe solvent solves the mixing job by dissolving the solute within it.
19
sulfur compoundsnoun phrase
Chemical substances containing sulfur combined with other elements
Volcanic emissions rich in sulfur compounds can lower global temperatures by reflecting sunlight back into space.
Memory cueSulfur compounds join sulfur atoms chemically with other elements.
20
valencenoun (used in testing/psychology sense)
The positive or negative quality/direction of something (e.g., positive valence = positive meaning)
The survey measured the emotional valence of participants' responses, distinguishing positive reactions from negative ones.
Memory cueValence gives an idea its positive or negative emotional charge.
Knowing the words is step one — reading the figure is step two.
Test Ninjas puts these terms back into real Science passages — full-length ACT practice tests, a complete Science course, and an explanation on every question.
