ACT Science Vocabulary

Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Taking the ACT? Check out this lesson to brush up on some science vocabulary that will help you review. You can expect to be tested on these terms, as well as many others.

Prepping for the ACT

So you're getting ready to take the ACT. That's quite a task! One important part of this test is the science section, but what's nice about it is that you don't have to know everything to pass it. In fact, with some basic knowledge and understanding of some terms you should have no trouble at all.

You may even be provided with definitions and explanations in the questions, but let's go ahead and go through some words you might need to know.

Temperature & Energy Transfer

Let's start by talking about temperature, which is a measure of the amount of heat an object has. Temperature is measured on three scales: Fahrenheit, Celsius, and Kelvin, all named after different people. Water freezes at 32° Fahrenheit, at 100° Celsius, and at 273 Kelvin (no degrees for this scale!). It boils at 212° F, 100° C, and 373 K. The lowest possible temperature is 0 K, at which point it is believed that matter has no energy left and stops moving completely.

This leads us nicely into our next topic, energy flow. When we add or remove energy to a substance we can change its phase (solid, liquid, gas), and there are three ways to this:

  1. We can transfer energy through direct contact or conduction, like when you touch a hot pan on the stove.
  2. Convection currents transfer heat with fluid motion, like when you see the bubbles of gas rising up in the pot of boiling water on the stove.
  3. Radiation transfers heat in the form of waves or particles, which you experience when you feel heat coming off a campfire in front of you.

Biology & Chemistry Terms


Let's move on to biology. For the ACT you'll need to know about the basic structure of molecules as well as the components of cells, the most basic structure of life. Cells have the following components:

  • a cell membrane that holds everything together,
  • a nucleus that holds the cell's DNA,
  • mitochondria, where chemical reactions occur that produce energy for cells,
  • lysosomes, which are responsible for breaking down dying cells, food particles, and more.

In plant cells we also find chloroplasts, cells where photosynthesis occurs. This is the process of converting light energy into usable energy or 'food' for the plant. Chloroplasts are green because they contain chlorophyll which is the pigment that captures light during photosynthesis.

Plant cells have chloroplasts that allow them to photosynthesize.
plant cell diagram

Cells may be the smallest unit of life, but atoms are the smallest unit of matter. They are made up of:

  • protons - positively charged particles in the nucleus
  • neutrons - neutrally charged particles in the nucleus
  • electrons - tiny, negatively charged particles orbiting the nucleus.

If an atom gains or loses an electron we call it an ion, and if it gains/loses a neutron we call it an isotope. Atoms will never gain or lose protons because that will turn them into a different element all together!


Elements are simply specific types of atoms, such as carbon, oxygen, or hydrogen. All things are made of some type of element and when we bond them together we get compounds. In our bodies these are sugars like glucose, a great source of energy for our bodies, but also:

  • proteins - made of amino acids and perform a variety of different functions in our cells
  • fats - essential for protecting organs and providing energy
  • nucleic acids - the building blocks of DNA, our genetic material.

pH Scale

Finally, we can't forget the pH scale, which runs from 1-14 and tells us how acidic or basic something is. Acids are low on the pH scale (like orange juice and vinegar). Bases are high on the scale (bleach and baking soda). The neutral mid-point of the scale lies at 7 (water).

The pH scale describes how acidic or basic substances are.
ph scale

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