DC CAPE 2.0 Science Grade 8 key terms

The smallest unit of an element that still keeps the properties of that element. It is made of protons and neutrons in a central nucleus with electrons around it.

A pure substance made of only one kind of atom. Each element has a unique number of protons and cannot be broken into simpler substances by ordinary chemical means.

A substance formed when atoms of two or more different elements bond together in fixed proportions. Its properties differ from those of the elements that make it up.

Two or more atoms held together by chemical bonds. A molecule can be made of the same kind of atom, like oxygen gas, or of different kinds, like water.

A process in which bonds between atoms break and new bonds form, turning starting substances into different substances. Atoms are rearranged but never created or destroyed.

A starting substance that is used up during a chemical reaction. Reactants appear on the left side of a chemical equation and are changed into products.

A new substance formed during a chemical reaction. Products appear on the right side of a chemical equation and have different properties from the reactants.

The rule that the total mass of matter does not change during a chemical reaction. The mass of the products equals the mass of the reactants because atoms are only rearranged.

A substance made by humans through chemical processes rather than taken directly from nature. Examples include many plastics and medicines, which are designed for specific useful properties.

A push or a pull that acts on an object and can change its motion or shape. Force has both size and direction and is measured in newtons.

The single overall force that results from combining all forces acting on an object. When forces balance, the net force is zero and motion does not change.

Two or more forces acting on an object that cancel each other out, giving a net force of zero. An object at rest or moving at constant speed has balanced forces acting on it.

Forces on an object that do not cancel out, producing a nonzero net force. These forces cause an object to speed up, slow down, or change direction.

A force that resists motion when two surfaces rub against each other. It acts in the direction opposite to the motion and produces heat.

How fast an object moves, calculated as distance traveled divided by time. Speed tells only the size of the motion, not the direction.

Speed in a given direction. Because direction is included, an object can move at constant speed but still change velocity by turning.

The rate at which velocity changes over time. An object accelerates when it speeds up, slows down, or changes direction, and the change can be positive or negative.

The energy an object has because of its motion. It increases with greater mass and with greater speed, and it is transferred when moving objects collide.

Stored energy that depends on an object's position or condition, such as height above the ground or being stretched. It can be converted into kinetic energy when released.

The movement of energy from one object or system to another, for example through motion, heat, light, sound, or electric current. Energy is not created or destroyed in the process.

The total energy of the moving particles in a substance. It depends on how fast the particles move and how many there are, and it flows from warmer to cooler objects.

A repeating disturbance that carries energy from place to place without carrying matter along with it. Waves can travel through a material or, for some types, through empty space.

The distance between one point on a wave and the same point on the next wave, such as from crest to crest. It is often measured in meters.

The number of complete waves that pass a point in one second, measured in hertz. A higher frequency means more waves pass each second.

The height of a wave measured from its resting position to its highest point. A larger amplitude carries more energy and, for sound, makes a louder sound.

The bouncing of a wave off a surface it cannot pass through. Light reflecting from an object is what allows us to see it.

The bending of a wave as it passes from one material into another where it travels at a different speed. It explains why objects look bent in water.

The smallest unit of life that can carry out all the basic activities of living things. Some organisms are a single cell, while others are made of many cells.

A specialized structure inside a cell that performs a particular job. Examples include the nucleus, which holds genetic material, and mitochondria, which release energy from food.

The thin outer covering of a cell that separates it from its surroundings. It controls which materials enter and leave the cell.

The process by which plants and other organisms use sunlight, water, and carbon dioxide to make sugar and release oxygen. It stores energy from sunlight in chemical form.

The process cells use to release energy from sugar, using oxygen and producing carbon dioxide and water. This energy powers the activities of the organism.

All the living organisms in an area together with the nonliving parts of their environment, such as water, soil, and air. These parts interact and depend on one another.

A model showing how energy and matter move through an ecosystem as organisms eat one another. It is made of overlapping food chains and shows many feeding relationships.

An organism that makes its own food, usually through photosynthesis, using energy from sunlight. Producers are the base of food webs and supply energy to other organisms.

An organism that cannot make its own food and must eat other organisms to get energy. Consumers may eat producers or other consumers.

All the individuals of the same species living in the same area at the same time. Populations change in size because of births, deaths, and movement.

A section of DNA that carries instructions for a trait. Genes are passed from parents to offspring and come in different versions that can produce different traits.

One of the different versions of a gene. An organism inherits two alleles for each gene, one from each parent, and these alleles can be dominant or recessive.

A trait that appears in an organism when at least one dominant allele is present. The dominant allele's effect hides the effect of a recessive allele.

A trait that appears only when an organism has two recessive alleles for that gene. Its effect is hidden whenever a dominant allele is also present.

A characteristic of an organism, such as eye color or plant height. Traits are influenced by genes, by the environment, or by both working together.

A change in the genetic material of an organism. Mutations can be harmful, helpful, or have no effect, and they are a source of variation in traits.

A trait that helps an organism survive and reproduce in its environment. Adaptations develop over many generations through natural selection.

The process by which organisms with helpful traits tend to survive and reproduce more than others. Over generations this causes helpful traits to become more common.

The attractive force that pulls objects toward one another. On Earth it pulls objects downward, and across space it keeps planets in orbit around the Sun.

The curved path one object follows as it travels around another because of gravity. Earth orbits the Sun, and the Moon orbits Earth.

The Sun together with all the objects that orbit it, including planets, moons, and asteroids. Gravity holds these objects in their paths around the Sun.

The ongoing set of processes that change rock from one type to another over time. Heat, pressure, melting, cooling, weathering, and erosion drive these changes.

The process that breaks down rock into smaller pieces where it sits. Water, wind, ice, and plant roots can all cause weathering.

The movement of weathered rock and soil from one place to another by water, wind, ice, or gravity. Erosion shapes landforms over long periods of time.