Identifying Physical and Chemical Changes & Classifying Matter

Comprehensive vocabulary flashcards covering Unit 3 Lesson 3 on physical and chemical changes, reaction rates, the Law of Conservation of Mass, and the classification of matter (elements, compounds, and mixtures).

Physical Change

A change that alters the appearance of a substance without transforming it into another substance, and which can be reversed (e.g., bending, crushing, cutting, and changes of state).

Sublimation

A physical change in state from a solid directly to a gas without passing through the liquid state (e.g., dry ice).

Chemical Change

A change that produces a new substance and cannot be reversed; also known as a chemical reaction (e.g., burning wood to ashes, metal rusting).

Reactant

The original substance that undergoes a chemical change during a chemical reaction.

Product

The new substance that is created as a result of a chemical reaction.

Law of Conservation of Mass

The scientific principle stating that matter is not created or destroyed in a physical or chemical change; atoms are never lost or gained, only rearranged (e.g., $2H_2 + O_2 \rightarrow 2H_2O$, or mixing $450\,g$ of milk with $15\,g$ of chocolate mix yielding $465\,g$ of chocolate milk).

Exothermic Change

A chemical energy change in which energy is released, causing an increase in temperature that you feel as heat.

Endothermic Change

A chemical energy change in which energy is absorbed, resulting in a temperature decrease that you feel as cold (e.g., an instant cold pack).

Precipitate

A solid substance that forms from liquids and separates out of a solution during a chemical reaction, often settling to the bottom (e.g., curdling milk).

Surface Area (Reaction Rate Factor)

The total amount of exposed area on a substance; greater surface area exposes more particles and causes a chemical reaction to occur faster.

Concentration

The amount of a substance in a given volume; having a greater concentration of reactant particles increases the reaction rate.

Catalyst

A substance that makes chemical reactions proceed faster by lowering the amount of energy needed.

Inhibitor

A substance that makes a chemical reaction happen slower (frequently used in food preservation).

Temperature (Reaction Rate Factor)

Increasing temperature makes particles move faster and collide more frequently, speeding up reactions; decreasing temperature slows particle motion and slows down reactions.

Element

A pure substance consisting of one single type of matter that cannot be broken down chemically (e.g., aluminum, $Al$).

Compound

A pure substance formed from two or more different elements that are chemically combined (bonded) together (e.g., sugar, $C_{12}H_{22}O_{11}$, or baking soda, $NaHCO_3$).

Mixture

Two or more different substances located together in the same place that are not chemically combined and can be separated by physical methods.

Molecule

Any group of two or more atoms chemically bonded together, which can be atoms of the same element (such as oxygen gas, $O_2$) or different elements (such as water, $H_2O$).

Heterogeneous Mixture

A mixture with non-uniform composition where the different components remain distinct and can be visibly differentiated (e.g., gravel, bean soup).

Homogeneous Mixture

A mixture that has a uniform composition throughout, where individual components are evenly blended (e.g., milk, coffee).