Physical science

Reactants

The starting substances in a chemical reaction. They are found on the left side of the chemical equation. During the reaction, the chemical bonds in the reactants are broken to form new substances.

Yields (The Arrow →)

The arrow in a chemical equation that separates the reactants from the products. It means "produces," "forms," or "yields," and indicates the direction in which the chemical reaction is occurring.

Products

The new substances formed as a result of a chemical reaction. They are found on the right side of the chemical equation. They have different chemical and physical properties than the original reactants.

Law of Conservation of Mass/Matter

Matter cannot be created or destroyed in a closed system; it only changes form. Therefore, the total mass of the reactants must equal the total mass of the products. This is why chemical equations must be balanced!

Subscripts

The small numbers written to the lower right of a chemical symbol (e.g., the "2" in H₂O). They indicate the number of atoms of that specific element in a molecule. Crucial rule: You can never change subscripts when balancing a chemical equation, as it would change the identity of the substance.

Coefficients

The large numbers placed in front of a chemical formula (e.g., the "2" in 2H₂O). They indicate the number of molecules or moles of that substance involved in the reaction. You only change coefficients to balance a chemical equation.

Signs of a Chemical Reaction

Observable evidence that a chemical change has occurred. The five main signs are:

Color change: Unexpected change in color.

Temperature change: Heat is absorbed or released.

Gas production: Bubbles or fizzing are observed.

Precipitate formation: A solid forms from mixing two liquids.

Emission of light/odor: A new smell or a glow/flash is produced.

Reaction Speeds

How fast or slow a chemical reaction takes place. It can be sped up or slowed down by altering four main factors:

Temperature: Higher temp = faster movement = more collisions.

Concentration: More particles in a given space = more collisions.

Surface Area: Crushing a solid into powder increases the area available to react.

Catalysts/Inhibitors: Catalysts speed up reactions without being consumed; inhibitors slow them down.

State Symbols

Abbreviations used in equations to show the physical state of a substance:

(s) = Solid

(l) = Liquid (usually pure substances like molten metals or liquid water)

(g) = Gas

(aq) = Aqueous (meaning the substance is dissolved in water)

Exothermic Reaction

A reaction that releases energy, usually in the form of heat or light. Because energy is leaving the system and entering the surroundings, the container feels hot to the touch. (Example: a campfire or a chemical hand warmer).

Endothermic Reaction

A reaction that absorbs energy (usually heat) from its surroundings. Because energy is being pulled into the system, the container feels cold to the touch. (Example: photosynthesis or a chemical ice pack).

Chemical Bonds

The attractive forces that hold atoms together to form compounds. The two main types covered in physical science are:

Ionic Bonds: Formed when electrons are transferred from a metal to a nonmetal, creating oppositely charged ions that attract.

Covalent Bonds: Formed when electrons are shared between two nonmetals.

Synthesis Reaction

A reaction where two or more simple substances combine to form a single, more complex product.

General Pattern: A + B → AB

Decomposition Reaction

A reaction where a single complex compound breaks down into two or more simpler substances. It is the opposite of synthesis.

General Pattern: AB → A + B

Single Replacement (or Displacement) Reaction

A reaction where one uncombined element replaces another element that is part of a compound.

General Pattern: A + BC → AC + B

Double Replacement (or Displacement) Reaction

A reaction where the positive ions (cations) of two different ionic compounds switch places to form two completely new compounds. Often results in a precipitate.

General Pattern: AB + CD → AD + CB

Combustion Reaction

A reaction where a substance (usually a hydrocarbon) reacts rapidly with Oxygen (O₂), releasing energy in the form of heat and light.

Key Identifier: The products are almost always Carbon Dioxide (CO₂) and Water (H₂O).

General Pattern: CₓHᵧ + O₂ → CO₂ + H₂O

What type of reaction is this? 2H₂ + O₂ → 2H₂O

Synthesis. Why? Two distinct elements (Hydrogen and Oxygen) are combining to form a single, more complex compound (Water).

What type of reaction is this? CH₄ + 2O₂ → CO₂ + 2H₂O

Combustion. Why? A hydrocarbon (CH₄) is reacting with Oxygen (O₂) to produce Carbon Dioxide (CO₂) and Water (H₂O).

What type of reaction is this? Zn + 2HCl → ZnCl₂ + H₂

Single Replacement. Why? A single element (Zinc) swaps places with the Hydrogen in the Hydrochloric Acid compound, leaving Hydrogen all by itself.

What type of reaction is this? 2H₂O₂ → 2H₂O + O₂

Decomposition. Why? One single reactant (Hydrogen Peroxide) is breaking apart into two simpler products (Water and Oxygen).

What type of reaction is this? AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

Double Replacement. Why? The Silver (Ag) and Sodium (Na) swap partners. You can also tell because two aqueous compounds react to form a solid precipitate (AgCl).