1. High School Physical Science Week 10 Chemical Reactions and the Law of Conservation of Mass

2. Chemical Reactions A chemical reaction is a process by which substances change to produce new substances with new chemical and physical properties. Signs that indicate a chemical reaction has taken place are a color change, formation of a gas or a solid, and production of energy. During a reaction, bonds are broken, atoms are rearranged, and new bonds are formed.

3. Chemical Formulas A chemical formula uses symbols and subscripts to describe the makeup of a compound. Chemical formulas can often be written from the names of covalent and ionic compounds. A chemical equation uses chemical formulas, chemical symbols, and coefficients to describe a reaction.

4. Balancing Equations Balancing an equation requires that the same numbers and kinds of atoms be on each side of the equation. A balanced equation illustrates the law of conservation of mass: mass is neither created nor destroyed during ordinary physical and chemical changes.

5. Types of Reactions A synthesis reaction is a reaction in which two or more substances combine to form a compound. A decomposition reaction is a reaction in which a compound breaks down to form two or more simpler substances. A single-displacement reaction is a reaction in which an element takes the place of another element that is part of a compound. A double-displacement reaction is a reaction in which ions in two compounds exchange places.

6. Energy & Reactions Energy is given off in exothermic reactions. Energy is used in an endothermic reaction. The law of conservation of energy states that energy is neither created nor destroyed. Activation energy is the energy needed for a reaction to occur. The rate of a chemical reaction is affected by temperature, concentration, surface area, and the presence of an inhibitor or catalyst.