Chapter 2.3 Elements and Compounds

-Pure Substances: Elements- simplest form of matter; unique properties; are on the Periodic Table; Au, Ag, Hg, K, N, O, C, etc…Names and Symbols: first letter always Capital Letter + Second letter (if the case) always lower case; Compound- two or more elements combined chemically; CaCl2; H20; H2SO4; CaCO3; HCl, etc…Properties of Compounds: different from the properties of the elements they are coming from.

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Physical and Chemical Change

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Some Examples of Chemical and Physical Changes

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Chapter 2.4 Chemical Reactions Reactants → Products

- new substances

- different physical properties

- different compositions

N° of atoms reactants = N° of atoms products

Mass reactants = Mass products

Old bonds are broken and new bonds are formed. Atoms in the reactants are rearranged to form one or more

different substances.

Evidences of chemical reactions:- 1. Change in the color- 2. Formation of a solid (precipitate)- 3. Formation of a gas (bubbles)- 4. Heat (or a flame) is produced or heat is absorbed

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Chapter 2.4 Chemical equations

Symbols used to represent a chemical equation:

“→” – means the reactants form products; shows which way the reaction goes“+” - separates two or more formulas on the same side“ ∆” - means reactants are heated to start the reaction“ s” - solid substance“ l “ - liquid “ g” - gas“ aq” - aqueous solution – substance dissolved in water

Example: C (s) + O2 (g) → CO2 (g)

Diatomic molecules: - only exist in nature as pairs of atoms, never alone H2 – hydrogen ; O2- oxygen; N2- nitrogen;

F2- fluorine; Cl2- Chlorine ; I2- iodine and Br2- bromine