Chemical vs. Physical Changes/Properties Chemical changes involve the making/breaking of chemical bonds. Chemical processes result in the formation of new substances. –These new substances formed have properties that are distinguishable from the initial substance or substances. Production of heat or light, formation of a gas, and formation of a precipitate and/or a color change are possible evidences that a chemical change has occurred. gas produced

Chemical Structure of Water

5 General Types of Chemical Reactions… (There are others.)

Chemical Reactions Involving Water 2 Na + 2 H 2 O  2 NaOH + H 2

For physical changes, the forces being overcome are weaker intermolecular interactions, the forces between molecules. The physical properties of a substance generally depend on the spacing between the particles (atoms, molecules, ions) that make up the substance and the forces of attraction among them. –Example: Dissolving sugar or salt into water makes the particles of the solid smaller, and intermolecular bonds form between the water and these particles. –Example: Breaking all of the intermolecular forces between the particles of a solid would make the substance turn into a gas.

How Salt Dissolves in Water

Physical Properties of Water

Cartoon explaining chemical vs. physical changes

Observations of physical properties are always made on collections of atoms, because some properties could actually change just based on the amount of atoms present: –Example: The color of the element sulfur is not yellow when you are dealing with just a handful of atoms. –Example: One single atom of silver would not have a “melting point”. Mass is always conserved during chemical or physical changes. –Law of Conservation of Mass: proposed by Antoine Lavoisier in –Nonradioactive atoms are neither lost nor gained as they cycle among land, water, atmosphere, and living organisms.

Conservation of Mass CaCl 2 + Na 2 SO 4  CaSO 4 + 2NaCl mass before = mass after # atoms before = # atoms after