Unit 2

In the previous slide you will see an example of burning a piece of wood. The atoms in the wood did not dissapear from the Earth, they only changed into a new form - carbon dioxide, water vapor and ash

The Law of Conservation of Energy tells us that energy is not created or destroyed, it only changes form. Most energy is released from a chemical reaction in the form of heat.

A chemical property is a property that tells us something about the reactivity ( or lack of reactivity ) for a given substance

A physical property is a property that involves all other characteristics of a substance (such as its color, density, conductivity, odor, state of matter)

• methane is highly flammable
• methane will not react with carbon dioxide
• copper turns green when mixed with an acid
• sulfur forms a rotton egg odor when burned
• sodium is a very reactive metal
• sodium is explosive in water

Exothermic reactions involve a net release of energy. They are putting energy (heat usually) out into the environment and are hot to the touch. Most reactions are exothermic to some extent. (Many are barely exothermic and as a result, it is not obvious)

Examples of exothermic reactions: burning methane, exploding sodium, etc.

Endothermic reactions involve a net absorption of energy. They take in energy from the environment and are cold to the touch. Not as common as exothermic. Example of endothermic: photosynthesis (net absorption of energy/sunlight)