Chemical Reactions and Changes

Physical Change: A change that does not affect the identity of a substance; no chemical bonds are broken or formed. Examples include phase changes of water ( $H_2O$ ) such as boiling, freezing, or melting.
Chemical Change: A change where the identity of the substances involved changes because chemical bonds are broken and/or formed. An example is the reaction of sodium ( $Na$ ) and chlorine ( $Cl_2$ ) to form sodium chloride ( $NaCl$ ).
Chemical Reaction: The process by which chemical change occurs, resulting in the transformation of one or more substances into different substances.

Color Change: Examples include iron turning into rust or clear air turning into brownish smog.
Smell Change: Examples include milk turning sour or wine smelling like vinegar.
Energy Change:
- Exothermic Reaction: A reaction that releases energy, often detectable as an increase in temperature or the emission of light (e.g., a burning candle).
- Endothermic Reaction: A reaction that absorbs energy, often detectable by a drop in temperature (e.g., mixing vinegar and baking soda) or used in processes like photosynthesis.
Gas Release: Indicated by fizzing or bubbling, such as $Alka-Seltzer$ tablets in water.
Precipitate Formation: The formation of a solid that separates out of a solution, such as the blue gelatinous copper(II) hydroxide ( $Cu(OH)_2$ ) formed from mixing sodium hydroxide and copper(II) sulfate.

Chemical Equation: A symbolic description of a chemical reaction.
Components:
- Reactants: Original substances on the left side of the equation.
- Products: New substances formed on the right side of the arrow.
- State Symbols: $(s)$ for solid, $(l)$ for liquid, $(g)$ for gas, and $(aq)$ for aqueous solution (dissolved in water).
- Chemical Coefficient: The number in front of a compound representing the whole-number ratio of reactants and products.
Law of Conservation of Mass: Matter is neither created nor destroyed; a balanced equation must have the same number of atoms for each element on both sides of the arrow.
Diatomic Elements: Elements that exist naturally in pairs: $H_2$ , $N_2$ , $O_2$ , $F_2$ , $Cl_2$ , $Br_2$ , and $I_2$ (Mnemonic: $BrINClHOF$ ).
Catalyst: A substance that speeds up a reaction without being consumed. It is written above the reaction arrow (e.g., $KI$ in the decomposition of $H_2O_2$ ).

Virgin Earth Challenge: A $25,000,000 award announced by Sir Richard Branson for technology that removes greenhouse gases from the atmosphere for at least 10 years.
Greenhouse Gases: Carbon dioxide ( $CO_2$ ) and methane ( $CH_4$ ) trap heat near the Earth.
Carbon Cycle: The movement of carbon through photosynthesis, combustion, decay, and respiration.
Atmospheric Trends:
- Historical $CO_2$ levels: $200$ to $300\,ppm$ .
- Recent average concentration: approximately $356\,ppm$ , increasing at $1.5\,ppm$ per year.
- Global temperature: Increased by $0.5^{\circ}C$ in the past century; predicted to rise by $4-5^{\circ}C$ if greenhouse gas levels are not managed.