Ch-Ch-Ch-Ch-Changes!

WHAT MATTERS?
Matter is anything that has mass and takes up space. It undergoes many changes. In chemistry, nearly all changes can be broken down into three categories:

1. Physical changes
2. Chemical changes
3. Nuclear changes

PHYSICAL CHANGES

Physical changes are changes from one state (solid/liquid/gas/aqueous) to another without a change in chemical composition. Other examples of physical changes include crushing, tearing, boiling water, cutting wood, and smashing cars. In any physical change, the basic chemical identity of the substances is unchanged. In other words, no new substances are formed.

 

Melting: solid → liquid
Freezing/Solidification: liquid → solid
Condensation: gas → liquid
Evaporation: liquid → gas
Sublimation/Deposition: gas → solid
Sublimation: solid → gas

The following graph demonstrates the process of melting that water follows:

 

 

And now here's a song that you can use to help you remember physical changes:

 

CHEMICAL CHANGES

Chemical changes are changes where bonds are broken and formed between different atoms. New substances are created, and the properties of matter change (conductivity, acidity, color, etc.). Examples of chemical changes include iron rusting, burning wood, and digesting food.




Iron rusting is a chemical change, because oxygen bonds to the iron atoms (which where not chemically bonded to anything before) to create an oxide of iron.

Frying an egg is a chemical change, because the chemical composition of the egg changes. This cannot be reversed easily.

2 NI₃ (s) → N₂ (g) + 3 I₂ (g) + energy

The decomposition of nitrogen triiodide is a chemical reaction. It is an exothermic reaction. 
 We witnessed an astonishing chemical reaction today in chemistry class. Mr. Doktor mixed lead (II) nitrate with potassium iodide. As a result, the two solutions reacted to form lead (II) iodide (precipitate) and potassium nitrate (salt).

Here is a video and a balanced equation of the chemical reaction:

Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

 
NUCLEAR CHANGES    
We will not go into great detail for this change just yet, but nuclear changes are changes in which the elemental identity of the atoms are changed.


BALANCING TIPS
There are a few things that you can do to make your life much easier when balancing chemical equations. Balance the elements in elemental form last (Fe, O_2,S_8,P₄). If they don't break apart, balance the polyatomic ions (eg. SO₄^2-)as a group. Finally, balance oxygen and hydrogen last.

We were also introduced to the concepts of exothermic and endothermic reactions. An exothermic reaction is a chemical reaction accompanied by the evolution of heat. Check that energy is on the right side of the equation. This indicates that the reaction is exothermic. An endothermic reaction is a chemical reaction accompanied by the absorption of heat. Check that energy is on the left side of the equation. This indicates that the reaction is endothermic.

The distinction between the two types of reactions:

   

Next class, classification of chemicals!