CHEMICAL REACTIONS
A chemical reaction is the transformation of one or more substances into one or more new substances. In a reaction the atoms
of the reacting substances will rearrange and bond (chemically combine) to give new substances.  The new substances will
have different physical and chemical properties from the initial substances.

Chemical reactions are symbolically represented by chemical equations.  In Equation 1, solid sodium metal reacts with liquid
water to produce aqueous sodium hydroxide and hydrogen gas.

        2 Na(s) + 2 H₂O(l) ----> 2NaOH(aq) + H₂(g)     (Equation 1)

REACTIONS OCCURRING IN AQUEOUS SOLUTIONS
Of the several types of reactions involving aqueous solutions, the three most common are:

1.    Neutralization.  In a neutralization reaction, an acid reacts with a base to produce a salt (or salt solution) and possibly
       water.    An example is indicated by Equation 2.

        H₂SO₄(aq) + 2 NaOH(aq) ---->Na₂SO₄(aq) + 2 H₂O(aq)     (Equation 2)

2.   Precipitation. In a precipitation reaction, two aqueous solutions react to form an insoluble product. Equation 3 is an
       example of a precipitation reaction.

        AgNO₃(aq) + NaCl(aq) ----> AgCl(s) + NaNO₃(aq)            (Equation 3)

3.   Oxidation. Chemical processes involving the transfer of electrons are called oxidation reactions (redox).  Equation 4 shows
       a typical redox reaction. Please note that one reactant and one product are in elemental form.

        Zn(s) + 2 HCl(aq) ----> ZnCl₂(aq) + H₂(g)                            (Equation 4)

OBSERVATIONS
Often when reactions occur there are accompanying visible changes. The most common observations are:

1.   Evolution of a gas. When one of the products of a reaction is a gas, bubbles of the gas may appear in the solution or
       on the surface of a solid. When a gas is produced, foam may form, or if the gas evolution is rapid enough a "fizzing"
       sound may be heard.

2.   The formation (or disappearance) of an insoluble solid called a precipitate.   Some reactions result with the production
       of an insoluble solid called a precipitate, which normally settles to the bottom of the reaction mixture as a collection of
       particles.  Describing a precipitate involves not only noting its color but also texture.  Precipitates can exist in the form of
       crystals, gelatinous or milky suspensions, or granular solids.  The disappearance of a solid can also indicate a reaction
       has taken place.

3.   A color change.   Both precipitation reactions and redox reactions can involve color change.  Also, color change can be
       associated with reactions that result in the formation of a species referred to as a complex ion.

4.   Evolution or absorption of heat. Energy is transferred in all reactions.  Sometimes the vessel in which the reaction occurs
       may get warm or cold, depending on whether the reaction evolves or absorbs heat.

LITMUS PAPER
Litmus paper is an indicator paper coated with a substance (litmus) which undergoes a color change in the presence of acids and bases. Red litmus paper is used to check for bases, blue litmus paper is used to check for acids. Red litmus paper dipped into a basic solution turns blue. Blue litmus paper dipped into an acidic solution turns red.

REACTION MATRICES
One effective method for tabulating the observations from chemical reactions involves the construction of a reaction matrix.
An example of a reaction matrix is shown in Matrix I. It provides a cell for entering the observation made from every possible
mixing of two solutions from a total of six different compounds.

Notice that just over half of the matrix cells are shaded. Cells where identical solutions are being mixed are shaded because
nothing results from this type mixture (e.g. CELL B2 - AlCl₃/AlCl₃). Cells with duplicate pairing of solutions
(i.e. CELL B3 = CELL C2) are also shaded.
Matrix I

	AlCl3	Ba(NO3)2	NiSO4	KOH	BiCl3	H2SO4
AlCl3	CELL B2	CELL C2
Ba(NO3)2	CELL B3
NiSO4
KOH
BiCl3
H2SO4

Matrix II is the completed matrix with legend. It provides the observations and information from the mixing of the 15 possible pairs. When recording observations made from chemical reactions, it is suggested that you devise a code such as the following. G = gas produced; Ppt = precipitate; Ppt(white) = white precipitate; H = evolution of heat; C = absorption of heat; NR = no reaction; D = dissolved

The first pair of solutions to be considered in Matrix I shows AlCl₃ mixed with Ba(NO₃)₂. The matrix shows that when these two solutions were mixed no reaction was observed, thus NR is entered into Cell C1. Cell D1 also has a NR entry because no reaction was observed when AlCl₃ was mixed with NiSO₄. Cell E1 indicates that when AlCl₃ and KOH were mixed a precipitate formed initially but dissolved upon addition of additional KOH.

Matrix II

	AlCl3	Ba(NO3)2	NiSO4	KOH	BiCl3	H2SO4
AlCl3		NR	NR	Ppt (white), D	NR	NR
Ba(NO3)2			Ppt(white)	Ppt (white)	NR	Ppt (white)
NiSO4				Ppt (green)	NR	NR
KOH					Ppt, H	H
BiCl3						NR
H2SO4

G = gas produced; Ppt = precipitate; Ppt(white) = white precipitate; H = evolution of heat; C = absorption of heat; NR = no reaction; D = dissolved

PART 2.    Write and balance equations for any reactions observed.

Reactions Which Produce Gases
Carbonates react with acids to produce carbon dioxide gas.
Na₂CO₃(aq) + 2 HCl(aq) --->  2 NaCl(aq)  + H₂O(l) + CO₂(g)

Sulfites react with acids to produce sulfur dioxide gas.
Na₂SO₃(aq) + 2 HCl(aq) --->  2 NaCl(aq) + H₂O(l) + SO₂(g)

Sulfides react with acids to produce hydrogen sulfide gas.
Na₂S(aq) + 2 HCl(aq) --->  2 NaCl(aq) + H₂O(l) + H₂S(g)

Ammonium Salts react with strong bases to produce ammonia gas.
NH₄Cl(aq) + NaOH(aq) ---> NaCl(aq) + H₂O(l) + NH₃(g)

BLUE litmus paper turns red in acidic solutions.