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Phase Notation

Phase Notation
Dr. MJ Patterson

The phase (solid, liquid, gas, aqueous solution) of a chemical plays a significant role in its reactivity.  Chemists have developed a shorthand to indicate phases by placing an abbreviation in parentheses as a subscript following the formula for the chemical.
 
 

Phase 

Symbol

solid

s

liquid

l

gas

g

aqueous solution

aq

(An aqueous solution is one in which the solvent is water.)

Example 1:
How would you indicate ethyl alcohol C2H6O as a solid, liquid, gas and aqueous solution?

Solution 1:
Use the phase symbol in parentheses and as a subscript following the compound's formula:

solid = C2H6O(s)
liquid = C2H6O(l)
gas = C2H6O(g)
aqueous solution = C2H6O(aq)

Example 2:
Write and balance the equation to show pure ethyl alcohol (normally a liquid) dissolving in water to make an aqueous solution.

Solution 2:
Since ethyl alcohol is a molecular compound (all nonmetals), when it dissolves in water, it stays intact as molecules.
C2H6O(l)   =>(H2O)   C2H6O(aq)

(Chemists frequently write reaction conditions, solvents, catalysts or other important information above the arrow in a reaction.  Since it is difficult to do that over the web, I will write any information like this in parentheses immediately after the arrow, such as the water in this example.)

Example 3:
Write and balance the equation to show Epsom''s salt MgSO4  (normally a solid) dissolving in water to make an aqueous solution.

Solution 3:
Since magnesium sulfate is an ionic compound (metal + polyatomic ion), when it dissolves in water, it dissolves as separate ions.

MgSO4(s)  =>(H2O)   Mg2+(aq) + SO42-(aq)

Review Question:
In the previous two examples, we encountered a difference in the way that molecular compounds and ionic compounds behave when dissolved in water.  What are the criteria for recognizing an ionic compound versus a molecular compound?

Review Solution:
An ionic compound is a metal + a nonmetal, or it contains one or more polyatomic ions.  A molecular compound consists of only nonmetals.