Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Writing Chemical Equations, Is It a Gas or Aqueous Solution?

  1. Oct 29, 2011 #1

    I understand that this question may seem rather obvious for some. Nonetheless, it is something that seems to have thrown me off a number of times.

    When writing chemical equations involving acidic or basic solutions, I know that the acid on the reactant side is written with the state (aq). i.e. HCl (aq) + H2O (l) → H3O+(aq) + Cl-(aq)

    However, if it is a gas that forms an acid upon contact with water, is the acid on the reactant side still written with the state (aq), or is it written with the state (g)? For example, let's say a question states "HCl (g) is dissolved in water to form an acidic solution..." What state would the hydrogen chloride gas be written as within the equation?

    Ideas are welcome, explanations are greatly appreciated. :)

    Thank you,

  2. jcsd
  3. Oct 29, 2011 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    HCl(aq) is the proper notation for Hydrochloric Acid (HCl dissolved in water).

    HCl is the proper notation for the gas. You don't usually specify the state unless there are very unusual conditions.

    HCl(g) + H20(l) ---> HCl(aq) but this is not really a chemical reaction. eg. you wouldn't normally use an equation to describe making salty water:

    NaCl(s) + H2O(l) ---> NaCl(aq)

    In general, you express the reactants in the state they start out in. You only need to do this if it is not obvious from the context. Your confusion is what happens when one of your reactants is the form of a gas that starts out dissolved in water (it's a solution - aq). This confusion is magnified by the common habit of leaving the (aq) off the acid.

    In practice, these notes are a shorthand to keep track of things which get explained in the preamble to the equations.
  4. Oct 29, 2011 #3
    You are asking about a dissolution reaction. The process is

    [tex]\text{HCl(g)} \rightleftharpoons \text{HCl(aq)}[/tex]

    but [itex]\text{HCl(aq)}[/itex] is very unstable and dissociates as you correctly note above. See http://dl.clackamas.edu/ch105-03/dissolut.htm
    Last edited: Oct 29, 2011
  5. Oct 29, 2011 #4
    Also, most gasses are things like N2 / O2 / H2, and they are more than likely on the products side of an irreversible reaction.

    Like, the reaction would form some product, and give off a gas as well.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook