So, litmus paper is a qualitative tool for assessing the acidity or basicity of a substance (usually a solution). In general, blue litmus turns red in the presence of an acid, and red litmus turns blue in the presence of a base. They can't really tell you much more than that.
The solutions that are most likely acids are those that turn blue litmus red <em>and </em>do not evoke a color change in red litmus. A solution that turns red litmus blue <em>or </em>does not evoke a color change in blue litmus is likely not an acid. Using these criteria, solutions 4 and 7 are most likely acids since they both turn blue litmus red (and they cause no color change in red litmus).
Equilibrium is achieved when the reaction rate of the forward and backward reaction are equal or the concentrations of the reactants and the products are in an unchanging ratio. Specifically, this system is in a dynamic equilibrium.
