A Demonstration of Acid Rain

The pH range over which the indicator changes from the acid color to the basic color for methyl orange is 3.2-4.4 (Lide, 2002). At pH > 4.4, the yellow basic form predominates. On the other hand, if methyl orange is in a solution of pH < 3.2, the red acidic form predominates.

The burning matches have produced a color change of the indicator from yellow (Fig. 1b) to red (Fig. 4b). It indicates that something acidic dissolved in the indicator solution. We can ask two questions?

We may consider carbonic acid as the source of the acid as it is a product of the combustion of wood. However, the burning of the wooden splint (Fig. 4a) did not cause a color change of the indicator. We have to conclude that the ignited match introduced something much more acidic than CO₂. The acid must have come from something in the flammable head of the match.

The head of a match contains sulfur and potassium chlorate(V). The match can be ignited when the head comes in contact with phosphorus, generally coated on the side of the box. The chemical reaction when we strike a match is the oxidation of a 'fuel' by potassium chlorate(V). Sulfur in the match head acts as the fuel, while phosphorus only initiates the reaction (Emsley, 2001). SO₂ is produced as a result of the oxidation of sulfur. The reaction of the burning of a safety match goes as follows:

SO₂ is significantly more soluble in water than CO₂, as indicated by its Henry's law constant of 1.2 molL^-1atm^-1 compared to 3.38 x 10^-2 molL^-1atm^-1 for CO₂ (Lide, 2002). The value of the acid dissociation constant of 1.7 x 10^-2 for SO₂ is more than four orders of magnitude higher than the value of 4.45 x 10^-7 for CO₂ (Lide, 2002). Thus, a small amount of dissolved SO₂ is able to lower the pH of a solution considerably resulting in the observed color change of the indicator. Dissolved CO₂ can also lower the pH of a solution, but to a much lesser extent that it is unable to cause a color change of the indicator.