Increasing amounts of carbon dioxide were dissolved in two beakers, one containing water and one a mixture of water and a buffer - HSC - SSCE Chemistry - Question 19 - 2018 - Paper 1

To determine which graph best represents the results of the experiment involving the dissolution of carbon dioxide in water and a water-buffer mixture, we need to consider the effects of CO2 on pH levels in these two conditions.

Understanding the Impact of CO2 on pH

1. Water Only (No Buffer):

   • When CO2 dissolves in pure water, it forms carbonic acid (H2CO3), which decreases the pH, making the solution more acidic as the concentration of CO2 increases.
   • The graph representing water only should show a steady decrease in pH as CO2 concentration increases.

2. Water + Buffer:

   • The presence of a buffer helps resist changes in pH. Buffers work by absorbing excess hydrogen ions (H+) that form from the dissolved CO2, thus maintaining a more stable pH level.
   • Therefore, while there will still be a decrease in pH as CO2 is added, this decrease will be less pronounced compared to the water-only scenario, showing a more gradual or plateau-like drop in pH.

Analyzing the Options

Upon reviewing the given options (A, B, C, D), the correct graph is:

• D: This graph shows a steeper decline in pH for the water-only scenario, while the buffered water shows a more stable pH despite increasing CO2 concentration.

Thus, the correct selection is Option D, as it accurately reflects the anticipated behavior of pH in both solutions when exposed to increasing CO2 levels.