To what extent does sulphuric acid decrease the mass of different materials?

My personal interest about this topic is that I am very passionate about the environment and investigating issues such as this one. Also because I am into historic architecture, I find it personally important to investigate how, why, and to what extent structures degrade, and also how they can be preserved.

The topic on the IB Chemistry syllabus that this investigation links to is topic 8.5.

Acid rain has a lower pH than normal rain (usually \( <5.00 \)) due to reactions with air pollutants such as sulphur dioxide and nitrogen oxides (which typically come from smoke fumes from factories). The coal used in the factories that produce smoke which causes the acidification of the rain can be washed with water to remove up to \( 90\% \) of the solid sulphur, therefore meaning that acid rain is less likely to be present or a concern. The equation for the formation of acid rain is sulphur trioxide + water \( \rightarrow \) sulphuric acid, or as a symbol equation: \( \mathrm{SO}_{3}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{2} \mathrm{SO}_{4} \). Its effects are that it can damage the surfaces of material and corrode metal, but it is also an environmental threat at it has harmful effects on fish and soil, and therefore plants, too.

What I aim to do in this experiment is investigate its effects, so to what extent it affects various materials, measured by the change in mass prior to and following the application of synthetic acid rain.

My experimental hypothesis is that the \( \mathrm{H}_{2} \mathrm{SO}_{4} \) (sulphuric acid) will cause a decrease in the mass of the objects, similar to how in real life, acid rain degrades materials over time, therefore causing them to lose mass.

Dangerous chemicals were used, namely sulphuric acid. Safety steps and precautions taken for this were that the teacher initially diluted it for me until it was reasonably safe, then I did the rest of it with the water until it got to the correct pH. The solutions were disposed of by doing down the sink. Acid rain has negative environmental effects, however because this was a lab-controlled simulation, this was not of great concern. A lab coat and safety goggles were used also as safety measures.

1. Calibrate the pH sensor with water (to pH 7.00).
2. Using distilled water, dilute sulfuric acid until it reaches pH 4.00.
3. Transfer this solution to a spray bottle.
4. Gather a 12 g sample of materials:
   1. Copper (to simulate building materials).
   2. Wood (to simulate gates and fences).
   3. Fabric (to simulate fabrics used in architecture).
   4. Paper (to simulate posters).
   5. Plastic (to simulate gutters).
5. Initially spray 3 times, repeat increasing by 3 each time at time intervals of 15 hours, 4 times.
6. Dry off any excess liquid where possible.
7. Measure their new masses and record this data in the table.
8. Dilute the acid before disposal.
9. Dispose of the materials - solids in the bin and liquids down the sink.

The materials were wet / damp, however there was no evidence that any significant reactions had occurred. The hard materials (plastic and copper) were more visually wet with water droplets, the paper was just soggy, and the wood and cloth were darker in places where the liquid was present. In all material samples, there was liquid in the bottom of the petri dish.

Average mass reduction: 0.06 g

Out of the 5 materials, only one of them had decreased in mass by the end of the experiment (the cloth). Two of them (plastic and copper) had remained at 12 g. The remaining two materials (wood and paper) actually increased in mass, contrary to what was expected. The plastic and copper are hard and non-absorbent materials, and the acid had no apparent effect on their masses as it is possible that this is because the pH was not low enough and it simply was able to be removed from the surface without reacting with them. The cloth is somewhat absorbent, and followed the hypothesis as its mass decreased with the application of the sulphuric acid, maybe due to it having biodegradable properties. The wood and paper increased in mass, likely due to them absorbing the acid without reacting how I anticipated (due to their physical properties, it is difficult to dry them / remove the solution from them, compared to solid materials such as the plastic and copper). The acid stayed in them and it is unclear if the mass of the paper and wood alone actually did decrease, and it was the presence of lingering acid that caused their mass readings to be higher than the initial ones. There were no significant visual observations that indicate the presence of chemical reactions.

The experiment was decently fair and controlled because the control variables were controlled properly. Something that went successfully in the experiment was how it provided me with an insight into the importance of the concentration of acid (over the pH) when it comes to reacting with materials to decrease their mass; it has shown me that to start seeing results it needs to be done over a longer period of time with much more solution. However, the experiment arguably failed as the results went against what was anticipated: that the masses would decrease. If I were to redo the experiment, I would change it so that both the concentration and the volume of applied acid is higher, and apply sprays at shorter time intervals to see results more quickly.

To conclude, according to data collected from my experiment, the extent to which sulphuric acid affects materials' weights is minimal / null, except for fabric. It is likely that this was because the concentration was too low to take effect, or even just because the materials were not exposed to it for a sufficient period of time for any noticeable chemical reactions or physical changes to occur.

To improve upon this experiment, I could have maybe reconducted it with a more concentrated solution of synthetic acid rain. Also, more repetitions of the experiment would benefit the reliability of the results. Additionally, more materials or a wider variety of materials (for example rocks (to simulate statues)), combined with leaving them exposed for a longer period of time (possibly 3 weeks) would also be beneficial in terms of reliability of results, providing an insight into real world effects of acid rain.

Air-Quality.org. (n.d.). Air Pollution. Retrieved from Air Quality: https://www.air-quality.org.uk/27.php

United States Environmental Protection Agency. (2023, June 1). Effects of Acid Rain. Retrieved from United States Environmental Protection Agency: https://www.epa.gov/acidrain/effects-acid-rain