What is Ocean Acidification, and how does it affect seashells in the ocean?
The excessive burning of fossil fuels, with other carbon dioxide emitting activities, causes the increase of carbon dioxide in the atmosphere, and hence causes ocean acidification. The carbon dioxide from the atmosphere dissolves into the ocean. The carbon dioxide reacts with seawater to form carbonic acid, which causes the ocean to increase in hydrogen ions. The hydrogen ions then decreases the ocean's pH.
Lower pH means that a certain substance is more acidic. Higher pH means that certain substance is more alkaline. Because of this, a lower pH in the ocean means that the ocean becomes more acidic.
Seashells are the exoskeletons (outer covering) of mollusks. A mollusk can be a gastropod like a snail or a bivalve such as a scallop or an oyster. The shells are mainly calcium carbonate (which is also what egg shells are made of).
When you add the calcium carbonate (neutral) to the ocean water (acid), a chemical reaction causes carbon dioxide to be formed. The carbon dioxide rise to the surface of the liquid in the form of bubbles.
Experiment
Things needed
1 Lemon - the juice squeezed out should be roughly 7 tablespoons (or pre-squeezed lemon juice)
Note: Lemon juice is not clear, hence the changes made to the shell is not very visible. However, bubbles can still be observed from the top.
7 tablespoons of vinegar
7 tablespoons of water
3 shells
3 (clear) cups
Procedure:
(for those with lemons only) Squeeze out lemon juice and measure 7 tablespoons of lemon juice into a clear cup. 
1. Measure 7 tablespoons of water and vinegar and pour them into separate cups.
2. Put shells into the liquids. Left to right: Water, Lemon, Vinegar
3. Observe. You will notice bubbles in the cup with vinegar.
4. Take out the shells from the liquids. Note: These shells were taken out after 10 minutes. However, if the shells are left in the vinegar for 24 hours or longer, the shells may have more visible changes - the shells should have partly dissolved.
The reason behind the results
Water is neutral, so the chemical reaction that releases carbon dioxide does not happen. Thus, no bubbles are produced.
Meanwhile, the lemon juice and vinegar are both acids, hence when it reacts to the shells, carbon dioxide is produced.
Why does this matter in the real world?
Ocean acidification is a real-life problem. Some animals use sea shells as their home until they outgrow the shell. With more carbon dioxide being produced every day, the ocean also gets more acidic. These animals' homes may be damaged. While the carbon dioxide dissolves in the water, more carbon dioxide is produced, hence contributing to global warming.
We can all play our part to slow global warming. Small changes like driving cars less and planting trees can become big if everyone does it.
References:
https://littlebinsforlittlehands.com/seashells-with-vinegar-ocean-chemistry/
https://en.wikipedia.org/wiki/Ocean_acidification