CO2 acidifies water

Add a couple of drops of indicator dye to the water in the cup to measure the pH of the water. It should be around 7.
Add some carbon dioxide to the water from breath: either blow through a straw into the water, or blow into the tube and then shake the exhaled air into the water (repeat blowing into the tube and shaking as necessary), until the colour changes.
Read the new colour of the water on the pH chart - it will have dropped i.e. become more acidic, to about pH 6.

The acidification of the water is reversible: waft in fresh air and shake it into the water. This air has less CO2 than our breath, and slowly, as more air is wafted into the tube and shaken into the water, the pH of the water will return to neutral. It will take longer than acidifying the water in the first place.

Climate change and ocean acidification
The water in the tube absorbs the CO2 from your breath, just as the oceans absorb increased CO2 in the Earth's atmosphere. As the CO2 in the water increases, the pH of the water falls - it becomes more acidic.
As we increase the amount of CO2 in our atmosphere through emissions, we are acidifying our oceans.
A dramatic climate change and ocean acidification event happened during Earth's history, and is evident in ocean bed core samples.
The Paleocene-Eocene Thermal Maxiumum (PETM) was a major greenhouse warming event 55 million years ago, resulting from a massive carbon release into the atmosphere (likely methane as well as CO2) which acidified the oceans. Ocean bed core samples show that this increase in greenhouse gases led to a sudden decrease in shell deposits (e.g. https://www.ecord.org/website/wp-content/uploads/2016/02/replica208a.jpg Also image here or here) - the shells of ocean animals dissolved in the more acidic ocean water before reaching the ocean floor, and the core samples showed just red clay. The greenhouse gases we are currently releasing into the atmosphere are at a much greater rate than this previous greenhouse event, and will lead to a similar effect on ocean life. Life in the oceans is linked to all life on Earth, so if we affect ocean life by acidifying ocean water, there will be knock on effects to all life on Earth.
But, there is hope!
As the students waft fresh air into their tubes, the acidification of the water will reverse and it will return to pH 7.
This shows that ocean acidification is reversible. If we can lower our emissions to reduce the CO2 in the atmosphere, the oceans will recover.

Blood CO2 levels affect our brain and heart response
When you exert yourself you need more oxygen. You also release more CO2, which makes your blood more acidic. A part of the brain called the medulla oblongata measures the pH of the blood, so can determine how much CO2 is in the blood. Once the acidity gets to a certain level, the medulla oblongata signals your body to speed up breathing and to increase heart rate. Increased heart rate will increase O2 levels in the blood again and also remove the excess CO2 from the blood.
(You can use the cortex of your brain to override and slow down breathing by thinking about it.)