The pH (acidity or basicity) level in organisms is generally an extremely important factor for growth. The reason that many organisms must maintain a near-constant level of pH is that life requires chemical reactions, and many chemical reactions in live organisms require certain pH levels in order to have the appropriate charges. For instance, the making of proteins requires the transfer of electrons, protons, and entire portions of molecules, which can only happen if the charges in different parts of protein-making machinery match. When the pH in an organism is disturbed too much, the organism cannot make molecules that it needs to live, so it dies. It logically follows (since plants are organisms) that plants will die if the pH of the water they're given is always too high or too low compared to what they need.

If the pH differs from ideal conditions by a certain amount, but not enough to kill the plant, then the plant's growth will be stunted, meaning that given the same amount of other nutrients and sunlight, it will grow much slower, mature much later, or not reach maturity, and this is readily testable with some seeds and a few planters!

You need to set up an experiment, where you take plants, subject them to different pH levels in the water, and observe what happens. This is how science is done, and is what makes science different from other fields of endeavor: we don't just read our answers from a book, because oftentimes the book of answers isn't written yet, so you need to write it yourself.

I'd suggest using plants that grow very quickly. Grass seed that you can buy in a nursery will do. You also need different pHs of water to give them. I'd suggest maybe a water-vinegar mixture for low pH, pure water for neutral pH, and water with baking soda dissolved in it for high pH (don't mix the baking soda and the vinegar, or you will get a frothy eruption as the acid and base react and carbon dioxide bubbles off). Vinegar and baking soda are both available at any supermarket. You need to measure the concentration of the vinegar or baking soda in the water that you use, and preferably have some kind of pH meter that you can use to know exactly what the pH you are using in each treatment (i.e. acid, neutral, alkaline).

If you're feeling really ambitious, you could try with different amounts of vinegar or baking soda in the water to have different levels of acid or basic pH. You could even do a large sample size with multiple copies of each treatment so that you can distinguish between the effect of pH and random chance, but that is probably beyond what you need for a high school project.

The pH of soils can absolutely affect the growth of plants. Too acidic of soil and too alkaline of soil can have adverse effects on growth. The optimal range for most plans if about 5.5 to 7pH. Too high or too low of a pH can affect nutrient availability and the concentrations of certain minerals that are harmful to plants.

This is a really interesting question because it turns out that pH plays a few important roles in plant growth.

Generally, plants absorb nutrients better in slightly acidic soils and with more nutrients plants can grow more. Also, nitrifying bacteria (microorganisms that break down unusable ammonia into usable nitrates) survive best in acidic soils . Plants need these nitrifying bacteria because the nitrates that they release are essential for plant growth. Overall, plants generally survive and grow best when placed in slightly acidic soils but that can differ depending on the species of plant. For example, asparagus actually grow best in neutral or slightly basic soil , but they are the exception rather than the rule. Thank you for your question!