Toxicity
Toxicity is how much harm a substance can cause to a living thing when it is exposed to that substance.
A drop of dish soap added to a full glass of water barely changes it. But that same drop added to a thimble of water overwhelms it completely. The amount of water compared to the soap determines how much harm the soap can do. Toxicity works the same way: the ratio of substance to living tissue is what matters.
Explaining toxicity by grade level
Some things are safe in small amounts but harmful in large amounts. A tiny drop of soap in a fish tank may not hurt the fish. A lot of soap in the same tank can make the fish sick. The amount matters just as much as what the substance is.
Projects that explore toxicity
Not all pollutants cause the same level of harm. In this experiment, daphnia pulex — tiny freshwater crustaceans — were exposed to motor oil, milk, and lignin sulfonate across several concentrations. After 24, 48, and 168 hours, survival counts told a clear story: motor oil was the most lethal of the three, lignin sulfonate ranked second, and milk was the least harmful. The same concentration of different substances can kill at very different rates.
Toxicity is not only about concentration. Even a weak dose can cause total harm over time. Brassica rapa plants watered with detergent at three strengths all died, while only the plain-water group survived.