Where a substance goes, and how much of it

Word	What it means
Organic / inorganic	Organic compounds contain carbon, usually with hydrogen: sugars, fats, plastics. Inorganic compounds are the rest — water, salt — plus a few simple carbon compounds such as carbon dioxide.
Nutrient	A substance a living thing needs to grow. Macronutrients, such as nitrogen (N), phosphorus (P) and potassium (K), are needed in large amounts; micronutrients, such as iron and zinc, in tiny amounts.
Fertilizer / pesticide	A fertilizer adds nutrients to soil. A pesticide kills pests: herbicides kill plants, insecticides insects, fungicides fungi.
Acid / base	An acid has a pH below 7 and turns blue litmus red. A base has a pH above 7, feels slippery and turns red litmus blue.
pH scale	0 to 14. 7 is neutral. Each step is a tenfold change.
Indicator	A substance that changes colour with pH.
Neutralization	An acid and a base reacting to make a salt (an ionic compound) and water.
Concentration	How much of a substance is in a mixture, here in ppm (parts per million) or mg/L — the same number for dilute solutions in water.
Dose / LD50	A dose is the amount taken into a body, per kg of body mass. The LD50 is the dose that kills half of a group of test animals. The smaller the LD50, the more toxic.
Bioaccumulation / biomagnification	A substance building up in one organism over its life / its concentration rising at each step up a food chain.
Biodegradation	Breakdown by living things, mostly bacteria and fungi.
Dissolved oxygen	Oxygen gas dissolved in water, which fish and water insects breathe.
Biological indicator	A species whose presence or absence tells you about the quality of its environment.

A plant's main three. Nitrogen for leaves and growth, phosphorus for roots, flowers and seeds, potassium for overall health. A fertilizer bag gives three numbers in that order.

Worked: reading a label. A 10 kg bag marked 20-10-5 is 20% nitrogen, so it holds 10 × 0.20 = 2 kg of nitrogen. The other two numbers are the percentages of phosphate and potash, the forms the phosphorus and potassium come in: 1 kg and 0.5 kg.

Two meanings of organic. In chemistry it means carbon-based; on a grocery label it means grown under particular farming rules.

When fertilizer runs off. Rain carries extra nitrogen and phosphorus into a lake, and algae grow explosively — an algal bloom. When the algae die, bacteria decomposing them use up the dissolved oxygen, and fish suffocate. Questions often give one link and ask for the next:

extra nutrients → algal bloom → algae die → decomposers use up oxygen → fish die

Pesticides trade a benefit — bigger harvests, fewer disease-carrying insects — for risks to organisms that were never the target. A good answer names both sides.

pH 4 is 10 × more acidic than pH 5, and 10 × 10 = 100 × more acidic than pH 6

Worked: acid rain. Ordinary rain is slightly acidic, about pH 5.6, because carbon dioxide dissolves in it. Rain at pH 4.6 is ten times more acidic than that; at pH 3.6, a hundred times. The extra acid comes from sulfur dioxide and nitrogen oxides, released by burning fossil fuels and processing natural gas.

Indicator	In an acid	In a base
Litmus paper	Blue turns red	Red turns blue
Bromothymol blue	Yellow	Blue (green near neutral)
Phenolphthalein	Colourless	Pink
Universal indicator	Red to yellow	Blue to purple (green at 7)

Neutralization. hydrochloric acid + sodium hydroxide → sodium chloride + water. The same idea is why an antacid settles an upset stomach, and why crushed limestone is sometimes added to an acidified lake.

On the test. A sample turns bromothymol blue yellow and leaves phenolphthalein colourless. Both say acidic, so it is below pH 7.

concentration (ppm, or mg/L) = mass of substance (mg) ÷ volume of water (L)

What one ppm means. A litre of water has a mass of one kilogram, which is a million milligrams, so 1 mg in 1 L is one part in a million — about one drop in 50 litres.

Worked. A 250 mL sample of well water contains 2.5 mg of nitrate. 250 mL is 0.250 L, so the concentration is 2.5 ÷ 0.250 = 10 mg/L, or 10 ppm. Leaving the volume in millilitres gives 0.01 — a thousand times too small.

Worked: LD50. Pesticide A has an LD50 of 200 mg/kg; pesticide B, 5 mg/kg. For a 20 kg animal, the LD50 dose of A is 200 × 20 = 4000 mg, while for B it is only 5 × 20 = 100 mg. B is far more toxic: the smaller number is the one to handle carefully, the reverse of what most people guess.

The dose makes the poison. Even table salt has an LD50 — about 3000 mg/kg in rats — and drinking a huge amount of water in a short time can kill. The real question is never whether something is toxic, but how much.

Three routes. Air carries smoke and spray far away; some pesticides turn up in the Arctic, where they were never used. Water carries runoff across land, and dissolved substances down through soil into groundwater (leaching). Soil can hold a substance for years. Spreading out lowers the concentration, and bacteria and fungi break many organic substances down, fastest when it is warm and moist — but some, like the insecticide DDT, last for years.

Food chain step	Concentration (ppm)	Times the step below
Lake water	0.0001	—
Algae	0.01	100
Small fish	0.1	10
Large fish	1	10
Fish-eating bird	10	10

Why the top predator gets the most. The numbers are illustrative, but real measurements follow the same pattern. The substance is stored in fat rather than removed, and each predator eats many prey and collects what all of them stored: from water to bird, 10 ÷ 0.0001 = 100 000 times. DDT built up this way in peregrine falcons and bald eagles, thinning their eggshells so eggs broke before hatching. After DDT was restricted in the 1970s, those birds recovered.

Monitoring water. Mayfly and stonefly nymphs need clean, oxygen-rich water; bloodworms (midge larvae) and sludge worms tolerate pollution.

Site	Dissolved oxygen	Most common animals
Upstream of a feedlot	10 mg/L	Stonefly and mayfly nymphs
Downstream	4 mg/L	Bloodworms and sludge worms

On the test: reading it. Oxygen fell and sensitive animals gave way to tolerant ones, so the water is poorer downstream — likely manure washing in, with decomposers using up the oxygen. If asked which evidence best supports that, choose the animals: they reflect weeks of conditions, while one oxygen reading is a single moment.

Air and waste. Canada's Air Quality Health Index combines readings of fine particles, ozone and nitrogen dioxide, and climbs in wildfire smoke. Paint, motor oil, batteries and pesticides go to a hazardous-waste drop-off, never down the drain.

• Reading a higher pH as more acidic. Lower is more acidic.
• Treating one pH step as small. One step is ten times; two is a hundred.
• Thinking a larger LD50 means more toxic. It is the other way round.
• Leaving the volume in millilitres when calculating ppm.
• Mixing up bioaccumulation and biomagnification. One organism over time, versus up a food chain.
• Using organic to mean natural or safe. In chemistry it only means carbon-based.