PH

APES Section 6 — pH (Acidity & Basicity)

1) The Core Concept (Most Important)

pH measures how acidic or basic water is.

It is based on the concentration of hydrogen ions (H⁺).

The pH scale runs:

0 → 14

  • 7 = neutral

  • Below 7 = acidic

  • Above 7 = basic

Important detail from the packet:

The pH scale is logarithmic, meaning each step is 10× more acidic or basic.

Example:

pH 6 is 10 times more acidic than pH 7.

That exact concept shows up frequently on quizzes.

2) Why pH Matters for Aquatic Life (Very Testable)

pH affects:

  • Chemical reactions

  • Toxicity of pollutants

  • Survival of organisms

Your packet explains that water that is too acidic or too basic can damage:

  • Gills

  • Fins

  • Exoskeletons

  • Fish eggs

Fish eggs often cannot hatch if pH drops below 5.

The chart on page 1 shows different animals dying off at different pH levels — demonstrating that organisms have specific tolerance ranges.

3) How pH Affects Toxicity (Important Connection)

This is a classic APES relationship.

Lower pH (more acidic):

  • Metals dissolve more easily

  • Metals become more toxic

The packet specifically lists:

  • Aluminum

  • Lead

  • Mercury

  • Copper

  • Arsenic

as becoming more soluble at low pH.

Also:

Higher pH (> 8.5):

  • Ammonia becomes more toxic

That’s another common test point.

4) Daily pH Cycle (Common Question)

pH changes during the day because of photosynthesis and respiration.

Daytime:

  • Plants use CO₂

  • Water becomes more basic

  • pH increases

Night:

  • Plants release CO₂

  • Water becomes more acidic

  • pH decreases

According to the graph on page 2, pH is:

Highest:

Late afternoon

Lowest:

Early morning

This pattern is very similar to dissolved oxygen cycles.

5) What Affects pH (Very Testable)

Natural factors:

  • Photosynthesis

  • Decomposition

  • Geology (rock type)

  • Rainfall

Human causes:

  • Acid rain

  • Mining

  • Pollution

  • Nutrient runoff

Your packet explains that acid rain is often caused by emissions from cars and coal-fired power plants.

6) Safe pH Range for Aquatic Life (Good Number to Know)

EPA recommendation:

pH range: 6.5 to 9.0

Outside this range, aquatic life may be harmed.

If You Remember Nothing Else, Remember This

These five points will likely cover most quiz questions:

  1. pH measures acidity or basicity

  2. The pH scale is logarithmic (10× change per unit)

  3. Low pH makes metals more toxic

  4. pH rises during the day and falls at night

  5. Healthy aquatic pH is about 6.5–9