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