PH — Science definition
Defined in 2 courses:
- A logarithmic measure of hydrogen-ion activity, approximated by concentration in dilute solutions. One unit corresponds to a tenfold difference; 0–14 is a common classroom range, not an absolute limit. (Life Science: Biology)
- Negative base-ten logarithm of hydronium activity, approximated using suitably normalized concentration in dilute solutions; 0–14 is not an absolute limit. (Physical Science: Chemistry)
Where it is taught
- Atoms, Bonds & Water — Investigating the Chemistry of Life, Life Science: Biology, Grade 10
- pH & Neutralization — Acids, Bases & Titration, Physical Science: Chemistry, Grade 11
In use
Finally, water itself can split, releasing a hydrogen ion (H⁺), and the balance of those ions is what we measure with pH.
Atoms, Bonds & Water
The exact definition uses dimensionless hydrogen-ion activity: pH = −log₁₀(aH⁺).
pH & Neutralization
Questions using ph
Problem 1
Answer and explanation
1000 times
Each pH unit is a factor of 10. The difference is 5 − 2 = 3 units, so 10³ = 1000 times more acidic.
Problem 2
Answer and explanation
The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.
The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Problem 3
- cohesion
- buffering
- evaporative cooling
- capillary action
Answer and explanation
buffering
A buffer accepts or releases hydrogen ions and resists pH change. Cohesion and cooling are also water properties but do not stabilize pH.
Problem 4
Answer and explanation
The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.
The pH 5 sample has 10³ = 1,000 times the hydrogen-ion concentration of the pH 8 sample in the dilute-solution comparison. A buffer has a finite working range and capacity; sufficiently large additions can exceed it.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the biological mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.
Taught alongside
- atom
- element
- covalent bond
- ionic bond
- hydrogen bond
- polar molecule
- pH scale
- neutralization
- salt
- indicator
- titration