Safety boundary
Use virtual indicators or teacher-supervised food-safe indicator only. Never mix cleaners, household acids/bases, or unknown chemicals.
Explicit chemistry instruction
An amphoteric/amphiprotic species can donate or accept a proton depending on context.
Water can act as a base with a stronger acid and as an acid with a stronger base.
Acid/base labels describe roles in a specific reaction, not permanent identities independent of context.
Vocabulary: amphoteric; amphiprotic; role; proton donor; proton acceptor.
Worked example
H2O accepts H+ from HCl but donates H+ to NH3 in the reverse conceptual comparison.
Guided question
Why can HCO3− act as either an acid or a base?
Independent work
- Classify water's role in four reactions.
- Identify two amphiprotic species.
- Write one role-dependent explanation.
Instructional depth extension · molecular structure and acid strength
Acid strength is not the same as acid concentration. Strength describes the extent of proton transfer for a particular acid in a given solvent; concentration describes how much acid is present per volume of solution.
When comparing related acids, molecular structure can change how stable the conjugate base is after proton donation. Bond polarity, bond strength, resonance delocalization, and inductive effects can all matter. A good explanation identifies which structural feature stabilizes or destabilizes the conjugate base instead of using a single electronegativity slogan.
Worked comparison: if two acids have equal concentration but one transfers protons more completely, their pH values may differ because strength and concentration are independent variables.
Misconception check
A substance can have different acid/base roles in different reactions.
Formative check
What determines acid/base role in Brønsted chemistry?
Reveal response
Whether the species donates or accepts a proton in that reaction.
Mentor evidence conference
Ask Archaemenes to audit proton-transfer roles, conjugate-pair logic, and model scope.
Teacher guidance
Foundation: Use reaction-role labels.
Core: Analyze amphiprotic behavior.
Extended: Compare relative proton-transfer tendencies.