Chemistry · 9th Grade

Active learning ideas

Properties of Acids and Bases (Arrhenius/Brønsted-Lowry)

Active learning helps students move beyond memorizing definitions by engaging them in tasks that require reasoning about acid-base behavior. Through annotation, sorting, and discussion, students connect abstract models to concrete reactions, which builds durable understanding of why chemists use different definitions.

• Common Core State Standards
• HS-PS1-2
• STD.CCSS.ELA-LITERACY.RST.9-10.4

• 15–25 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Inquiry Circle
• 20 min · Pairs

Annotation Activity: Mapping Proton Transfer

Students receive five Bronsted-Lowry reactions and annotate each: identify the acid, base, conjugate acid, and conjugate base by drawing arrows showing proton transfer. Pairs compare annotations, discuss any disagreements, and then reconcile in a brief class discussion.

Differentiate between Arrhenius and Brønsted-Lowry definitions of acids and bases.

Facilitation Tip

During the Annotation Activity, ask students to label each proton-transfer arrow with 'donor' or 'acceptor' before identifying the acid or base to slow down automatic labeling.

What to look for

Provide students with the reaction: NH3 + H2O <=> NH4+ + OH-. Ask them to identify the Brønsted-Lowry acid, the Brønsted-Lowry base, and one conjugate acid-base pair. Also, ask them to explain water's role in this specific reaction.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 02

• Inquiry Circle
• 25 min · Small Groups

Card Sort: Arrhenius vs. Bronsted-Lowry

Provide cards with acid-base reactions, some in aqueous solution and some in non-aqueous solvents. Students sort them into three categories: explained by Arrhenius, requires Bronsted-Lowry, or covered by both. Discussion focuses on what each model cannot explain.

Identify conjugate acid-base pairs in a chemical reaction.

Facilitation Tip

For the Card Sort, include at least two non-aqueous examples so students see that Brønsted-Lowry applies beyond water and challenge the idea that Arrhenius definitions cover all cases.

What to look for

Present students with a list of substances (e.g., HCl, NaOH, NH3, H2SO4, KOH). Ask them to classify each as an Arrhenius acid, Arrhenius base, or neither, and then as a Brønsted-Lowry acid, Brønsted-Lowry base, or amphoteric substance, justifying their choices.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 03

• Think-Pair-Share
• 15 min · Pairs

Think-Pair-Share: Is Water an Acid or a Base?

Present two reactions featuring water -- one where it donates a proton (acts as acid) and one where it accepts a proton (acts as base). Students decide which role water plays in each, explain to a partner, and then define 'amphoteric' based on both reactions.

Explain the role of water as an amphoteric substance.

Facilitation Tip

In the Think-Pair-Share, assign specific reaction pairs to each pair so students analyze different cases and report back to the class, making the range of water's behavior visible.

What to look for

Pose the question: 'Why is the Brønsted-Lowry definition considered more useful than the Arrhenius definition in chemistry?' Facilitate a class discussion where students compare the limitations of the Arrhenius model with the broader applicability of the Brønsted-Lowry model, referencing examples like reactions not in water.

• Understand
• Apply
• Analyze
• Self-Awareness
• Relationship Skills

Activity 04

• Gallery Walk
• 25 min · Small Groups

Gallery Walk: Acids and Bases in the Real World

Post six stations with common substances (vinegar, baking soda, stomach acid, ammonia, blood, lemon juice) and their measurable properties. Students classify each using both Arrhenius and Bronsted-Lowry models and identify conjugate acid-base pairs where applicable.

Differentiate between Arrhenius and Brønsted-Lowry definitions of acids and bases.

What to look for

Provide students with the reaction: NH3 + H2O <=> NH4+ + OH-. Ask them to identify the Brønsted-Lowry acid, the Brønsted-Lowry base, and one conjugate acid-base pair. Also, ask them to explain water's role in this specific reaction.

• Understand
• Apply
• Analyze
• Create
• Relationship Skills
• Social Awareness

A few notes on teaching this unit

Experienced teachers treat Arrhenius and Brønsted-Lowry as tools rather than truths, emphasizing context over memorization. They avoid rushing to the broader definition and instead let students experience the limitations of Arrhenius firsthand. Research shows that students learn definitions best when they repeatedly classify reactions where water's role changes or where the reaction medium is not water.

Students will confidently distinguish between Arrhenius and Brønsted-Lowry definitions and explain when each applies. They will identify acids, bases, and conjugate pairs in written reactions and apply these ideas to unfamiliar substances. Misconceptions will surface through student talk and written justifications during activities.

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Watch Out for These Misconceptions

• During Card Sort: Arrhenius vs. Bronsted-Lowry, watch for students who assume ammonia (NH3) is only a base in water. Redirect them by asking them to mark where water appears in the reaction NH3(g) + HCl(g) → NH4Cl(s), and identify proton transfer without aqueous ions.

  During Card Sort: Arrhenius vs. Bronsted-Lowry, have students physically place the gas-phase reaction NH3 + HCl on the Brønsted-Lowry side and annotate the proton transfer arrow. Ask them to explain why this reaction cannot be classified using Arrhenius definitions, making the limitation visible on the table.

• During Think-Pair-Share: Is Water an Acid or a Base?, watch for students who insist the conjugate base of a strong acid must also be strong. Redirect by asking them to examine the reaction HCl + H2O → H3O+ + Cl- and predict how Cl- will react with water.

  During Think-Pair-Share: Is Water an Acid or a Base?, give each pair the reaction HCl + H2O → H3O+ + Cl- and the separate reaction NH3 + H2O → NH4+ + OH-. Ask them to rank the strength of Cl- and OH- as bases based on how much they react with water, using the evidence from both reactions.

• During Annotation Activity: Mapping Proton Transfer, watch for students who label water as neutral and stop there. Redirect by asking them to trace the proton transfer in both directions for the same molecule.

  During Annotation Activity: Mapping Proton Transfer, require students to draw two separate reactions on the same page: one where water accepts a proton (acting as a base) and one where it donates a proton (acting as an acid). Ask them to label each with the acid and base in that specific context.

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Methods used in this brief

• Inquiry Circle
• Think-Pair-Share
• Gallery Walk