Acids & Bases — Print · Grade 11 Science

Chapter: Acids, Bases & Titration

Acids & Bases

Learning goals

Readings for this unit

Before this unit

Protons, conjugate pairs, and ionization.

Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.

Start with something to explain

Does a lower pH always mean the acid species is stronger?

Illustrative evidence case. A dilute strong-acid solution and a concentrated weak-acid solution have similar modeled pH.

Name the acid–base model before assigning rolesName the acid–base model before assigning rolesHCl + H₂O → H₃O⁺ + Cl⁻Acid donates a protonBase accepts a protonH₂O + H⁺ → H₃O⁺ is a species bookkeeping shorthand.A proton transfer is not an electron-transfer definition.
Original selected aqueous proton-transfer representation. The arrow tracks a proton transfer conceptually, not a free bare proton’s measured trajectory.

Notice and wonder

Record two observations using the stated quantities or relationships. Keep observations separate from possible causes. Write one question that a new measurement could help answer.

Does a lower pH always mean the acid species is stronger?

Your first model

Keep formal amount and equilibrium ionization on separate lines of the account.

Build the language from the idea

Arrhenius acid
A substance that releases hydrogen ions (H⁺, which form H₃O⁺) when dissolved in water.
Arrhenius base
A substance that releases hydroxide ions (OH⁻) when dissolved in water.
Brønsted-Lowry acid
A proton (H⁺) donor in a chemical reaction.
Brønsted-Lowry base
A proton (H⁺) acceptor in a chemical reaction.
strong acid
An acid that ionizes completely in water, releasing nearly all of its hydrogen ions.
weak acid
An acid that ionizes only partially in water, so most of its molecules stay intact at any moment.

Acids and bases are two of the most important families of compounds in all of chemistry, and people had sorted substances into 'sour' and 'bitter' groups for centuries before anyone understood why. The first modern explanation came from Svante Arrhenius, who defined acids and bases by the ions they release in water. An Arrhenius acid produces hydrogen ions, H⁺ (which immediately grab onto water molecules to form hydronium, H₃O⁺), when dissolved in water. An Arrhenius base produces hydroxide ions, OH⁻. Lemon juice, vinegar, and stomach acid fit the first group; lye, ammonia solution, and dissolved baking soda fit the second.

Arrhenius's picture is powerful but a little narrow — it describes aqueous hydronium/hydroxide behavior; a base can increase OH⁻ through reaction with water without initially containing that ion. A broader definition came from Brønsted and Lowry, who shifted the focus from ions to protons. (H⁺ denotes a proton, but in water the proton is hydrated rather than remaining an isolated bare particle.) A Brønsted-Lowry acid is any proton donor, and a Brønsted-Lowry base is any proton acceptor. When HCl dissolves, it *donates* a proton to a water molecule: HCl + H₂O → H₃O⁺ + Cl⁻. Here HCl is the acid and water is the base — a general way of thinking that covers reactions Arrhenius's model can't.

Name the acid–base model before assigning rolesName the acid–base model before assigning rolesHCl + H₂O → H₃O⁺ + Cl⁻Acid donates a protonBase accepts a protonH₂O + H⁺ → H₃O⁺ is a species bookkeeping shorthand.A proton transfer is not an electron-transfer definition.
Original selected aqueous proton-transfer representation. The arrow tracks a proton transfer conceptually, not a free bare proton’s measured trajectory.

One fascinating consequence is that water can play *either* role, depending on its partner. With hydrochloric acid, water accepts a proton and acts as a base; but with ammonia (NH₃ + H₂O → NH₄⁺ + OH⁻), water donates a proton and acts as an acid. A substance that can behave as either an acid or a base is called amphoteric, and water is the most famous example — a quiet hint that being an acid or a base is less a fixed label than a description of how two substances behave toward each other.

Conjugate acid and base pairs differ by one protonConjugate acid and base pairs differ by one protonProton-transfer pairDifferenceCharge changeHA / A⁻One H⁺0 → −1 on lossNH₄⁺ / NH₃One H⁺+1 → 0 on lossH₃O⁺ / H₂OOne H⁺+1 → 0 on lossA conjugate pair differs by exactly one proton, not any H atom.
Original selected conjugate-pair table; HA is a hypothetical monoprotic acid, not a universal formula for every acid.

Acids and bases also announce themselves through characteristic properties. Acids taste sour (think of lemons and vinegar), react with many metals to release hydrogen gas, and turn blue litmus paper red. Bases taste bitter, feel slippery or soapy, and turn red litmus paper blue. Both kinds conduct electricity in solution, because both release ions that can carry charge. One firm safety rule, though: you never taste or touch laboratory chemicals to identify them — strong acids and strong bases are both corrosive enough to cause serious burns.

Strong and weak describe proton-transfer equilibrium, separate from concentration. A strong monoprotic acid such as HCl is essentially fully ionized in the usual dilute aqueous account; a weak acid has an equilibrium involving appreciable un-ionized species under appropriate conditions. Polyprotic acids require separate steps: sulfuric acid’s first ionization is strong, while its second step must be treated with its own equilibrium. A soluble strong hydroxide base such as NaOH dissociates into ions, while NH₃ produces hydroxide through a weak-base reaction with water.

Strength and concentration answer different questionsStrength and concentration answer different questionsSelected solution at 25°CFormalconcentrationApproximate pHStrong monoprotic acid0.0010 M3.00Weak acid, Ka=1.0×10⁻⁵0.10 M3.00Strength concerns equilibrium ionization, not simply the pH value.The more concentrated weak acid can match a dilute strong acid.
Original rounded comparison with Ka=1.0×10⁻⁵ for a hypothetical weak acid. It is not an instruction to identify unknown chemicals by contact or taste.

Use proton-transfer and ionization models to interpret acids and bases rather than product labels alone. Strength, concentration, temperature, and the reacting species determine behavior. A weak acid can still be hazardous, and a stronger acid is not automatically more concentrated. Work with supplied equations and known values for quantitative comparisons.

Model check: Acid strength and concentration are independent; neither alone fixes pH. It accepts a proton from water, forming NH₄⁺ and OH⁻; Brønsted bases need not contain hydroxide initially.

Build your understanding

Use the detailed parent plan: two core 50–60-minute sessions per focused unit, with a third assigned for selected calculation/modeling topics. First study the core explanation and Parts 1–2 with their figures/checks; next study Parts 3–4, the whole-unit application, worked cases, varied practice and exit check. Attempt reasoning before opening guidance. Extend time when actual understanding requires it; investigations and chapter assessments have separate sessions.

Part 1 · Distinguish acid-base models

In the Brønsted–Lowry model, an acid donates a proton and a base accepts one. HCl transfers a proton to water in this selected aqueous account; water acts as the base and forms hydronium, H₃O⁺. The Arrhenius model concerns increased hydronium or hydroxide in water, while the Lewis acid–base model uses electron-pair acceptance and donation. These models have different scopes. A substance’s role can depend on its reaction partner rather than an immutable acid label in every context.

Name the acid–base model before assigning rolesName the acid–base model before assigning rolesHCl + H₂O → H₃O⁺ + Cl⁻Acid donates a protonBase accepts a protonH₂O + H⁺ → H₃O⁺ is a species bookkeeping shorthand.A proton transfer is not an electron-transfer definition.
Original selected aqueous proton-transfer representation. The arrow tracks a proton transfer conceptually, not a free bare proton’s measured trajectory.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

In the displayed proton transfer, which species is the base and what does it become?

Visual reasoning — after your attempt

H₂O accepts a proton and becomes H₃O⁺, its conjugate acid in this account.

An Arrhenius acid increases hydrogen-ion-related species in aqueous solution, while a Brønsted–Lowry acid donates a proton and a base accepts one. In water, H₃O⁺ is a useful representation rather than an isolated bare proton. Broader models extend beyond aqueous hydroxide formation.

Ammonia can accept a proton from water to form NH₄⁺ and OH⁻ despite containing no hydroxide ion in its starting formula.

Pause and explain

Why can ammonia be a base without an OH group?

Check your reasoning after answering

It accepts a proton. Its reaction with water can generate OH⁻, so the starting formula need not contain hydroxide.

Part 2 · Match conjugate pairs by one proton

A conjugate base remains after an acid donates H⁺, and a conjugate acid forms when a base accepts it. Losing H⁺ reduces the species’ charge by one unit; gaining it raises the charge by one. NH₄⁺ and NH₃ form a conjugate pair, while species differing by two protons are not one adjacent pair. Use proton and charge inventories together; adding a neutral H atom or an electron is a different transformation from the proton-transfer definition.

Conjugate acid and base pairs differ by one protonConjugate acid and base pairs differ by one protonProton-transfer pairDifferenceCharge changeHA / A⁻One H⁺0 → −1 on lossNH₄⁺ / NH₃One H⁺+1 → 0 on lossH₃O⁺ / H₂OOne H⁺+1 → 0 on lossA conjugate pair differs by exactly one proton, not any H atom.
Original selected conjugate-pair table; HA is a hypothetical monoprotic acid, not a universal formula for every acid.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

Why are NH₄⁺ and NH₃ a conjugate pair but NH₄⁺ and NH₂⁻ not one adjacent pair?

Visual reasoning — after your attempt

The first pair differs by one H⁺. The second differs by two proton steps and therefore includes an intermediate conjugate relationship.

A conjugate acid/base pair differs by one H⁺ with corresponding charge change. Identify the donor and acceptor in the specific reaction; a species can play different roles with different partners. Proton transfer is not a change in the nucleus of an existing atom through a nuclear reaction.

NH₃/NH₄⁺ and H₂O/OH⁻ are conjugate pairs in NH₃+H₂O⇌NH₄⁺+OH⁻.

Pause and explain

Why are NH₃ and OH⁻ not conjugate partners?

Check your reasoning after answering

They do not differ by one proton on the same chemical framework. Their conjugates in the stated reaction are NH₄⁺ and H₂O respectively.

Part 3 · Separate strength from concentration

Acid strength describes the extent of proton-transfer equilibrium under defined conditions, commonly summarized by Ka. Concentration specifies how much acid is present per solution quantity. A dilute strong acid can have a similar pH to a more concentrated weak acid because they produce similar hydronium activity. The selected comparison uses a monoprotic ideal model at 25°C. Strong does not mean concentrated, corrosive in every context or guaranteed to have the lowest pH among unspecified samples.

Strength and concentration answer different questionsStrength and concentration answer different questionsSelected solution at 25°CFormalconcentrationApproximate pHStrong monoprotic acid0.0010 M3.00Weak acid, Ka=1.0×10⁻⁵0.10 M3.00Strength concerns equilibrium ionization, not simply the pH value.The more concentrated weak acid can match a dilute strong acid.
Original rounded comparison with Ka=1.0×10⁻⁵ for a hypothetical weak acid. It is not an instruction to identify unknown chemicals by contact or taste.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

Can pH near 3 alone distinguish the two selected acid solutions?

Visual reasoning — after your attempt

No. Their formal concentrations and ionization equilibria differ but give similar hydronium concentration under the model.

Strength concerns the extent of acid/base reaction with solvent under defined conditions; concentration is amount per volume. A dilute strong acid can contain less acid amount than a concentrated weak acid. Hazard also depends on identity, amount, and exposure, so one strength label is not a safety ranking.

A weak acid is not necessarily dilute. Use equilibrium information and initial concentration separately when predicting pH.

Pause and explain

Can a weak acid be concentrated?

Check your reasoning after answering

Yes. Weak refers to limited ionization under the stated equilibrium conditions, while concentration describes the supplied amount per solution volume.

Part 4 · Use equilibrium and multiple proton steps

A polyprotic acid can donate more than one proton through distinct steps, each with its own equilibrium constant. The species between steps can act as both a conjugate base and a further acid; amphiprotic means capable of donating or accepting a proton in suitable reactions. A base such as NH₃ can increase hydroxide through reaction with water without containing OH⁻ in its original formula. Do not infer complete multi-proton release merely from counting hydrogens in a formula; some hydrogens are not comparably acidic.

Several transferable protons require separate equilibrium stepsSeveral transferable protons require separate equilibrium stepsH₂A ⇌ H⁺ + HA⁻HA⁻ ⇌ H⁺ + A²⁻Ka₁ and Ka₂ describe separate steps.A base can accept a proton without containing OH⁻ initially.
Original hypothetical diprotic-acid sequence; H⁺ is aqueous proton shorthand and no equality of Ka values is implied.

Read the visual model

Identify the labeled structures and units, then trace the arrows or compare values under the caption’s stated conditions. Separate the displayed evidence from a further inference. Write your reasoning before opening the guidance.

Why does writing two H atoms in H₂A not justify treating both proton steps as equally strong?

Visual reasoning — after your attempt

Each step has a different species, charge environment and equilibrium constant; their extents must be evaluated separately.

Weak-acid/base reactions are reversible and their conjugates matter. Polyprotic acids lose protons in successive equilibria that need not have equal strengths. A full neutralization stoichiometry and a pH calculation can require different levels of detail.

For a diprotic acid, the second proton may not be fully released in the initial solution. Do not automatically set initial [H₃O⁺]=2C without an appropriate model.

Pause and explain

Why does two acidic hydrogens not always mean [H₃O⁺]=2C initially?

Check your reasoning after answering

Each deprotonation has its own equilibrium and may be incomplete. A stoichiometric total neutralization count is not the same as initial equilibrium hydronium concentration.

Apply the whole unit · Identify roles in ammonia equilibrium

Illustrative equation: NH₃+H₂O⇌NH₄⁺+OH⁻ in water. Treat this as a Brønsted–Lowry acid-base reaction rather than complete conversion.

Your task

Identify acid, base, conjugate pairs, and charge/atom conservation. Explain why the reaction does not require ammonia to contain hydroxide and why a weak-base label does not determine initial concentration.

What your response should show

  • Choose the appropriate chemical model or relationship and state its conditions.
  • Show substitutions, units, particle or charge counts, and intermediate reasoning as relevant.
  • Use supplied evidence to justify the conclusion; distinguish observation from inference.
  • Identify a meaningful limitation, uncertainty, or next measurement.
Response guidance — after your attempt

  1. Water donates H⁺ and is the acid in this reaction; ammonia accepts and is the base.

  2. Pairs: NH₃/NH₄⁺ and H₂O/OH⁻.

  3. Check total charge zero on each side and retain N,O,H counts.

The equation generates hydroxide through proton transfer, not through release of an OH group from ammonia. Weak-base behavior concerns equilibrium conversion and requires Kb for quantitative prediction; concentration must be supplied independently. The role assignments concern this reaction and need not describe water’s role with every other partner.

Scientific references and source-model foundations

Parent / teacher teaching notes

Have the learner mark the transferred proton and track charges. Avoid using acidity as an automatic identity or exposure-safety conclusion. Ask the learner to explain each particle symbol, axis, unit and arrow before calculating. Distinguish hypothetical models and calculated curves from measured records. Public OpenSciEd excerpts here are selected foundation prompts, not complete high-school teacher editions or an endorsement. All required advanced explanations and practice are provided locally.

Strong acid (HCl)ionizes completelyWeak acid (CH₃COOH)ionizes only partiallyH+Cl-H+Cl-H+Cl-HAHAHAHAH+A-
Selected strong/weak monoprotic-acid particle comparison at matched formal concentration. Ionization extent depends on the species and conditions; weakness does not mean every possible weak-acid sample is mostly un-ionized or has the same pH.

The key idea

Acid–base roles depend on the chosen model and partner. Proton-transfer strength, formal concentration and measured pH are distinct; polyprotic steps and base reactions with water require the appropriate equilibrium account.

Worked examples

Example 1

In the reaction HCl + H₂O → H₃O⁺ + Cl⁻, identify the Brønsted-Lowry acid and the Brønsted-Lowry base.
  1. A Brønsted-Lowry acid donates a proton (H⁺); a base accepts one.
  2. HCl gives up a proton to become Cl⁻, so HCl is the acid (proton donor).
  3. H₂O gains that proton to become H₃O⁺, so water is the base (proton acceptor).
Answer: HCl is the acid (proton donor); H₂O is the base (proton acceptor).

Example 2 · Worked application

In NH₃+H₂O⇌NH₄⁺+OH⁻, identify acid, base and conjugate pairs.
  1. Water donates a proton to NH₃, so water acts as acid and NH₃ as base.
  2. NH₄⁺ is the conjugate acid of NH₃; OH⁻ is water's conjugate base.
  3. Conjugate pairs differ by one proton.
Answer: Acid H₂O; base NH₃; pairs NH₃/NH₄⁺ and H₂O/OH⁻.
Check the reasoning

Acid/base roles depend on the stated reaction.

Example 3

Hydrochloric acid (HCl) ionizes completely in water, while acetic acid (CH₃COOH) ionizes only slightly. Which is the strong acid and which is the weak acid, and which solution has more H⁺ ions at the same concentration?
  1. A strong acid ionizes completely; a weak acid ionizes only partially.
  2. HCl ionizes completely, so it is the strong acid; acetic acid ionizes only partially, so it is the weak acid.
  3. At the same concentration, the strong acid (HCl) releases far more of its H⁺ ions into solution.
Answer: HCl is the strong acid and acetic acid is the weak acid; HCl gives more H⁺ at the same concentration.

Example 4 · Find and fix the mistake

Context: In NH₃+H₂O⇌NH₄⁺+OH⁻, identify acid, base and conjugate pairs. A student claims: “A base must contain OH in its formula.” Evaluate the claim and write a corrected explanation.
  1. Test the claim against the stated conditions: Acid/base roles depend on the stated reaction.
  2. NH₃ accepts a proton and increases OH⁻ through reaction with water without containing OH initially.
Answer: NH₃ accepts a proton and increases OH⁻ through reaction with water without containing OH initially.
Check the reasoning

Acid/base roles depend on the stated reaction.

Example 5 · Reason from the visual model

Strength and concentration answer different questionsStrength and concentration answer different questionsSelected solution at 25°CFormalconcentrationApproximate pHStrong monoprotic acid0.0010 M3.00Weak acid, Ka=1.0×10⁻⁵0.10 M3.00Strength concerns equilibrium ionization, not simply the pH value.The more concentrated weak acid can match a dilute strong acid.
Original rounded comparison with Ka=1.0×10⁻⁵ for a hypothetical weak acid. It is not an instruction to identify unknown chemicals by contact or taste.
Can pH near 3 alone distinguish the two selected acid solutions?
  1. Acid strength describes the extent of proton-transfer equilibrium under defined conditions, commonly summarized by Ka. Concentration specifies how much acid is present per solution quantity. A dilute strong acid can have a similar pH to a more concentrated weak acid because they produce similar hydronium activity. The selected comparison uses a monoprotic ideal model at 25°C. Strong does not mean concentrated, corrosive in every context or guaranteed to have the lowest pH among unspecified samples.
  2. No. Their formal concentrations and ionization equilibria differ but give similar hydronium concentration under the model.
Answer: No. Their formal concentrations and ionization equilibria differ but give similar hydronium concentration under the model.
Check the reasoning

Original rounded comparison with Ka=1.0×10⁻⁵ for a hypothetical weak acid. It is not an instruction to identify unknown chemicals by contact or taste.

Avoid these mistakes

Check your understanding

Explain why NH₃ is a base without containing OH in its formula.

Discuss the answer

It accepts a proton from water, forming NH₄⁺ and OH⁻; Brønsted bases need not contain hydroxide initially.

Return to the phenomenon

Return to the initial case. Does a lower pH always mean the acid species is stronger? Revise your drawing or explanation using two named ideas and one stated limitation.

Compare explanations after your attempt

pH describes hydronium activity, while strength concerns the species’ equilibrium behavior. Concentration and conditions are needed before using pH to compare solutions.

What should we investigate next?

Which comparison would test a remaining uncertainty? State what you would change, keep comparable and measure.

Investigate, calculate and revise

Keep a first model and an evidence-supported revision. Attempt each task before opening guidance; all required explanations and supplied-data routes are available locally.

Activity 1 · Return to the opening case

Assigned reading and preparation

Use the opening phenomenon, unit definitions and explained Parts 1–4.

Your task

Return to the initial case. Does a lower pH always mean the acid species is stronger? Revise your drawing or explanation using two named ideas and one stated limitation.

Success criteria

  • Use the stated observations and appropriate units.
  • Connect a labeled model with the chemical mechanism.
  • State assumptions and an evidence limitation.
  • Explain a substantive revision or next test.
Hint

Keep species identity, material transfer and energy account distinct.

Response guidance — discuss after your attempt

pH describes hydronium activity, while strength concerns the species’ equilibrium behavior. Concentration and conditions are needed before using pH to compare solutions.

Activity 2 · Transfer to a changed case

Assigned reading and preparation

Cover the worked answer and use the explained models, relationships and notebook.

Your task

In NH₃+H₂O⇌NH₄⁺+OH⁻, identify the acid, base and both conjugate pairs. Explain why NH₃ can act as a base without OH in its formula.

Success criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.
Hint

Name known quantities and conditions, show intermediate steps and check conservation.

Response guidance — discuss after your attempt

H₂O donates a proton and NH₃ accepts it. Pairs are H₂O/OH⁻ and NH₄⁺/NH₃. NH₃ produces hydroxide through proton transfer with water rather than having to release pre-existing OH⁻ groups.

Words to know

Arrhenius acid
A substance that releases hydrogen ions (H⁺, which form H₃O⁺) when dissolved in water.
Arrhenius base
A substance that releases hydroxide ions (OH⁻) when dissolved in water.
Brønsted-Lowry acid
A proton (H⁺) donor in a chemical reaction.
Brønsted-Lowry base
A proton (H⁺) acceptor in a chemical reaction.
strong acid
An acid that ionizes completely in water, releasing nearly all of its hydrogen ions.
weak acid
An acid that ionizes only partially in water, so most of its molecules stay intact at any moment.

A third definition: Lewis acids and bases

Arrhenius focused on ions in water and Brønsted-Lowry on proton transfer, but the broadest view came from Gilbert Lewis, who shifted attention to electrons. A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. Because a proton (H⁺) is an electron-pair acceptor, every Brønsted acid is also a Lewis acid — but the Lewis definition reaches further, covering reactions with no hydrogen at all, such as metal ions bonding to ammonia. Each definition is a wider lens: Arrhenius fits inside Brønsted-Lowry, which fits inside Lewis, and chemists pick whichever level of generality a problem needs.

Conjugate acid-base pairs: the partners in every proton transfer

Every Brønsted-Lowry reaction actually involves two acid-base pairs. When an acid donates a proton, what's left behind is a conjugate base; when a base accepts a proton, it becomes a conjugate acid. In HCl + H₂O → H₃O⁺ + Cl⁻, HCl and Cl⁻ are one conjugate pair, and H₂O and H₃O⁺ are the other. A useful rule follows: the stronger an acid, the weaker its conjugate base, because a substance that lets go of its proton easily has little tendency to grab it back. Recognizing conjugate pairs turns a confusing equation into a simple story of a proton changing hands.

Strong vs. weak, concentrated vs. dilute: untangling four words

These four words trip up nearly every chemistry student because two of them describe ionization and two describe amount. Strong and weak describe how completely an acid or base breaks apart into ions: a strong acid ionizes almost 100%, a weak acid only a few percent. Concentrated and dilute describe how much acid is dissolved per liter — the molarity. The two axes are independent, so all four combinations exist: dilute strong acid, concentrated strong acid, dilute weak acid, and concentrated weak acid. Vinegar is a dilute weak acid; the acid in a car battery is a concentrated strong acid. Keeping the axes separate is the key to the whole topic.

How your tongue tastes an acid

The sour taste of acids isn't a coincidence — it's your biology detecting hydrogen ions directly. Specialized taste receptor cells on your tongue respond to the H⁺ that acids release, which is why lemon juice (citric acid) and vinegar (acetic acid) taste sharp and sour. Evolution likely favored sour detection to warn against spoiled or unripe food, both of which tend to be acidic. Bitterness, the hallmark of many bases and alkaloids, is detected by a completely different family of receptors and probably evolved to flag potential toxins. Your sense of taste is, quite literally, a portable acid-base indicator — though, unlike litmus, one you should never use on lab chemicals.

The acids and bases in your home — and one dangerous mix

Household chemistry is full of acids and bases: vinegar and lemon juice are mild acids, while baking soda, ammonia, oven cleaner, and bleach are bases. Mixing an acid and a base intentionally can be handy — baking soda neutralizes an acid spill or a bee sting. But some combinations are genuinely dangerous. Mixing bleach with ammonia (or with acidic cleaners) releases toxic chloramine and chlorine gases that can severely damage the lungs. This is why cleaning-product labels warn 'do not mix,' and it's a real-world reminder that acid-base reactions are powerful chemistry, not just textbook color changes.

In the real world

Your stomach makes strong hydrochloric acid to digest food, while the weak citric and acetic acids in fruit and vinegar are mild enough to eat every day.

Exit check

Explain why NH₃ is a base without containing OH in its formula.

Check your explanation after answering

It accepts a proton from water, forming NH₄⁺ and OH⁻; Brønsted bases need not contain hydroxide initially.

Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.

Problem 1

An Arrhenius acid produces which ion when dissolved in water?
  1. hydrogen ion, H⁺ (as H₃O⁺)
  2. sodium ion, Na⁺
  3. chloride ion, Cl⁻
  4. hydroxide, OH⁻
Answer and explanation

hydrogen ion, H⁺ (as H₃O⁺)

Arrhenius acids release H⁺ ions, which combine with water to form hydronium, H₃O⁺.

Problem 2

In the Brønsted-Lowry model, a base is defined as a…
  1. proton donor
  2. proton acceptor
  3. electron donor
  4. hydroxide crystal
Answer and explanation

proton acceptor

A Brønsted-Lowry base accepts a proton (H⁺).

Problem 3

Which statement correctly describes a strong acid?
  1. It is safe to taste because it is common
  2. It cannot conduct electricity
  3. It ionizes completely in water, releasing nearly all its H⁺
  4. It always has a pH above 7
Answer and explanation

It ionizes completely in water, releasing nearly all its H⁺

A strong acid ionizes essentially completely, releasing almost all of its hydrogen ions into solution.

Problem 4

Explain the difference between a strong acid and a concentrated acid.
Answer and explanation

'Strong' describes how completely the acid ionizes in water — a strong acid ionizes almost 100%. 'Concentrated' describes how much acid is dissolved per volume (a high molarity). The two are independent: you can have a dilute solution of a strong acid or a concentrated solution of a weak acid.

Problem 5

Water reacts as a base in HCl + H₂O → H₃O⁺ + Cl⁻ but as an acid in NH₃ + H₂O → NH₄⁺ + OH⁻. Use the Brønsted-Lowry definitions to explain water's role in each reaction.
Answer and explanation

In the first reaction water accepts a proton from HCl (becoming H₃O⁺), so it acts as a Brønsted-Lowry base. In the second reaction water donates a proton to ammonia (becoming OH⁻), so it acts as a Brønsted-Lowry acid. A substance that can do both is called amphoteric.

Problem 6

Identify the conjugate base of H₂CO₃ after loss of one proton. Show the reasoning, not just the conclusion.
Hints

Acid/base roles depend on the stated reaction.

Answer and explanation

HCO₃⁻.

Problem 7

Can water act as both acid and base in different reactions?
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Yes. It can donate or accept a proton depending on its reaction partner.

Problem 8

Strength and concentration answer different questionsStrength and concentration answer different questionsSelected solution at 25°CFormalconcentrationApproximate pHStrong monoprotic acid0.0010 M3.00Weak acid, Ka=1.0×10⁻⁵0.10 M3.00Strength concerns equilibrium ionization, not simply the pH value.The more concentrated weak acid can match a dilute strong acid.
Original rounded comparison with Ka=1.0×10⁻⁵ for a hypothetical weak acid. It is not an instruction to identify unknown chemicals by contact or taste.
Interpret the supplied visual. Can pH near 3 alone distinguish the two selected acid solutions?
Answer and explanation

No. Their formal concentrations and ionization equilibria differ but give similar hydronium concentration under the model.

Acid strength describes the extent of proton-transfer equilibrium under defined conditions, commonly summarized by Ka. Concentration specifies how much acid is present per solution quantity. A dilute strong acid can have a similar pH to a more concentrated weak acid because they produce similar hydronium activity. The selected comparison uses a monoprotic ideal model at 25°C. Strong does not mean concentrated, corrosive in every context or guaranteed to have the lowest pH among unspecified samples.

No. Their formal concentrations and ionization equilibria differ but give similar hydronium concentration under the model.

Response criteria

  • Interpret labels, structures, axes, units and directions correctly.
  • Explain the chemistry relationship or mechanism.
  • Respect stated assumptions and distinguish observation from inference.

Problem 9

In NH₃+H₂O⇌NH₄⁺+OH⁻, identify the acid, base and both conjugate pairs. Explain why NH₃ can act as a base without OH in its formula.
Answer and explanation

H₂O donates a proton and NH₃ accepts it. Pairs are H₂O/OH⁻ and NH₄⁺/NH₃. NH₃ produces hydroxide through proton transfer with water rather than having to release pre-existing OH⁻ groups.

H₂O donates a proton and NH₃ accepts it. Pairs are H₂O/OH⁻ and NH₄⁺/NH₃. NH₃ produces hydroxide through proton transfer with water rather than having to release pre-existing OH⁻ groups.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the chemical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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