Which Substances Are Acids Check All That Apply

You're staring at a multiple-choice question. Plus, check all that apply. " The options stare back: HCl, NaOH, H₂SO, NH, CHCOOH, H₂O. Your palm sweats. But you know some* of these. On the flip side, "Which substances are acids? But which ones count*?

This isn't just a test question. It's a fundamental chemistry skill — recognizing acids by formula, by name, by behavior. And most people overcomplicate it.

What Is an Acid, Really

Forget the textbook definition for a second. But an acid is a substance that donates protons (H ions) when dissolved in water. That's the Brønsted-Lowry definition, and it's the one that actually helps you identify* acids in practice.

The Arrhenius definition — "produces H in aqueous solution" — works for most introductory purposes. The Lewis definition — "electron pair acceptor" — matters more in organic mechanisms. But for "check all that apply" questions? Brønsted-Lowry is your workhorse.

Acids taste sour (don't test this). Day to day, they turn blue litmus red. On the flip side, they react with metals to produce hydrogen gas. They neutralize bases. But the fastest* way to spot them on a list? Look at the formula.

The Hydrogen-at-the-Front Rule

Most common acids start with hydrogen. HCl. H₂SO. HNO. HPO. CHCOOH (acetic acid — hydrogen's there, just not first). This pattern holds for the vast majority of acids you'll encounter in general chemistry.

But — and this trips people up — not everything starting with H is an acid. On the flip side, cH (methane) isn't. In real terms, h₂ (hydrogen gas) isn't. NH (ammonia) starts with N but acts as a base. The hydrogen has to be ionizable* — attached to something electronegative enough to let it go.

Strong vs. Weak: The Distinction That Matters

Seven strong acids. Memorize them. Everything else is weak.

The seven strong acids:

• HCl (hydrochloric)
• HBr (hydrobromic)
• HI (hydroiodic)
• HNO (nitric)
• HClO (perchloric)
• HClO (chloric)
• H₂SO (sulfuric — first proton only)

These dissociate completely in water. In practice, no equilibrium. Because of that, the rest — acetic acid, carbonic acid, phosphoric acid (second and third protons), hydrofluoric acid, and thousands of organic acids — exist in equilibrium. Consider this: 100%. Mostly undissociated.

This distinction changes how you answer "check all that apply." If the question asks "which are strong* acids," you check only the seven. If it asks "which are acids," you check the seven plus* every weak acid on the list.

Why It Matters / Why People Care

You're not learning this to pass a quiz. You're learning it because acid identification shows up everywhere.

In biology: stomach acid (HCl), lactic acid in muscles, DNA's phosphoric acid backbone. In cooking: vinegar (acetic acid), lemon juice (citric acid), buttermilk (lactic acid). In environmental science: acid rain (H₂SO, HNO), ocean acidification (carbonic acid). In industry: sulfuric acid production exceeds any other chemical* by volume — fertilizers, petroleum refining, batteries.

Misidentify an acid in a lab? You grab the wrong bottle. That's why mix concentrated sulfuric acid with water the wrong way (acid into* water, always) and it erupts. Confuse a weak acid for a strong one in a buffer calculation? Your pH is off by orders of magnitude.

And on exams? "Check all that apply" questions are partial credit traps. Miss one acid on a five-option question — you lose the whole thing. Know the patterns, and you get full credit every time.

How to Identify Acids on a List

Step 1: Memorize the Seven Strong Acids

No shortcut here. On top of that, say them aloud. Here's the thing — these are always* acids. Day to day, flashcards. Always strong. Consider this: hCl, HBr, HI, HNO, HClO, HClO, H₂SO. Write them out. If you see them, check the box.

Step 2: Recognize the "Hydrogen + Anion" Pattern

See a formula starting with H followed by a nonmetal or polyatomic ion? Probably an acid.

• HClO₂ (chlorous acid)
• H₂CO (carbonic acid)
• HPO (phosphoric acid)
• H₂S (hydrosulfuric acid)
• HCN (hydrocyanic acid)

The name usually gives it away: "hydro___ic acid" for binary acids, "___ic acid" or "___ous acid" for oxyacids.

Step 3: Know the Common Weak Acids by Name and Formula

These appear constantly on exams:

Name	Formula	Context
Acetic acid	CHCOOH / HC₂HO₂	Vinegar, buffers
Carbonic acid	H₂CO	Blood buffer, sodas
Phosphoric acid	HPO	Colas, fertilizers
Citric acid	CHO	Citrus fruits
Lactic acid	CHO	Muscle fatigue, yogurt
Formic acid	HCOOH	Ant stings
Hydrofluoric acid	HF	Glass etching, weak* despite halogen
Benzoic acid	CHCOOH	Preservative
Oxalic acid	H₂C₂O	Spinach, rhubarb

HF deserves a callout. It's a weak* acid (Ka ≈ 6.8 × 10). Students assume all hydrohalic acids are strong. HCl, HBr, HI are. HF isn't. Fluorine's small size and high electronegativity make the H–F bond surprisingly strong.

Step 4: Spot Organic Acids — The Carboxylic Acid Group

Any molecule with –COOH (carboxyl group) is an acid. Doesn't matter how big the rest of the molecule is.

CHCOOH (acetic) — acid
CHCOOH (stearic) — acid
HCOOH (formic) — acid
CHCOOH (benzoic) — acid

The hydrogen on the –OH of the carboxyl group is the acidic one. The other hydrogens on the carbon chain? Not acidic under normal conditions.

Step 5: Recognize Polyprotic Acids

Some acids have more than one ionizable hydrogen.

• H₂SO (diprotic) — first proton strong, second weak (Ka₂ = 1.2 × 10²)
• H₂CO (diprotic) — both weak
• HPO (triprotic) — all three weak, successively weaker
• H₂C₂O (oxalic, diprotic) — both weak
• H₂S (diprotic) — both very weak

On a "check all that apply" question, H₂SO counts as an acid. So does HSO (bisulfate) — it can still donate a proton.

Common Acid-Base Pairs You Must Know

Strong Acid + Strong Base → Salt + Water

HCl + NaOH → NaCl + H₂O

Weak Acid + Strong Base → Conjugate Base + Water

CH₃COOH + NaOH → CH₃COO⁻Na⁺ + H₂O

Strong Acid + Weak Base → Conjugate Acid + Salt

HCl + NH₃ → NH₄⁺Cl⁻

Weak Acid + Weak Base → Salt (no reaction in most cases)

CH₃COOH + NH₃ → CH₃COO⁻NH₄⁺

Memorize these four scenarios. They cover 90% of acid-base reaction questions.

The pH Scale and What It Means

pH = -log[H⁺]

• pH 0-3: Strong acids (completely dissociated)
• pH 4-6: Weak acids (partially dissociated)
• pH 7: Neutral (pure water)
• pH 8-10: Weak bases (partially dissociated)
• pH 11-14: Strong bases (completely dissociated)

Key insight: pH doesn't tell you concentration. 0.1 M HCl and 1 M acetic acid both have pH ≈ 1.

Buffer Solutions: Your Safety Net

Buffers resist pH changes when you add acid or base.

Continue exploring with our guides on 6 protons 6 neutrons 6 electrons atomic mass and aluminum metal reacts with hydrochloric acid.

Buffer equation: pH = pKa + log([A⁻]/[HA])

Example: Acetic acid + sodium acetate

• pKa = 4.76
• If [A⁻]/[HA] = 1, then pH = 4.76
• Add strong base → [A⁻] increases, pH rises slightly
• Add strong acid → [HA] increases, pH drops slightly

Exam tip: When asked to calculate buffer pH, identify the conjugate pair first, then plug into Henderson-Hasselbalch.

Redox Reactions: Oxidation State Changes

Oxidation = loss of electrons (increase in oxidation state) Reduction = gain of electrons (decrease in oxidation state)

Identifying Oxidation States

• Group 1 metals = +1
• Group 2 metals = +2
• Hydrogen = +1 (except in metal hydrides)
• Oxygen = -2 (except in peroxides)
• Sum of oxidation states = 0 (neutral compound) or charge (ion)

Common Redox Pairs

• MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺
• Cr₂O₇²⁻ + H⁺ + NO₃⁻ → Cr³⁺ + H₂O + NO₂
• Cl₂ + OH⁻ → Cl⁻ + ClO⁻ + H₂O

Half-reaction method:

1. Split into oxidation and reduction half-reactions
2. Balance atoms other than O and H
3. Add H₂O to balance O
4. Add H⁺ to balance H
5. Add electrons to balance charge
6. Multiply and combine

Thermodynamics: When Reactions Happen

ΔG = ΔH - TΔS

• ΔG < 0: Spontaneous
• ΔG > 0: Non-spontaneous
• ΔG = 0: Equilibrium

Entropy (S): Disordered systems favor spontaneity Enthalpy (H): Exothermic (ΔH < 0) favors spontaneity

Le Chatelier's Principle: If you stress a system at equilibrium, it shifts to minimize the stress.

• Add reactant → shift toward products
• Add product → shift toward reactants
• Increase temperature → shift toward endothermic direction
• Increase pressure → shift toward fewer gas moles

Equilibrium Constants: The Numbers Game

Kc = [products]ᶜ/[reactants]ʳ

Rule: Concentrations of solids and liquids = 1 (don't include in expression)

Example: CaCO₃(s) ⇌ Ca²⁺(aq) + CO₃²⁻(aq) Kc = [Ca²⁺][CO₃²⁻] (solid omitted)

Small K (K < 10⁻³): Reactants favored Large K (K > 10³): Products favored K ≈ 10⁻³ to 10³: Significant amounts of both

ICE Tables (Initial, Change, Equilibrium): Use when given K and asked to find concentrations, or vice versa.

Electrochemistry: Batteries in a Beaker

Cell potential: E°cell = E°cathode - E°anode

Standard reduction potentials: Look up in table

• Higher E° = stronger oxidizing agent
• Lower E° = stronger reducing agent

Nernst Equation: E = E° - (RT/nF)ln(Q)

At 25°C: E = E° - (0.0592/n)log(Q)

Example: Zn²⁺ + Cu → Zn + Cu²⁺ E°cell = 0.34 - (-0.76) = 1.10 V

Kinetics: Speed Matters

Rate = k[Reactants]ᵐ[Products]ⁿ

Factors affecting rate:

• Concentration (higher = faster)
• Temperature (higher = faster)
• Catalysts (lower activation energy)
• Surface area (more = faster)

Activation energy (Ea): Energy barrier molecules must overcome

Arrhenius equation: k = Ae^(-Ea/RT)

Organic Chemistry: Functional Group Recognition

Alcohol

Alcohol

• General formula: R-OH
• Identification: Broad IR peak around 3200-3600 cm⁻¹
• Reactivity: Can be oxidized to aldehydes/ketones (primary/secondary), esterification with carboxylic acids

Carbonyl Compounds

• Aldehydes: R-CHO (terminal carbonyl)
• Ketones: R-CO-R' (internal carbonyl)
• Carboxylic Acids: R-COOH (broad OH peak + C=O stretch ~1710 cm⁻¹)

Amines

• Primary: R-NH₂
• Secondary: R-NH-R'
• Tertiary: R-N-R'
• Identification: N-H stretches (~3300 cm⁻¹) in primary/secondary amines

Nuclear Chemistry: Breaking Apart Atoms

Types of decay:

• Alpha (α): Loss of ₂He⁴ (2 protons, 2 neutrons)
• Beta (β): Neutron → proton + electron (increases atomic number by 1)
• Gamma (γ): High-energy photon emission (no change in composition)

Half-life: Time for half the radioactive sample to decay

• First-order kinetics: ln([A]/[A]₀) = -kt
• Relationship: t₁/₂ = 0.693/k

Final Thoughts: Connecting the Concepts

Chemistry isn't just memorizing reactions—it's understanding the fundamental principles that govern matter's behavior. Also, equilibrium helps us optimize industrial processes. Kinetics explains why some reactions need a spark while others proceed slowly. Redox reactions power our devices and sustain life. Electrochemistry bridges chemistry and electricity. Consider this: thermodynamics tells us whether reactions will occur spontaneously. Organic chemistry reveals the complexity of carbon-based molecules. Nuclear chemistry shows us the energy stored within atoms themselves.

Mastering these interconnected concepts allows you to predict reaction outcomes, design new materials, and understand the world at the molecular level. Whether you're troubleshooting a failed reaction, designing a battery, or synthesizing a new drug, these principles provide the foundation for success in chemistry and beyond.