Chemistry Clarity · chemistryclarity.com

What Are Acids and Bases?

What is the difference between an acid and a base?

BeginnerAcids & BasesLast reviewed 3 October 2026
Download lesson (PDF)

What is it?

Acids and bases are two important families of substances, and they react with each other. Two definitions are used at this level:

AcidBase
Arrhenius (in water)Produces HX+\ce{H+} ions (more precisely HX3OX+\ce{H3O+}) when dissolved in waterProduces OHX−\ce{OH-} ions when dissolved in water
Brønsted–Lowry (more general)Donates a proton (HX+\ce{H+})Accepts a proton (HX+\ce{H+})

A hydrogen ion, HX+\ce{H+}, is just a proton: a hydrogen atom that has lost its only electron.

Key idea

In the Brønsted–Lowry picture, an acid–base reaction is a proton transfer: the acid gives an HX+\ce{H+} and the base takes it.

Why does it matter?

Acids and bases are everywhere:

  • In your body: stomach acid contains hydrochloric acid, HCl\ce{HCl}, which helps digest food, and the pH of blood is kept within a narrow range.
  • At home: vinegar contains acetic acid; baking soda (sodium hydrogen carbonate) and many cleaning products are basic; antacid tablets neutralize excess stomach acid.
  • In the environment and industry: soil acidity affects which crops grow; acid–base reactions are used to make fertilisers, medicines and many other products.

How does it work?

1. Acids donate protons

When hydrogen chloride dissolves in water, it gives a proton to a water molecule:

HCl+HX2O→HX3OX++ClX−\ce{HCl + H2O -> H3O+ + Cl-}

HCl\ce{HCl} is the acid (proton donor) and water is the base (proton acceptor). The ion HX3OX+\ce{H3O+} is called the hydronium ion.

2. Bases accept protons

Some bases contain hydroxide ions, such as sodium hydroxide: NaOH→NaX++OHX−\ce{NaOH -> Na+ + OH-}. The OHX−\ce{OH-} ion accepts protons.

Other bases contain no OHX−\ce{OH-} at all. Ammonia accepts a proton from water and produces OHX−\ce{OH-}:

NHX3+HX2O⇌NHX4X++OHX−\ce{NH3 + H2O <=> NH4+ + OH-}

3. Conjugate acid–base pairs

When an acid gives away a proton, what is left can take a proton back, so it is a base: its conjugate base. When a base accepts a proton it becomes its conjugate acid. The two members of a conjugate pair differ by exactly one HX+\ce{H+}:

  • HCl\ce{HCl} / ClX−\ce{Cl-}
  • HX3OX+\ce{H3O+} / HX2O\ce{H2O}
  • NHX4X+\ce{NH4+} / NHX3\ce{NH3}

4. Strong and weak acids and bases

  • A strong acid ionizes completely in water: essentially every molecule gives up its proton. Examples: HCl\ce{HCl}, HNOX3\ce{HNO3}, HX2SOX4\ce{H2SO4} (its first proton).
  • A weak acid ionizes only partly: most molecules stay intact. Example: acetic acid, CHX3COOH\ce{CH3COOH}.
  • Strong bases include the hydroxides of Group 1 metals, such as NaOH\ce{NaOH} and KOH\ce{KOH}. Weak bases include ammonia, NHX3\ce{NH3}.

A double arrow (⇌\ce{<=>}) shows a reaction that goes only partly; a single arrow (→\ce{->}) shows one that goes essentially to completion.

5. Neutralization

An acid and a base react to form a salt and water:

HCl+NaOH→NaCl+HX2O\ce{HCl + NaOH -> NaCl + H2O}

The essential change is that HX+\ce{H+} and OHX−\ce{OH-} combine to make water: HX++OHX−→HX2O\ce{H+ + OH- -> H2O}.

Think of it like this

Think of the proton as a ball. The acid throws the ball and the base catches it. After the throw, the thrower is empty-handed but could catch a ball back (the conjugate base), and the catcher now holds a ball it could throw again (the conjugate acid).

More precisely

A bare HX+\ce{H+} ion doesn’t float freely in water: it is always attached to water molecules. Writing HX+(aq)\ce{H+(aq)} is a convenient shorthand for HX3OX+(aq)\ce{H3O+(aq)}. There is also a broader definition, the Lewis definition, in which an acid accepts a pair of electrons and a base donates one. It covers reactions with no proton transfer at all.

Visualise it

Proton transfer diagram. HCl, the acid, gives an H plus ion to H2O, the base. The products are Cl minus, the conjugate base, and H3O plus, the conjugate acid. Dashed lines join the conjugate pairs: HCl with Cl minus, and H2O with H3O plus; each pair differs by one H plus.
An acid–base reaction is a proton (H⁺) transfer.

Worked example

Worked example: Identifying acid, base and conjugate pairs

Question: In the reaction NHX3+HX2O⇌NHX4X++OHX−\ce{NH3 + H2O <=> NH4+ + OH-}, identify the acid, the base and the two conjugate pairs.

  1. Water loses a proton (it becomes OHX−\ce{OH-}), so water is the acid.
  2. Ammonia gains a proton (it becomes NHX4X+\ce{NH4+}), so ammonia is the base.
  3. Conjugate pairs differ by one HX+\ce{H+}: NHX4X+\ce{NH4+} / NHX3\ce{NH3} and HX2O\ce{H2O} / OHX−\ce{OH-}.

Note: in the HCl reaction, water acted as a base; here it acts as an acid. Substances that can do both are called amphoteric.

Worked example: Writing a neutralization equation

Question: Write the equation for the neutralization of nitric acid, HNOX3\ce{HNO3}, by potassium hydroxide, KOH\ce{KOH}.

  1. Acid + base → salt + water.
  2. The salt combines the metal ion from the base (KX+\ce{K+}) with the anion from the acid (NOX3X−\ce{NO3-}): KNOX3\ce{KNO3}.

Answer: HNOX3+KOH→KNOX3+HX2O\ce{HNO3 + KOH -> KNO3 + H2O}

Common mistake

Common mistake: Confusing 'strong' with 'concentrated'

Strong describes how completely an acid ionizes. Concentrated describes how much acid is dissolved in a given volume. A dilute solution of a strong acid (HCl) and a concentrated solution of a weak acid (acetic acid) are both possible.

Common mistake: Thinking every base contains OH⁻

Ammonia, NHX3\ce{NH3}, has no hydroxide ion, but it is a base because it accepts a proton. The Brønsted–Lowry definition covers it; the Arrhenius definition, strictly applied, does not.

Common mistake: Mixing up conjugate pairs

A conjugate pair differs by exactly one HX+\ce{H+}. HX3OX+\ce{H3O+} and OHX−\ce{OH-} are not a conjugate pair: they differ by two protons.

Notation note

  • HX+(aq)\ce{H+(aq)} and HX3OX+(aq)\ce{H3O+(aq)} are both used for the hydrated proton; many textbooks switch between them.
  • “Ionize” and “dissociate” are often used interchangeably for acids in water.
  • Some courses spell the term neutralisation (British spelling).

Remember this

Remember this

  • Acid = proton (H⁺) donor; base = proton (H⁺) acceptor.
  • Conjugate pairs differ by exactly one H⁺.
  • Strong = ionizes completely; weak = ionizes partly. Strong ≠ concentrated.
  • Acid + base → salt + water.

Test yourself

Check your understanding before moving on.

Flashcards

11 cards

  1. Question
    What is a Brønsted–Lowry acid?
    Answer

    A proton (HX+\ce{H+}) donor.

  2. Question
    What is a Brønsted–Lowry base?
    Answer

    A proton (HX+\ce{H+}) acceptor.

  3. Question
    What is an Arrhenius acid?
    Answer

    A substance that produces HX+\ce{H+} (more precisely HX3OX+\ce{H3O+}) ions when dissolved in water.

  4. Question
    What is the hydronium ion?
    Answer

    HX3OX+\ce{H3O+}: a proton attached to a water molecule.

  5. Question
    What is the conjugate base of HCl\ce{HCl}?
    Answer

    ClX−\ce{Cl-} (HCl minus one H⁺)

  6. Question
    What is the conjugate acid of NHX3\ce{NH3}?
    Answer

    NHX4X+\ce{NH4+} (NH₃ plus one H⁺)

  7. Question
    What is the difference between a strong acid and a weak acid?
    Answer

    A strong acid ionizes completely in water; a weak acid ionizes only partly.

  8. Question
    Is "strong" the same as "concentrated"?
    Answer

    No. Strong = how completely it ionizes. Concentrated = how much is dissolved per volume.

  9. Question
    Why is ammonia a base even though it has no OHX−\ce{OH-}?
    Answer

    It accepts a proton: NHX3+HX2O⇌NHX4X++OHX−\ce{NH3 + H2O <=> NH4+ + OH-}

  10. Question
    What are the products of a neutralization reaction?
    Answer

    A salt and water, e.g. HCl+NaOH→NaCl+HX2O\ce{HCl + NaOH -> NaCl + H2O}

  11. Question
    What does "amphoteric" mean?
    Answer

    Able to act as an acid or as a base. Water is amphoteric.

Quiz

7 questions

  1. Question 1EasyAccording to the Brønsted–Lowry definition, an acid is a substance that…
    Show answer

    Answer: donates a proton

    A Brønsted–Lowry acid donates a proton (HX+\ce{H+}) and a base accepts one. Accepting an electron pair is the Lewis definition of an acid; producing OHX−\ce{OH-} describes an Arrhenius base.

  2. Question 2EasyIn HCl+HX2O→HX3OX++ClX−\ce{HCl + H2O -> H3O+ + Cl-}, which substance acts as the base?
    Show answer

    Answer: HX2O\ce{H2O}

    Water accepts the proton from HCl\ce{HCl} and becomes HX3OX+\ce{H3O+}, so water is the base here.

  3. Question 3MediumWhat is the conjugate acid of NHX3\ce{NH3}?
    Show answer

    Answer: NHX4X+\ce{NH4+}

    A conjugate acid has one more HX+\ce{H+} than its base: NHX3\ce{NH3} + HX+\ce{H+} → NHX4X+\ce{NH4+}. NHX2X−\ce{NH2-} is the conjugate base of NHX3\ce{NH3}.

  4. Question 4MediumWhich pair is a conjugate acid–base pair?
    Show answer

    Answer: HNOX3\ce{HNO3} and NOX3X−\ce{NO3-}

    A conjugate pair differs by exactly one HX+\ce{H+}. HNOX3\ce{HNO3} and NOX3X−\ce{NO3-} do. HX3OX+\ce{H3O+}/OHX−\ce{OH-} and NHX4X+\ce{NH4+}/NHX2X−\ce{NH2-} differ by two protons.

  5. Question 5EasyWhat does it mean to say that acetic acid is a weak acid?
    Show answer

    Answer: It ionizes only partly in water

    Weak refers to how much the acid ionizes, not how much is dissolved. Weak acids still react with bases; a concentrated solution of a weak acid is possible.

  6. Question 6EasyWhat are the products when nitric acid, HNOX3\ce{HNO3}, is neutralized by potassium hydroxide, KOH\ce{KOH}?
    Show answer

    Answer: KNOX3+HX2O\ce{KNO3 + H2O}

    Acid + base → salt + water. The salt combines KX+\ce{K+} from the base with NOX3X−\ce{NO3-} from the acid: HNOX3+KOH→KNOX3+HX2O\ce{HNO3 + KOH -> KNO3 + H2O}.

  7. Question 7HardIn NHX3+HX2O⇌NHX4X++OHX−\ce{NH3 + H2O <=> NH4+ + OH-}, what role does water play?
    Show answer

    Answer: Acid

    Water donates a proton to ammonia and becomes OHX−\ce{OH-}, so it acts as an acid. With HCl, water acts as a base, which is why water is called amphoteric.

Notes and downloads

  • Worksheet

    Acids and Bases Worksheet

    8 questions on Brønsted–Lowry acids and bases, conjugate pairs, strong vs weak and neutralization equations. Answer key included.

    BeginnerFree

References

  1. Brown, T. L.; LeMay, H. E., Jr.; Bursten, B. E.; Murphy, C. J.; Woodward, P. M.; Stoltzfus, M. W. Chemistry: The Central Science, 15th ed.; Pearson, 2022.

Practise this topic with flashcards and a quiz at chemistryclarity.com/chemistry/acids-and-bases/

Spotted a mistake? Let us know and we'll fix it.