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6.1: What is an Acid and a Base?

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    General Properties of Acids and Bases

    We commonly encounter acids and bases in our foods –some foods are acidic, and others are basic (alkaline) as illustrated in Fig. 6.1.1.

    clipboard_e5c801c95f9e59160611ade1634fd6fe0.png
    Figure \(\PageIndex{1}\): Acidic and alkaline foods along with a pH scale, Copyright; Public domain

    The general properties of acids and bases are the following.

    1. Acids taste sour, e.g., citrus fruits taste source because of citrus acid and ascorbic acid, i.e., vitamin C, in them. Basic (alkaline) substances, on the other hand, taste bitter.
    2. Basic (alkaline) substances feel soupy, while acidic substances may sting.
    3. The acids turn blue litmus paper to read but do not change the color of red litmus paper. Bases turn red litmus paper blue but do not change the color of blue litmus paper, as illustrated in Fig. 6.1.2.
    4. Phenolphthalein indicator turns colorless in acid and turns pink in basic solution, as illustrated in Fig. 6.1.3.
    5. Acids and bases neutralize each other. Hydrochloric acid is found in the stomach that helps digestion. Excess hydrochloric acid may cause acid burns—antacids like milk of magnesia are bases that help by neutralizing excess acid in the stomach.
    Blue and red litmus strips shown with sodium hydroxide, ammonia, acetic acid, and hydrochloric acid applied to them.
    Figure \(\PageIndex{2}\): Demonstration of acids turn red litmus paper blue and bases turn blue litmus paper red - solution listed at the top of the image was spotted on the red litmus paper (top) or blue litmus paper (bottom).
    clipboard_e94d6744533486299a93e163c06bab4d0.png
    Figure \(\PageIndex{3}\): Colours of phenolphthalein in acid(left) and base (right) solutions. Source: User:Siegert / Public domain.

    Arrhenius's Definition of Acids and Bases

    The earliest definition of acids and bases is Arrhenius's definition which states that:

    • An acid is a substance that forms hydrogen ions H+ when dissolved in water, and
    • A base is a substance that forms hydroxide ions OH- when dissolved in water.

    For example, hydrochloric acid (\(\ce{HCl}\)) is an acid because it forms \(\ce{H^{+}}\) when it dissolves in water.

    \[\mathrm{HCl}(\mathrm{g}) \stackrel{\text { Water }}{\longrightarrow} \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})\nonumber\]

    Similarly, NaOH is a base because it forms OH- when it dissolves in water.

    \[\mathrm{NaOH}(\mathrm{s}) \stackrel{\text { Water }}{\longrightarrow} \mathrm{Na}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})\nonumber\]
    Note that hydrogen ion H+ does not exist in reality. It bonds with water molecules and exists as hydronium ion H3O+(aq).
    \[\mathrm{H}^{+}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})\nonumber\]

    However, \(\ce{H^{+}(aq)}\) is often written in the place of (H3O+(aq).

    Naming Arrhenius acids and bases

    Table 1 lists the names and formulas of some of the common acids and their anions.

    Rules for naming acids

    1. The names end with the word “acid.”
    2. If the anion is not an oxyanion, then add the prefix hydro- to the name of the anion and change the last syllable of the anion name to –ic. For example, Cl- is a chloride ion, and HCl is hydrochloric acid.
    3. If the anion is an oxyanion with the last syllable –ate, change the last syllable with –ic. Do not use the prefix hydro-, but add the last word “acid.” If there is a prefix per- in the name of the oxyanion, retain the prefix in the acid name. For example, NO3- is a nitrate, and HNO3 is nitric acid. Another example, ClO4- is a perchlorate, and HClO3 is perchloric acid.
    4. If the anion is an oxyanion with the last syllable –ite, change the last syllable with –ous. Do not use the prefix hydro-, but add the last word “acid.” If there is a prefix hypo- in the name of the oxyanion, retain the prefix in the acid name. For example, NO2- is nitrite, and HNO2 is nitrous acid. Another example, ClO- is hypochlorite, and HClO is hypochlorous acid.
    Table 1: Names of some common acids and their anions.
    Acid formula Acid name Anion Anion name
    HCl Hydrochloric acid Cl- Chloride
    HBr Hydrobromic acid Br- Bromide
    HI Hydroiodic acid I- Iodide
    HCN Hydrocyanic acid CN- Cyanide
    HNO3 Nitric acid NO3- Nitrate
    HNO2 Nitrous acid NO2- Nitrite
    H2SO4 Sulfuric acid SO42- Sulfate
    H2SO3 Sulfurous acid SO32- Sulfite
    H2CO3 Carbonic acid CO3- Carbonate
    CH3COOH Acetic acid CH3COO- Acetate
    H3PO4 Phosphoric acid PO43- Phosphate
    H3PO3 Phosphorous acid PO33- Phosphite
    HClO4 Perchloric acid ClO4- Perchlorate
    HClO3 Chloric acid ClO3- Chlorate
    HClO2 Chlorous acid ClO2- Chlorite
    HClO Hypoclorous acid ClO- Hypochlorite

    Table 2 lists the names and formulas of some of the common Arrhenius bases.

    Naming Arrhenius Bases

    The Arrhenius bases are ionic compounds of metal and hydroxide ion, and their name starts with the name of the metal element followed by the name of the anion, i.e., hydroxide. For example, NaOH is sodium hydroxide.

    Table 2: Names of some of the common Arrhenius bases
    Formula Name
    LiOH Lithium hydroxide
    NaOH Sodium hydroxide
    KOH Potassium hydroxide
    Ca(OH)2 Calcium hydroxide
    Sr(OH)2 Strontium hydroxide
    Ba(OH)2 Barium hydroxide

    This page titled 6.1: What is an Acid and a Base? is shared under a Public Domain license and was authored, remixed, and/or curated by Muhammad Arif Malik.

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