1.06:_Acids_and_Bases

<span id="Acids"></span><span id="Acids"></span><h2 style="background-color: unset;" class="lt-bio-3746">Acids</h2>

<p class="lt-bio-3746" style="background-color: unset;">Acids are substances that donate protons (hydrogen ions, H<sup><font size="3">+</font></sup>) to bases.</p>

<span id="Bases"></span><span id="Bases"></span><h2 style="background-color: unset;" class="lt-bio-3746">Bases</h2>

<p class="lt-bio-3746" style="background-color: unset;">Bases are substances that accept protons from acids.</p>

<span id="Example_of_Acid_and_Base_formation"></span><span id="Example_of_Acid_and_Base_formation"></span><h2 style="background-color: unset;" class="lt-bio-3746">Example of Acid and Base formation</h2>

<p class="lt-bio-3746" style="background-color: unset;"><img class="internal right" style="width: 169px; height: 550px; float: right;" alt="Stacked geometric shapes including squares, rectangles, and triangles in various orientations." loading="lazy" width="170" height="550" src="https://bio.libretexts.org/@api/deki/files/5114/acids_bases.gif?revision=1&amp;size=bestfit&amp;width=169&amp;height=550" />Hydrogen chloride (HCl) is a gas. Its two atoms are held together by a shared pair of electrons. However, the chlorine atom is so much more <strong>electronegative</strong> than hydrogen, that the bond between them is <strong>polar covalent</strong>.</p>

<p class="lt-bio-3746" style="background-color: unset;">When hydrogen chloride is bubbled through water, the nucleus of the hydrogen atom leaves and takes up residence at one of the unshared pairs of electrons in the water molecule. However, its electron remains behind still attached to the chlorine atom. <strong>&quot;1&quot;</strong> This ionization produces:</p>

<ul>
    <li class="lt-bio-3746" style="background-color: unset;">a <strong>chloride ion</strong> (Cl<sup><font size="2">&minus;</font></sup>)</li>
    <li class="lt-bio-3746" style="background-color: unset;">a <strong>hydronium ion</strong> (H<sub><font size="2">3</font></sub>O<sup><font size="2">+</font></sup>). <strong>&quot;2&quot;</strong></li>
</ul>

<p class="lt-bio-3746" style="background-color: unset;">The resulting mixture is called <strong>hydrochloric acid</strong>.</p>

<p class="lt-bio-3746" style="background-color: unset;">Now let us bubble <strong>ammonia</strong> gas (NH<sub><font size="2">3</font></sub>) through the hydrochloric acid. Ammonia molecules have one pair of unshared electrons and these have a greater affinity for a proton than do the unshared electrons in the water molecule. Consequently, the proton shifts again (<strong>&quot;3&quot;</strong>) to form a new ion, the <strong>ammonium ion</strong> (NH<sub><font size="2">4</font></sub><sup><font size="2">+</font></sup>) and water (<strong>&quot;4&quot;</strong>).</p>

<p class="lt-bio-3746" style="background-color: unset;">Because both the HCl molecule and the hydronium ion are proton donors, they meet the definition of an <strong>acid</strong>.</p>

<p class="lt-bio-3746" style="background-color: unset;">The water molecule in the first example and the ammonia in the second example accept protons; therefore each is a <strong>base</strong>.</p>

<p class="lt-bio-3746" style="background-color: unset;">While HCl is found in living systems (e.g., the gastric juice secreted by the stomach), the most common acids in biology are those containing the <strong>carboxyl group</strong> (<strong>&quot;5&quot;</strong>).</p>

<p class="lt-bio-3746" style="background-color: unset;">The proton of the carboxyl group is easily removed forming the <strong>carboxyl ion</strong> (<strong>&quot;6&quot;</strong>).</p>

<p class="lt-bio-3746" style="background-color: unset;">Acetic acid (CH<sub><font size="2">3</font></sub>COOH) is a common example of a carboxylic acid. When mixed with water, some of the protons on its -COOH group are attracted to the unshared electron pairs of water molecules. Hydronium ions (H<sub><font size="2">3</font></sub>O<sup><font size="2">+</font></sup>) and acetate ions (CH<sub><font size="2">3</font></sub>COO<sup><font size="2">&minus;</font></sup>) result. Vinegar is a dilute solution of acetic acid.</p>

<p class="lt-bio-3746" style="background-color: unset;">Ammonia is also found (in low concentrations) in living matter. But the most common bases are those molecules that contain an <strong>amino group</strong> (<strong>&quot;7&quot;</strong>). The unshared pair of electrons serves as a proton acceptor, as it does in the ammonia molecule.</p>

<p class="lt-bio-3746" style="background-color: unset;"><strong>Bicarbonate ions</strong> (<strong>&quot;8&quot;</strong>) also serve as an important base in living tissue.</p>

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<hr class="autoattribution-divider" /><div class="autoattribution"><p>This page titled <a target="_blank" class="internal mt-self-link" href="/Sandboxes/johnnyphung/biology/01:_The_Chemical_Basis_of_Life/1.06:_Acids_and_Bases">1.6: Acids and Bases</a> is shared under a <a rel="nofollow" href="https://creativecommons.org/licenses/by/3.0" target="_blank">CC BY  3.0</a>  license and was authored, remixed, and/or curated by <a rel="nofollow" target="_blank" href="http://www.biology-pages.info/">John W. Kimball</a> via <a rel="nofollow" href="https://www.biology-pages.info/" target="_blank">source content</a> that was edited to the style and standards of the LibreTexts platform.</p></div>

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