The key difference between acid and base is that acids have pH values ranging from 1 to 7 whereas bases have pH values ranging from 7 to 14.
pH value is the minus logarithm of H+ ion concentration. pH 7 is considered as the neutral pH. pH values higher than 7 indicates the presence of a base while values below 7 indicate the presence of acids. According to the Brønsted-Lowry theory, acids can release H+ ions whereas bases can accept H+ ions.
CONTENTS
1. Overview and Key Difference
2. What is an Acid
3. What is a Base
4. Side by Side Comparison – Acid vs Base in Tabular Form
5. Summary
What is an Acid?
Acids are substances having pH less than 7 in their aqueous solution. The presence of an acidic medium can be determined using litmus papers. Acids can turn blue litmus red. However, there is no change in color if red litmus is used. Compounds having easily ionizable Hydrogen atoms are often acids.
According to the Brønsted-Lowry theory, an acid is a substance that can release protons (H+ ions) to the medium when it is in an aqueous medium. When H+ ions are released, these ions cannot exist alone in the aqueous medium. Therefore these ions combine with water molecules, forming H3O+ ions (hydronium ions). The presence of hydronium ions, thus, indicates the presence of an acid.
According to the Arrhenius theory, an acid is a substance that can increase the amount of hydronium ions in an aqueous medium. This happens due to the increase of H+ ions. In other words, acids release H+ ions, which can interact with water molecules to form hydronium ions.
When considering the Lewis theory, an acid is a compound that is able to accept a pair of electrons from a covalent chemical bond. Based on this definition, substances that have no hydrogen atoms are also categorized as acids due to their ability to accept electron pairs.
Properties of an Acid
An acid typically has a sour taste. The pH of an acid is always less than 7. Almost all acids have a burning odor. The texture of an acid is sticky rather than slippery. Furthermore, acids can react with metals (even highly unreactive metals) to form metal hydride and hydrogen gas.
What is a Base?
A base is a substance that shows a pH value higher than 7 when it is in an aqueous solution. The basicity of a solution causes the color of red litmus to turn into blue color. Therefore, the presence of a base can be determined using red litmus. However, when blue litmus is used, there is no color change with a base. Compounds having easily ionizable hydroxyl groups are often bases.
According to the Brønsted-Lowry theory, a base is a proton acceptor; in other words, a base can accept protons from an aqueous medium. However, Arrhenius theory also gives a similar definition: a base is a substance that decreases the amount of hydronium ions present in a medium. The hydronium ion concentration is decreased because the base obtains H+ ions or protons from the aqueous medium. These ions are required for the formation of hydronium ions.
Figure 1: Comparison of Acids and Bases
When considering the Lewis theory, a base is a substance that can donate electron pairs. These substances donate electron pairs and form coordinate covalent bonds. According to this theory, most compounds which have no OH- groups become bases.
Properties of a Base
Bases have a bitter taste. These substances always show pH values higher than 7. Almost all bases are odorless, except for ammonia. Ammonia has a pungent odor. Unlike acids, bases feel slippery. Bases get neutralized when reacted with acids.
What is the Difference Between Acid and Base?
Acid vs Base |
|
| An acid is a substance that shows pH value lower than 7 when it is in an aqueous solution | A base is a substance that shows pH value higher than 7 when it is in an aqueous solution |
| Definition based on Brønsted-Lowry Theory | |
| An acid is a substance that can release protons (H+ ions) to the medium when it is in an aqueous medium. | A base is a proton acceptor; in other words, a base can accept protons from an aqueous medium. |
| Definition based on Arrhenius Theory | |
| An acid is a substance that can increase the amount of hydronium ions in an aqueous medium. | A base is a substance that can decrease the amount of hydronium ions in an aqueous medium. |
| Definition based on Lewis Theory | |
| An acid is a compound that is able to accept a pair of electrons from a covalent chemical bond. | A base is a substance that can donate electron pairs. |
| Color Change in Litmus | |
| Acids can turn blue litmus red, but there is no color change in red litmus. | Bases can turn red litmus blue, but there is no color change in blue litmus. |
| Taste | |
| Aids have a sour taste. | Bases have a bitter taste. |
| Odor | |
| Acids have a burning odor. | Bases are odorless, except for ammonia. |
| Ionization | |
| Acids can form hydronium ions when ionized. | Bases can form hydroxyl ions when ionized. |
| Conjugate Chemical Species | |
| The conjugate species of an acid is its conjugate base. | The conjugate species of a base is its conjugate acid. |
| Neutralization | |
| An acid can be neutralized using a base. | A base can be neutralized using an acid. |
| Titration | |
| Acids are titrated with bases for the neutralization. | Bases are reacted with acids for the neutralization. |
Summary – Acid vs Base
All compounds can be categorized into acids, bases and neutral compounds. The key difference between acid and base is that acids have pH values ranging from 1 to 7 whereas bases have pH values ranging from 7 to 14.
Reference:
1. Helmenstine, Anne Marie. “Here’s What an Acid Is in Chemistry.” ThoughtCo, Available here.
2. Helmenstine, Anne Marie. “Key Acids and Bases Definitions.” ThoughtCo, Available here.
3. Libretexts. “Overview of Acids and Bases.” Chemistry LibreTexts, Libretexts, 13 Feb. 2017, Available here.
Image Courtesy:
1. “215 Acids and Bases-01” By OpenStax College – Anatomy & Physiology, Connexions Web site, Jun 19, 2013 (CC BY 3.0) via Commons Wikimedia