Difference Between Acid and Alkaline

The key difference between acid and alkaline is that the pH of acids lies below pH 7 whereas the pH of alkaline is above pH 7.

Acids and bases are two important concepts in chemistry. They have contradictory properties. Alkaline is a subset of bases. Thus, it has all the basic characteristics. Besides, there are various methods to differentiate between acid and alkali which we will discuss below.

CONTENTS

1. Overview and Key Difference
2. What is Acid
3. What is Alkaline
4. Side by Side Comparison – Acid vs Alkaline in Tabular Form
5. Summary

What is Acid?

There are several definitions for acids from different scientists. Arrhenius defines an acid as a substance that donates H₃O⁺ ions in the solution. Whereas, Bronsted- Lowry defines an acid as a substance that can donate a proton. However, Lewis acid definition is far more common than the above two. According to it, any electron pair acceptor is an acid. According to the Arrhenius or Bronsted-Lowry definition, a compound, to name it as an acid, should have hydrogen and the ability to donate it as a proton. But according to Lewis, there are molecules, which don’t possess hydrogen but can act as an acid. For example, BCl₃ is a Lewis acid, because it can accept an electron pair. Alcohol is a Bronsted-Lowry acid because it can donate a proton, however according to Lewis, it is a base.

Regardless of the above definitions, we normally identify an acid as a proton donor. Acids have a sour taste. Lime juice, vinegar are two acids we come across at our homes. They react with bases producing water, and also they react with metals to form H₂, thus increase metal corrosion rate. We can categorize acids into two classes, depending on their ability to dissociate and produce protons. They are strong acids and weak acids.

Strong Acids and Weak Acids

Strong acids like HCl, HNO₃ can completely ionize in a solution to give protons. Weak acids like CH₃COOH can dissociate partially to give fewer amounts of protons. Ka is the acid dissociation constant. It gives an indication of the ability to lose a proton of a weak acid. To check whether a substance is an acid or not we can use several indicators like litmus paper or pH paper. In the pH scale from 1-6 acids are represented (below pH 7). An acid with pH 1 is a very strong and as the pH value increases, acidity decreases. Moreover, acids turn blue litmus to red.

What is Alkaline?

Alkaline is having pH above 7. Therefore, the pH of an alkaline substance lies above pH 7. Group 1 and group 2 elements, which we name as alkali metals and alkaline earth metals are common alkaline substances, and they give alkaline solutions when we dissolve them in water. Sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium carbonate are some of the examples of these alkaline substances.

Arrhenius defines the bases as the substances that produce OH^– in solutions. Above molecules form OH^– when we dissolve them in water, therefore, act as bases. Alkaline solutions react easily with acids producing water and salt molecules. They show a pH value higher than 7 and turn red litmus to blue. There are other bases except for alkaline bases like NH₃. They also have the same basic properties.

What is the Difference Between Acid and Alkaline?

Acids and alkaline are two forms of compounds that we categorize according to the pH of them. Therefore, the key difference between acid and alkaline is that the pH of acids lies below pH 7 whereas the pH of alkaline is above pH 7. Moreover, acids can ionize to form protons or H⁺ ions while alkaline compounds can ionize to form hydroxide ions. When considering the taste and texture of those two compounds, we can get another difference between acid and alkaline compounds. That is, acids taste sour and have a sticky feeling while alkaline tastes bitter and has a slippery feeling.

The below infographic summarizes the difference between acid and alkaline in tabular form.

Summary – Acid vs Alkaline

Acids and alkalines have opposing chemical and physical properties due to their different chemical behaviours. The key difference between acid and alkaline is that the pH of acids lies below pH 7 whereas the pH of alkaline is above pH 7.

Reference:

1. Britannica, The Editors of Encyclopaedia. “Acid.” Encyclopædia Britannica, Encyclopædia Britannica, Inc., 27 Dec. 2017. Available here
2. “Alkaline | Definition of Alkaline in English by Oxford Dictionaries.” Oxford Dictionaries | English, Oxford Dictionaries. Available here 

Image Courtesy:

1.”Litmus paper”By Chemicalinterest – Own work, (Public Domain) via Commons Wikimedia  
2.”PH Scale”By Christinelmiller – Own work, (CC BY-SA 4.0) via Commons Wikimedia