Difference Between Acid and Alkaline

Main Difference – Acid vs Alkaline

Acids and alkaline are two terms often found in aquatic chemistry. Acids are chemical species that show acidic characteristics. Alkaline is a type of base. Therefore, alkaline solutions show basic properties. The main difference between Acid and Alkaline is that the pH of acids lies below pH 7 whereas the pH of alkaline is above pH 7.

Key Areas Covered

1. What is Acid
      – Definition, Properties, Examples
2. What is Alkaline
      – Definition, Properties, Examples
3. What is the difference between Acid and Alkaline
     – Comparison of Key Differences

Key Terms: Acid, Alkali, Alkali Earth Metals, Alkaline, Aquatic Chemistry, Bases, Lewis Acid, pH, Strong Acids, Weak Acids

What is an Acid

An acid is a chemical species that can be neutralized by a base. Acids are capable of releasing protons or H⁺ ions in aqueous solutions. An acid can be dissociated into its conjugated base and H⁺ ions. According to their dissociation strength, there are two types of acids.

• Strong acids
• Weak acids

Strong acids are compounds that can dissociate completely into its ions. Weak acids are compounds that partially dissociates into its ions. Therefore, weak acids are in equilibrium with their conjugated base and H⁺ ions in aqueous solutions.

The pH of an acid is always below 7. pH 7 is the neutral pH of a solution such as pure water. Very low pH values are given by strong acids and pH values that are near to pH 7 are given by weak acids. However, they have this low pH values since acids give high amounts of H⁺ ions to the aqueous solution and pH is the logarithm of the inverse of H⁺ concentration.

In addition, compounds that are able to accept an electron pair from a donor is known as a Lewis acid. According to this definition, some compounds that do not contain H⁺ are also categorized as acids. For example, AlCl₃ (Aluminium trichloride) is a Lewis acid. This is because the Al atom has empty p orbitals that can accept an incoming electron pair from a donor.

Figure 01: The reaction between a Lewis acid and a Lewis base

In the above image, “A” is a chemical compound. It can accept the lone electron pair of “B”. Therefore, “A” is a Lewis acid.

Normally, acids are sour tasting and are corrosive in their concentrated form. They can turn blue litmus into red color. Most acids can react with metals, forming a salt and hydrogen gas (H₂).

What is Alkaline

Alkaline is a solution that has pH values above 7. This means alkaline solutions show basic properties. Therefore, alkaline solutions can be neutralized by acids. Alkaline solutions are bitter tasting and feel slippery when touched.

An alkaline solution is formed when an alkali is dissolved in water. Alkali refers to any basic hydroxide or a salt of alkali metals or alkaline earth metals. The term alkaline is derived from the metal elements of group 1 and group 2 in the periodic table of elements. The group1 elements are called alkali metals. The group 2 metals are called alkali earth metals. Both types of metals cause the pH of the water to be increased when they are added to water. That is because, when an alkali metal or alkali earth metal is added to water, it reacts with water molecules, forming the metal hydroxide and H₂ gas. This metal hydroxide causes the pH of the solution to be increased since the concentration of OH^– ions increase.  

Figure 02: The reaction between sodium (Na) which is an alkali metal and water (H2O)

Apart from that, alkaline solutions can turn red litmus into blue color. That is an indication of basicity of alkaline solutions. Some alkaline solutions are corrosive but some are not. All alkaline solutions are bases that are dissolved in water. But all bases are not alkaline because some bases are not dissolved in water.

Ammonia (NH3) is also categorized as alkaline though it is not a hydroxide or a salt of alkali metals. This is because it is very well dissolved in water and its aqueous solution shows basic properties.

Difference Between Acid and Alkaline

Definition

Acid: An acid is a chemical species that has a pH below 7.

Alkaline: An alkaline solution is a chemical that has a pH above 7.

Ions Present

Acid: Aqueous solutions of acids are composed of H⁺ ions.

Alkaline: Aqueous solutions of alkaline are composed of OH^– ions.

Taste

Acid: Acids are sour in taste.

Alkaline: Alkalines are bitter tasting.

Texture

Acid: Acids are sticky.

Alkaline: Alkalines are slippery.

Neutralization

Acid: Acids can be neutralized by bases.

Alkaline: Alkalines can be neutralized by acids.

Conclusion

Acids and alkalines are solutions having lower and higher pH values, respectively. The main difference between Acid and Alkaline is that the pH of acids lies below pH 7 whereas the pH of alkaline is above pH 7. They play a very important role in determining the acidity and alkalinity of water. Therefore they are valuable parameters in checking the water quality for drinking and other purposes.

References:

1. “Alkaline Solution.” What is an Alkali? | Alkaline Solution. N.p., n.d. Web. Available here. 10 July 2017. 
2. Helmenstine, Ph.D. Anne Marie. “Here’s What an Acid Is in Chemistry.” ThoughtCo. N.p., n.d. Web. Available here. 10 July 2017. 

Image Courtesy:

1. “Lewis acid-base equilibrium” By Su-no-G assumed (Public Domain) via Commons Wikimedia