What is the Difference Between an Acid and a Base?

by Kate Onissiphorou

The main difference between an acid and a base is that they are polar opposites; one is either a proton acceptor or a proton donor.

Similarly, one can either donate a pair of electrons or it can receive them. There are three main theories that define and describe the differences between acids and bases. These are known as the Arrhenius theory, the Brønsted-Lowry theory and the Lewis theory.

Acids and bases are among the most interesting and useful topics in chemistry. Not only are these substances present in nature, but they’re widely used in industry, too. Whether separately or together, acids and bases can be used to synthesise new substances, including organic ones. They’re also used to determine the composition of unknown samples.

Neutralisation is the reaction between acids and bases. It’s exothermic (gives off heat) in nature, especially when a strong acid and a strong base are involved. Neutralisation is an example of a double displacement reaction in which the ions switch places.

How to Distinguish Acids and Bases

The main distinguishing characteristic that sets acids and bases apart is their respective pH levels. Acids have pH levels that are below 7, while bases have pH levels that are above 7. This difference can be detected by either using a pH meter or an indicator, such as litmus paper.

Scientist using litmus paper to test whether a substance is an acid or a base.

Litmus paper is the most commonly used qualitative indicator for acids and bases. A blue litmus paper strip turns red if it’s immersed in an acidic solution. Conversely, a red litmus paper strip turns blue if it’s submerged in a basic solution. However, litmus paper is only sensitive within the limited range of pH 4.5 to 8.3.

Other examples of indicators commonly used in the laboratory for testing acids and bases include:

  • Thymol Blue (1st change) changes to red in acidic solutions and yellow in basic solutions. It has a pH range of 1.2 – 2.8
  • Methyl Orange this indicator turns red in acids and yellow in bases. It has a pH range of 3.2 – 4.4
  • Bromocresol Green changes to yellow in acid and blue in base solutions. Its pH range is 3.8 – 5.4
  • Methyl Red the pH range of this indicator is 4.8 – 6.0. It turns yellow when the solution is an acid and red when the solution is alkaline or basic
  • Bromothymol Blue this indicator has a pH range of 6.0 – 7.6. It turns yellow when mixed with acidic solutions and blue when mixed with basic solutions

Taste is another distinguishing characteristic of acids and bases. While we definitely don’t recommend doing a taste test for strong inorganic acids and bases due to their toxicity, you can do such a test with weak organic acids and bases. For example, you can taste vinegar, which is acetic acid. You can also try baking soda, or sodium bicarbonate, which is a base. Generally, acidic substances taste sour, while basic substances are bitter.

When it comes to chemical formulas, acids start with a hydrogen atom as a distinct group that becomes an ion when dissolved in water. Meanwhile, most bases have hydroxyl groups (-OH).

Some examples of acids include:

Some examples of bases include:

  • Calcium hydroxide Ca(OH)2 
  • Magnesium hydroxide Mg(OH)2
  • Sodium hydroxide NaOH

The Properties of Acids and Bases

Acids and bases are chemically opposite and highly reactive with each other. They both form ions in aqueous solutions, making the solutions electrically conductive. For example, the lead-acid batteries used in cars contain sulphuric acid, while the alkaline rechargeable batteries found in electronic gadgets contain potassium hydroxide.

When acids and bases react, they neutralise each other out and form salts. Some salts produced by acid-base neutralisation reactions may either be slightly basic or slightly acidic, depending on the strength of the base or the acid reactant.

Acids can easily strip away the valence electrons of metals and other compounds. This property makes acids very corrosive, especially when strong and highly concentrated. Acids react with most metals and the reactions are exothermic.

When an acid reacts with a metal, it’s classified as a displacement reaction. Hydrogen is liberated and a salt is produced. For example, the reaction of magnesium with sulphuric acid produces magnesium sulphate salt and hydrogen gas.

Mg + H₂SO₄ → MgSO₄ + H₂

Metals can be arranged in a series based on how much or how little they react with acid, as shown in the chart below. This also determines their order of displacement. More reactive metals can displace the less reactive ones.

A table showing the reactivity of metals

However, when a base reacts with a metal, hydrogen is liberated but the metal in the original base substance is not displaced. For example, when sodium hydroxide reacts with zinc, hydrogen and sodium zincate salt are formed.

2NaOH + Zn ⇨ Na2ZnO2 + H2

When a base reacts with a non-metal oxide, the reaction produces salt and water. This is true in the example below, where the reaction between sodium hydroxide and carbon dioxide produces sodium carbonate and water.

2NaOH + CO2 ⇨ Na2CO3 + H2O

Why do Acids and Bases React?

As previously mentioned, acids and bases are chemically opposite and, as the saying goes, opposites attract. The more technical explanation centres on the affinity of bases with protons and the affinity of acids with electron pairs.

When acids and bases dissolve in water they form ions. According to the Brønsted-Lowry theory, the hydrogen ions in the acid have a strong affinity towards the hydroxyl ions in the base. Hence, an acid is a proton donor and a base is a proton acceptor.

How do Acids and Bases Neutralise Each Other?

The ions formed by acids and bases in an aqueous solution ‘bump into’ each other to form more stable compounds. These compounds are salt and water. The general chemical formula for acid-base reactions can be written as:

Acid(aq) + Base(aq) ⇨ Salt + Water

How to Calculate the pH of an Acid/Base Mixture

Scientist identifying the pH of an acid/base mixture.

Not all acid-base neutralisation reactions are completely balanced and therefore do not form completely neutral solutions. The pH level depends on several factors, such as the proportional concentrations of the reactants and their respective strengths. You can calculate the pH of the resultant products in the solution by following these steps.

  • Step 1: You must first balance the chemical equation of the reaction. Write it down as a guide for the next step.
  • Step 2: Compute the moles of the hydrogen ions based on the balanced chemical equation.
  • Step 3: Compute the moles of the hydroxide ions based on the balanced chemical equation.
  • Step 4: You can then use the mole ratio of the reaction to calculate which of the two reactants is in excess.
  • Step 5: You can now compute the moles of the ions of the reactant that are in excess.
  • Step 6: Measure the total volume of the combined acid and base solutions.
  • Step 7: Compute the excess ions in the mixed solution.
  • Step 8: Calculate the pH of the resultant solution based on the excess ions in proportion to the total products.


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