Acids are one of the most important groups of chemicals, found everywhere from household products to industrial processes. They are substances that can release hydrogen ions (H⁺) in water, and they play a key role in countless chemical reactions.
Acids are not only essential in laboratories, but also in industries such as fertiliser production, pharmaceuticals, and manufacturing. From the sour taste of lemon juice to the powerful chemicals used in industry, acids are all around us.
Understanding how they work, what makes them strong or weak, and what some of the most common kinds are helps us understand how to get the best of these chemicals.
In this post:
Key Takeaways
Acids are substances that release hydrogen ions (H⁺) in water
They can be defined by Arrhenius, Brønsted–Lowry, or Lewis theories, depending on how they interact with ions, protons, or electron pairs
Acids vary in strength, with strong acids dissociating completely in water and weak acids only partially dissociating
They are widely used in daily life and industry, from food preservation and cleaning to pharmaceuticals and manufacturing
What is the Definition of Acid in Chemistry?
An acid has a pH level of less than 7 and it reacts with bases in neutralisation. Similar to the definition of bases, the definition of acid and the concepts related to it are categorised into three main theories: the Arrhenius definition, the Brønsted–Lowry definition, and the Lewis definition.
Arrhenius Definition
Named after the Swedish scientist, Svante Arrhenius, this definition is based on the observation that when acid is dissolved in water, it increases the concentration of hydrogen ions. That means an Arrhenius acid is any substance that increases hydrogen ions (H+) when dissolved in water.
It’s also a substance that produces other types of hydrogen ions in water, for example, hydronium ions (H3O+) or other forms, such as H5O2+, H9O4+.
Brønsted–Lowry Theory
According to the Brønsted–Lowry theory, an acid can only act as an acid when a base is present. Conversely, a base can only act as a base in the presence of an acid. This thesis defines an acid as a proton donor to a base, which serves as the proton acceptor.
The Arrhenius theory is insufficient in describing many organic reactions that do not involve hydronium ions, such as the dynamic equilibrium reactions between acetic acid and ammonia, as shown below.
CH3COOH+NH3⇌CH3COO–+NH+4
The Lewis Definition
In 1923, Gilbert N. Lewis proposed an improvement on the previous theories regarding acids and bases. The Lewis definition does not involve hydrogen ions or proton transfers. Instead, it states that an acid is a substance that readily accepts a pair of electrons from another substance in a solution.
Physical and Chemical Properties of Acids and Bases
Acids and bases have distinct physical and chemical properties that are important in chemistry:
- Acids typically taste sour (although we would not recommend you taste them), they react with metals to release hydrogen gas, and make blue litmus paper turn red
- Bases (alkalis) often feel slippery, taste more bitter than sour, and make red litmus paper turn blue
- Both acids and bases react with each other in neutralisation reactions, producing a salt and water
How Acids Work
Referring to the three definitions of acids and bases, an acid forms hydrogen or hydronium ions in water, donates a proton to a base, and accepts a pair of electrons. An acid is chemically reactive and, as a result, can corrode metals and produce exothermic reactions with bases.
When an acid is dissolved in water, it dissociates into positively charged hydrogen or cations and negatively charged anions. The dissociation increases the number of hydrogen or hydronium ions in the water, which reduces the pH level of the solution. If a metal, base or other reactive substances are present, they may react with the acid by either donating electron pairs or accepting protons.
Different Types of Acids and Their Uses
Acids are common in our daily lives and can be found in everything ranging from the physiological or biological functions in our body to the products we consume.
They can be classified according to the number of dissociable protons they contain:
- Monoprotic acids release one proton per molecule (e.g. hydrochloric acid)
- Diprotic acids release two protons per molecule (e.g. sulphuric acid)
- Triprotic acids release three protons per molecule (e.g. phosphoric acid)
Some common examples of acids and their uses are outlined below.
Citric acid
Citric acid is an organic acid that occurs naturally.
It’s found in citrus fruits such as lemons and oranges, and is often used as a food flavouring and food preservative, and citric acid can also help in cleaning products to help break down soap scum.
Ascorbic acid
This is another naturally occurring organic acid commonly found in citric fruits. Otherwise known as vitamin C, it’s a powerful antioxidant that’s also used to treat scurvy and bone marrow diseases.
Hydrochloric acid
Hydrochloric acid is a strong, corrosive acid used in labs as well as in industry. It cleans metals, removes rust, and aids in producing chemicals like fertilisers.
In labs, it’s used for pH control, chemical reactions, and titrations. It’s also naturally found in the stomach, helping digest food and kill bacteria.
Acetic acid
Acetic acid is an organic acid that’s often used to flavour food. It’s also used as a food preservative, such as in pickling among many other common uses.
Sulphuric acid
Sulphuric acid acts as a basic raw material and reagent in various processes.
Consequently, it’s used in a wide range of sectors including the automotive, pharmaceutical, steel manufacturing, and fabric industries.
Boric acid
Boric acid is a versatile chemical used in the manufacture of glass, where it improves heat resistance, in paper production as a preservative and quality enhancer, and in leather processing to soften hides and prevent microbial growth.
It is also valued for its antiseptic and insecticidal properties, making it useful in both industrial and household products.
Strong vs Weak Acids
The strength or weakness of acid in terms of its pH level mainly depends on the concentration of the acid solution.
The higher the concentration, the stronger the acid. However, other factors, such as the temperature of the solution, also affect the pH level and strength of an acid. Hence, the pH is not a particularly good measure of an acid’s strength.
A more objective measure of acid strength is to look at the dissociation constant, which is the ratio between the ions and the amount of acid in a solution. Strong acids dissociate more completely in an aqueous solution compared to weak acids. The constant can be expressed in this formula:
Some examples of strong acids include:
- Hydrochloric acid – HCl
- Nitric acid – HNO3
- Perchloric acid – HClO4.
Examples of weak acids include:
- Sulphurous acid – H2SO3
- Phosphoric acid – H3PO4
- Nitrous acid – HNO2
Conclusion
Acids are much more than just a sour taste in food. They are vital to chemistry, industry, and everyday life. By understanding their properties, types, and strength, as well as their practical applications, we can appreciate how these versatile chemicals shape everything from laboratory reactions to household products. reactions.
