Acids and bases might sound like something straight out of a chemistry textbook, but they’re actually part of our daily lives in more ways than you might realise.
From the tang of a lemon to the cleaning power of soap, these substances play a crucial role in everything from food and health to industry and nature.
But what exactly makes an acid different from a base?
In this post:
Key Takeaways
Acids release hydrogen ions, while bases accept hydrogen ions or release hydroxide ions
The pH scale sets acids (below 7) apart from bases (above 7), with indicators like litmus paper used to detect the differences
Acids react with metals in what are mainly exothermic reactions and create hydrogen gas, while bases react with non-metal oxides to form salts and water
Acid-base reactions result in neutralisation, forming salts and water with varying pH levels
Everyday interactions include antacids neutralising stomach acid, toothpaste protecting enamel, and bases balancing water pH
Defining Acids and Bases
Acids and bases are classified according to how they behave in solutions.
Acids release hydrogen ions (H⁺) when dissolved in water, giving them their characteristic properties such as the ability to react with metals.
Bases, on the other hand, accept hydrogen ions or release hydroxide ions (OH⁻), often feeling slippery and tasting bitter.
There are three main theories that define and describe the differences between acids and bases:
- The Arrhenius theory – Proposed by Svante Arrhenius in 1884, this theory defines acids as “substances that increase the concentration of hydrogen ions (H⁺) in water-rich, or aqueous, solutions”, while bases increase hydroxide ions (OH⁻). It is limited to aqueous environments and does not account for acid-base behaviour in non-aqueous systems.
- The Brønsted-Lowry theory – Developed by Johannes Brønsted and Thomas Lowry in 1923, this theory expands on Arrhenius by defining acids as proton (H⁺) donors and bases as proton acceptors. It applies to reactions in both aqueous and non-aqueous systems and includes conjugate acid-base pairs.
- The Lewis theory – Put forward by Gilbert N. Lewis in 1923, this theory defines acids as electron pair acceptors and bases as electron pair donors. It explains acid-base interactions without requiring the presence of hydrogen ions, making it useful for understanding complex reactions, including those in organic and inorganic chemistry.
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.
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:
- Citric acid – HNO3
- Boric acid – H3BO3
- Hydrochloric acid – HCl
- Sulphuric acid – H2SO4
Meanwhile, 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, with most reactions being exothermic in nature.
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.
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. In short, 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
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.
Common Examples of Interactions Between Acids and Bases in Real Life
Acids and bases are constantly reacting with each other in ways that shape our daily lives. Here’s some examples that you may not have realised before.
Antacids and Stomach Acid
Heartburn and indigestion are often caused by excess stomach acid, which contains hydrochloric acid. To relieve discomfort, people take antacids, which are mild bases such as magnesium hydroxide or sodium bicarbonate.
These bases react with stomach acid to form water and neutral salts, reducing acidity and providing relief.
Soil Treatment in Agriculture
Soil pH plays a major role in plant health. If soil is too acidic, plants can only get limited nutrients and their growth slows down.
Farmers and gardeners use lime, a basic substance, to neutralise the acidity and bring the soil to a more balanced pH level. This process helps improve crop yields and ensures plants get their required nutrients.
Cleaning Products and Grease Removal
Many cleaning products rely on bases to cut through grease and grime. Soap, for example, is made from alkaline substances that react with fatty acids to break down oils and dirt.
This is why washing up liquid and other household cleaners feel slippery, because they contain bases that chemically interact with grease to make it easier to rinse away.
Toothpaste and Oral Health
The food we eat can sometimes produce acids that weaken tooth enamel over time.
Toothpaste, which often contains mild bases like sodium fluoride or baking soda, helps neutralise these acids and protect teeth from decay. This everyday acid-base reaction is an important part of dental care and maintaining healthy teeth.
Water Treatment and Safe Drinking Water
Water treatment facilities use bases like sodium hydroxide to balance pH levels in drinking water. If water is too acidic, it can corrode pipes and leach harmful metals into the supply.
By adding a base, water treatment plants prevent this issue and ensure that the water remains safe for consumption.
Conclusion
Acids and bases influence countless aspects of daily life, shaping the way we cook, clean, and maintain health. Their ability to react with one another in neutralisation makes them particularly important in balancing chemical processes, both in nature and industry.
