In chemistry, acidic or alkaline properties are fundamental to a substance, profoundly affecting its chemical behavior. We encounter various acidic and alkaline substances in daily life – lemon juice is acidic, soap is alkaline. These features mainly depend on the ability to donate or receive protons (hydrogen ions, H$^+) and the presence of hydroxide ions (OH^-$). The acidity or alkalinity of a substance is measured using the pH scale, which extends from 1 to 5. pH 1 indicates neutral, acidic below 5, and alkaline above 1.
This idea is not only limited to the laboratory but also has a huge impact on our daily life and biology. The pH of the soil affects crop growth, the pH of blood plasma is vital for the human body, and even the pH of rainwater affects the environment. Acid-base reactions serve as a fundamental process in many areas of industry, medicine, agriculture, and environmental science. Understanding these features is important not only for chemists but for everyone, because it helps us understand the world around us.
What Makes Something Acidic or Alkaline?
- Arrhenius Theory: The Basic Idea
The first concept of acid and alkali was given by the Swedish chemist, Svante Arrhenius. According to his theory, an acid is a substance that produces hydrogen ions (H$^+) when dissolved in water.span_3 span_4For example, hydrochloric acid (HCl) dissolves in water and creates H^+$ and Cl$^-$ ions. The more H$^+$ ions produced, the more acidic the substance.
On the other hand, according to Arrhenius theory, an alkali is a substance that produces hydroxide ions (OH$^-) when dissolved in water.span_5 span_6When sodium hydroxide (NaOH) dissolves in water, Na^+$ and OH$^-$ ions form. The more OH$^-$ ions produced, the more alkaline the substance will be. In an aqueous solution, the Arrhenius theory explains the acidity or alkalinity based on the relative concentration of H$^+$ and OH$^-$ ions. Although this theory was straightforward, it had some limitations, as it applies only to aqueous solutions.
What Makes Something Acidic or Alkaline?
- Brønsted-Lowry Theory: Proton Transfer
To address the limitations of Arrhenius theory, Johannes Brønsted and Thomas Lowry suggested a new theory, known as the Brønsted-Lowry theory. According to this theory, an acid is a proton (H$^+$) donor, and an alkali (or base) is a proton acceptor. This definition is more extensive than Arrhenius theory, as it applies to any solution, not just aqueous solutions.
For example, when hydrochloric acid (HCl) dissolves in water (H$_2O), HCl donates a proton (H^+) to water, forming H_3O^+$ (hydronium ion) and Cl$^-$ ion. Here HCl acts as an acid and H$_2O as an alkali (base). span_13Similarly, when ammonia (NH_3$) dissolves in water, NH$_3$ takes a proton from water and produces NH$_4^+$ (ammonium ion) and OH$^-$ ion. Here NH$_3$ acts as an alkaline (base) and H$_2$O as an acid. This theory also introduces the concept of conjugate acid-base pairs, which helps understand acid-base reactions in more detail.
What Makes Something Acidic or Alkaline?
- Lewis Theory: Electron Pair Transfer
One of the most widespread concepts, more general than the Brønsted-Lowry theory, is the Lewis theory, proposed by Gilbert N. Lewis in 1923. According to this theory, a Lewis acid is a species that can accept an electron pair, and a Lewis alkali (base) is a species that can donate an electron pair. This definition does not depend on proton transfer but is based on the donation or acceptance of electron pairs, which is capable of explaining many more chemical reactions as acid-base reactions.
For example, boron trifluoride (BF$_3$) is a Lewis acid because it can accept an electron pair. Ammonia (NH$_3$) is a Lewis alkali (base) because it can donate a lone pair of electrons from its nitrogen atom. The reaction between BF$_3$ and NH$_3$ is an acid-base reaction, where the ammonia donates its electron pair to BF$_3$ to create an adduct. This theory is extremely helpful in explaining reactions in various organic chemistry contexts, such as with carbonyl compounds and the formation of complex ions.
What Makes Something Acidic or Alkaline?
- pH Scale: Measure of Acidity and Alkalinity
The pH scale is used to measure the acidity or alkalinity of a substance. The pH scale is a logarithmic scale, which expresses the concentration of hydrogen ions in an aqueous solution. pH stands for “potential of hydrogen.” Mathematically, pH is defined as $ -\log_{10}[H^+] $, where [H^+] is the concentration of hydrogen ions per liter.
pH 1 is considered neutral, such as pure water. The solution is acidic when below pH 1; the lower the pH, the higher the acidity. For example, the pH of lemon juice is about 2, which indicates it is quite acidic. On the other hand, the solution is alkaline when above pH 1; the higher the pH, the higher the alkalinity. For example, the pH of soapy water is about 1, which indicates it is alkaline. Although the pH scale commonly extends from 0 to 14, it can go beyond this range in the case of extremely strong acids or alkalis. This scale is widely used in measuring the acidity of various substances in science, industry, and daily life.
What Makes Something Acidic or Alkaline?
- Influencing Factors: Temperature, Concentration, and Solvent
The acidic or alkaline properties of a substance can depend not only on its chemical structure but also on some external influences. Temperature is an important influencer. For example, the ionic product of water increases as the temperature rises, so that neutral water can deviate from pH 1. At high temperatures, water produces more H$^+$ and OH$^-$ ions, so the pH goes below 7 (the standard neutral pH at 25°C), though the solution is still neutral.
The concentration of acid or alkali in the solution also affects pH. High-concentration acid solutions are more acidic than lower-concentration acid solutions. Similarly, high-concentration alkaline solutions are more alkaline. The nature of the solvent also affects acidity. According to the Brønsted-Lowry theory, one substance can act as an acid or alkali (base) in a certain solvent, but it may behave differently in another. For example, ammonia (NH$_3$) is an alkali (base) in water, but in liquid ammonia, it can act as an acid. Thus, these influencing factors are essential for a proper understanding of acid-base reactions.
What Makes Something Acidic or Alkaline?
Conclusion:
Arrhenius, Brønsted-Lowry, and Lewis theories provide the fundamental framework to understand the idea of a substance being acidic or alkaline in our daily life. The Arrhenius theory gives the initial idea based on H$^+$ and OH$^-$ in water, the Brønsted-Lowry theory expands it through proton transfer, and the Lewis theory further generalizes it by electron pair transfer. The pH scale provides an effective measure of these features, which helps compare the acidity or alkalinity of substances in various cases. Also, external factors such as temperature, concentration, and the solvent affect the acidic or alkaline properties of a substance, making it very important to understand chemical reactions and their environmental effects properly. This knowledge helps us better understand the world around us and solve various practical problems.