Identification of Acidic and Alkaline Solutions: We all know that water is neutral, i.e. it is neither acidic nor alkaline. This is because hydroxyl and hydronium ions are in equilibrium. However, sodium hydroxide is basic, and \({\text{HCl}}\) is acidic. But how to identify whether a solution is acidic or alkaline. Alkaline solutions will feel soapy or slippery, but acidic solutions will feel slightly stinging. Acid-base indicators show whether a solution is acidic, neutral or alkaline. Let’s explore more about it in this article.

What is an Acid?

A substance that donates hydrogen ions on dissolution is known as an acid. For example, hydrochloric acid produces hydrogen ions:

\({\text{HCl}}\left({{\text{aq}}} \right) \to {{\text{H}}^ + }\left({{\text{aq}}} \right) + {\text{C}}{{\text{l}}^ – }\left({{\text{aq}}} \right)\)

Hydrochloric acid is a strong acid. The ionic bond between the hydrogen and the chloride ions is polar and can be easily dissolved in water, generating many hydrogen ions. Hence, the solution is strongly acidic.

However, all substances that contain hydrogen are not acidic. This is because the hydrogen must be present in a state that is easily released. Organic compounds are mostly made up of carbon and hydrogen atoms, but they are not acidic. The hydrogen atom binds to carbon atoms very tightly and cannot be released on dissolution.

Certain compounds donate hydrogen but not very readily. These compounds are known as a weak acids. Acetic acid \({\text{C}}{{\text{H}}_3}{\text{COOH,}}\) found in vinegar, contains four hydrogens, but only one of the hydrogens is readily available to leave the molecule.

What is a Base?

A substance that donates hydroxyl ions on dissolution is known as a base. These substances accept hydrogen ions released by acids. The release of hydroxyl ions makes the solution alkaline or basic. For example, sodium hydroxide on dissolution produces hydroxide ions:

\({\text{NaOH}}\left({{\text{aq}}} \right) \to {\text{N}}{{\text{a}}^ + }\left({{\text{aq}}} \right) + {\text{O}}{{\text{H}}^ – }\left({{\text{aq}}} \right)\)

Hence, sodium hydroxide is a base.

A base soluble in water is known as an alkali. All alkalis are bases, but all bases are not alkalies.

When an acid and an alkali are present in equimolar concentrations, the hydrogen and hydroxyl ions react readily with each other, producing salt and water in a reaction called neutralization.

What does it Mean for a Solution to be Acidic or Basic (Alkaline)?

A solution can be differentiated as acidic or alkaline with respect to hydrogen ions (abbreviated with the chemical symbol \({{\text{H}}^ + }\)). Pure water consists of an equal number of hydrogen ions and hydroxide ions; hence, it is neither acidic nor basic.

When an acid dissolves in water, the balance between \({{\text{H}}^ + }\) ions  and \({\text{O}}{{\text{H}}^ – }\) ions are lost, and the equilibrium is shifted towards the hydrogen ions. This means there are more \({{\text{H}}^ + }\) ions than \({\text{O}}{{\text{H}}^ – }\) ions and the solution becomes acidic.

When a base is dissolved in water, the balance between \({{\text{H}}^ + }\) ions and \({\text{O}}{{\text{H}}^ – }\) ions are lost again and is shifted towards the hydroxyl ions. There are more \({\text{O}}{{\text{H}}^ – }\) ions than \({{\text{H}}^ + }\) ions and the solution becomes basic.

Acidity and alkalinity are measured with respect to a logarithmic scale called \({\text{pH}}{\text{.}}\) A strongly basic solution can have one hundred million, or one hundred trillion \(\left({100,000,000,000,000} \right)\) times more hydroxyl ions than a strongly acidic solution!

To handle these large numbers easily, a logarithmic scale commonly known as the \({\text{pH}}\) scale is used. The \({\text{pH}}\) scale is defined as the negative logarithm of the hydrogen ion concentration.

\({\text{pH=}}\,{\text{-log}}\left[{{{\text{H}}^ + }} \right]\)

Therefore, the more \({{\text{H}}^ + }\) ions present, the lower the \({\text{pH}}\) and vice versa.

A change in the concentration of \({{\text{H}}^ + }\) ions by a factor of \(10\) represent a change of one unit on the \({\text{pH}}\) scale. Similarly, a change in the concentration of hydrogen ions by a factor of \(100\) represents a change of two units on the \({\text{pH}}\) scale. Thus, large changes in the concentrations of hydrogen ions represent small changes in \({\text{pH}}\) value. The \({\text{pH}}\) values for most of the substances are in the range from \(0\) to \(14.\)

Pure water has a \({\text{pH}}\) of \(7,\) which is neutral. The \({\text{pH}}\) values lower than seven are acidic, and those higher than \(7\) are alkaline (basic).

Indicators

Acid-alkali indicators show whether a solution is acidic, neutral \(\left({{\text{pH}} = 7} \right)\) or alkaline. Some common acid-base indicators are listed below.

Indicator	Acidic	Neutral	Alkaline
Litmus paper	Red	Purple	Blue
Methyl orange	Red	Yellow	Yellow
Phenolphthalein	Colourless	Colourless	Pink

The \({{\text{pH}}}\) of a solution measures its acidity or alkalinity (base) and ranges from \(0\) to \(14.\) It measures how much acid or base (alkalinity) exists in a solution. Litmus paper, methyl orange, phenolphthalein indicate whether a solution is acidic, neutral \(\left({{\text{pH}} = 7} \right)\) or alkaline. Still, they do not tell us anything about the extent of alkalinity or acidity of a solution. Hence, \({{\text{pH}}}\) paper is used.

Low concentrations of \({{\text{H}}^ + }\) ions yield a high \({{\text{pH}}}\) (basic substances), whereas high levels of \({{\text{H}}^ + }\) ions result in a low \({{\text{pH}}}\) (acidic substances). The blood in our veins is slightly alkaline \(\left({{\text{pH}} = 7.4} \right),\) whereas the environment in our stomach is highly acidic (\({\text{pH}} = 1\) to \(2\)). Baking soda is mildly basic \(\left({{\text{pH}} = 9.0}\right),\) whereas orange juice is mildly acidic (\({{\text{pH}} = }\) approximately \(3.5\)).

The substances that provide hydrogen ions \(\left({{{\text{H}}^ + }} \right)\) and lower \({\text{pH}}\) are acids, whereas the substances that provide hydroxide ions \(\left({{\text{O}}{{\text{H}}^ – }} \right)\) and raise the \({\text{pH}}\) are bases. The stronger the acid, the more readily it donates \({{\text{H}}^ + }.\) The stronger the base, the more readily it donates \({\text{O}}{{\text{H}}^ – }\) ions. The \({{\text{H}}^ + }\)  ions and \({\text{O}}{{\text{H}}^ – }\) ions combine to produce water, which raises a substance’s \({\text{pH}}.\)

Buffers

Most cells in our bodies operate within a \({\text{pH}}\) range of \(7.2\) to \(7.6.\) If the body’s pH is outside this range, cells no longer function properly and can even be fatal.

So how to manage the \({\text{pH}}\) level of the body without ingesting acidic or basic substances? Thanks to buffers. These substances readily absorb excess \({{\text{H}}^ + }\) or \({\text{O}}{{\text{H}}^ – },\) maintaining the body’s \({\text{pH}}\) around \(7.2\) to \(7.6.\) Carbon dioxide is an important buffer in the human body that maintains the \({\text{pH}}\) within the proper range.

The carbon dioxide buffer system involves carbonic acid \(\left({{{\text{H}}_2}{\text{C}}{{\text{O}}_3}} \right)\) and bicarbonate \(\left({{\text{HCO}}_3^ – } \right)\) anion. If the body becomes too much acidic, bicarbonate will combine with the H⁺ to create carbonic acid and limit the further increase in the system’s acidity.

Likewise, carbonic acid will rapidly dissociate into bicarbonate and \({{\text{H}}^ + }\) ions if the body becomes too basic. The \({{\text{H}}^ + }\) ions can combine with the \({\text{O}}{{\text{H}}^ – }\) ions, limiting the increase in \({\text{pH}}.\)

Without this buffer system, the \({\text{pH}}\) in our bodies would fluctuate too much and could be fatal.

Summary

A solution with a high number of \({{\text{H}}^ + }\) ions are acidic and have a low \({\text{pH}}\) value, whereas a solution with a high number of \({\text{O}}{{\text{H}}^ – }\) ions or less number of \({{\text{H}}^ + }\) is basic and has a high \({\text{pH}}\) value. The \({\text{pH}}\) of a solution is a measure of the concentration of \({{\text{H}}^ + }\) ions in the solution. The \({\text{pH}}\) scale ranges from \(0\) to \(14,\) with a \({\text{pH}}\) of \(7\) being neutral. Buffers are solutions that regulate \({\text{pH}}\) changes when an acid or base is added to an alkaline or acidic system. These are crucial to biological systems due to their ability to maintain constant \({\text{pH}}\) conditions. This page explains what acidic and basic substances are and how they can be identified. It also explains the concept of \({\text{pH}}\) and how it is used to determine the acidity and alkalinity of a substance.

Frequently Asked Questions

Q.1. How can you identify an acid?
Ans: The most common tests used to identify acids and bases are litmus paper tests. It is a chemical indicator that changes its colour in response to the \({\text{pH}}\) of the solution. The blue litmus paper is used to detect the acidic solution. It changes to red.

Q.2. How does an alkaline solution affect \({\text{pH}}\) ?
Ans: \({\text{pH}}\) is the concentration of \({{\text{H}}^ + }\) ions. An alkaline solution has less number of \({{\text{H}}^ + }\) ions. Therefore, these have high \({\text{pH}}\)  values. These substances are used to neutralize acids.

Q.3. What are the properties of an alkali?
Ans: Alkalis are soluble bases. Some common alkalis are sodium hydroxide and potassium hydroxide. Its properties are-
1. They are bitter and soapy or slippery to touch.
2. On the \({\text{pH}}\) scale, they rank above \(7.\) The higher the number, the stronger will be the alkali.

Q.4. What is \({\text{pH}}\) ?
Ans: \({\text{pH}}\) s stands for the term “power of Hydrogen”. It represents the hydrogen ion concentration present in a given solution. It is used to measure the acidity or alkalinity of a substance.

Q.5. If for two solutions, the \({\text{pH}}\) values are \(1\) and \(3,\) then what information can you infer?
Ans: The solution with \({\text{p}}{{\text{H}}_1}\) has \(100\) times as many hydrogen ions as the solution with \({\text{p}}{{\text{H}}_3}\) It is therefore \(100\) times less acidic than a solution with \({\text{p}}{{\text{H}}_3}.\)

Learn About Ionic Bond Here

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