Which of the following solutions is alkaline?

Question

Prepp Team · Accepted Answer

Understanding Acidic, Basic, and Neutral Solutions

The question asks us to identify which of the given solutions is alkaline. Solutions can be classified as acidic, basic (alkaline), or neutral based on their hydrogen ion concentration, denoted as $[H^+]$. This concentration is directly related to the pH scale, which is commonly used to measure the acidity or alkalinity of a solution.

Defining Acidity, Alkalinity, and Neutrality based on [H⁺]

In pure water at a standard temperature of $25^\circ\text{C}$, there is a natural equilibrium where water molecules dissociate into hydrogen ions ($H^+$) and hydroxide ions ($OH^-$). The concentration of these ions is equal:

$$[H^+] = [OH^-] = 1 \times 10^{-7} \text{ Mol/l}$$

This is the point of neutrality. Based on this, we define the types of solutions:

Neutral Solution: A solution where the concentration of hydrogen ions equals the concentration in pure water at $25^\circ\text{C}$.
- $[H^+] = 1 \times 10^{-7} \text{ Mol/l}$
Acidic Solution: A solution where the concentration of hydrogen ions is higher than in pure water at $25^\circ\text{C}$. This means there are relatively more $H^+$ ions than $OH^-$ ions.
- $[H^+] > 1 \times 10^{-7} \text{ Mol/l}$
Alkaline (Basic) Solution: A solution where the concentration of hydrogen ions is lower than in pure water at $25^\circ\text{C}$. This implies that the concentration of hydroxide ions ($[OH^-]$) is higher than $1 \times 10^{-7}$ Mol/l (since $[H^+][OH^-] = 1 \times 10^{-14}$ at $25^\circ\text{C}$). There are relatively more $OH^-$ ions than $H^+$ ions.
- $[H^+] < 1 \times 10^{-7} \text{ Mol/l}$

Analyzing the Given Options

Let's examine each option provided in the question based on the definitions above:

Option	$[H^+]$ Value	Comparison to $1 \times 10^{-7}$ Mol/l	Type of Solution
1	$1 \times 10^{-4}$ Mol/l	$10^{-4}$ is greater than $10^{-7}$ ($10^{-4} > 10^{-7}$)	Acidic
2	$[H^+] < 1 \times 10^{-7}$ Mol/l	$[H^+]$ is less than $10^{-7}$ ($[H^+] < 10^{-7}$)	Alkaline
3	$[H^+] > 1 \times 10^{-7}$ Mol/l	$[H^+]$ is greater than $10^{-7}$ ($[H^+] > 10^{-7}$)	Acidic
4	$1 \times 10^{-7}$ Mol/l	$[H^+]$ is equal to $10^{-7}$ ($[H^+] = 10^{-7}$)	Neutral

From the analysis, we can see that the solution with a hydrogen ion concentration $[H^+]$ less than $1 \times 10^{-7}$ Mol/l is classified as alkaline.

Conclusion on Alkaline Solution

Comparing our analysis with the options, Option 2, which states $[H^+] < 1 \times 10^{-7}$ Mol/l, correctly describes an alkaline solution according to the standard definition at $25^\circ\text{C}$.

Revision Table: Acidity, Neutrality, Alkalinity

Solution Type	Hydrogen Ion Concentration ($[H^+]$)	pH Range
Acidic	$[H^+] > 1 \times 10^{-7}$ Mol/l	pH < 7
Neutral	$[H^+] = 1 \times 10^{-7}$ Mol/l	pH = 7
Alkaline (Basic)	$[H^+] < 1 \times 10^{-7}$ Mol/l	pH > 7

Additional Information: The pH Scale and $[H^+]$

The pH scale is a logarithmic scale used to specify the acidity or basicity of an aqueous solution. pH is defined as the negative common logarithm of the hydrogen ion concentration:

$$\text{pH} = -\log_{10}[H^+]$$

This formula shows the inverse relationship between $[H^+]$ and pH. As $[H^+]$ increases, pH decreases, indicating increasing acidity. Conversely, as $[H^+]$ decreases, pH increases, indicating increasing alkalinity.

If $[H^+] = 1 \times 10^{-7}$, $\text{pH} = -\log(10^{-7}) = 7$.
If $[H^+] = 1 \times 10^{-4}$, $\text{pH} = -\log(10^{-4}) = 4$ (Acidic).
If $[H^+] = 1 \times 10^{-9}$, $\text{pH} = -\log(10^{-9}) = 9$ (Alkaline). Note that $10^{-9}$ is less than $10^{-7}$.

Understanding the relationship between hydrogen ion concentration and the pH scale is fundamental to classifying solutions as acidic, neutral, or alkaline.