CBSE Class 10 Answered

The pH scale is an inverse logarithmic representation of hydrogen proton (H⁺) concentration. Unlike linear scales, which have a constant relationship between the item being measured (H⁺ concentration in this case) and the value reported, each individual pH unit is a factor of 10 different than the next higher or lower unit. For example, a change in pH from 2 to 3 represents a 10-fold decrease in H⁺ concentration, and a shift from 2 to 4 represents a one-hundred (10 × 10)-fold decrease in H⁺ concentration. The formula for calculating pH is:

Where α_H⁺ denotes the activity of H⁺ ions, and is dimensionless. In solutions containing other ions, activity and concentration will not generally be the same. Activity is a measure of the effective concentration of hydrogen ions, rather than the actual concentration; it includes the fact that other ions surrounding hydrogen ions will shield them and affect their ability to participate in chemical reactions. These other ions change the effective amount of hydrogen ion concentration in any process that involves H⁺.

In dilute solutions such as tap water, activity is approximately equal to the numeric value of the concentration of the H⁺ ion, denoted as [H⁺] ([H₃O⁺]), measured in moles per litre (also known as molarity). Therefore, it is often convenient to define pH as:The pH scale is an inverse logarithmic representation of hydrogen proton (H⁺) concentration. Unlike linear scales, which have a constant relationship between the item being measured (H⁺ concentration in this case) and the value reported, each individual pH unit is a factor of 10 different than the next higher or lower unit. For example, a change in pH from 2 to 3 represents a 10-fold decrease in H⁺ concentration, and a shift from 2 to 4 represents a one-hundred (10 × 10)-fold decrease in H⁺ concentration. The formula for calculating pH is:

Where α_H⁺ denotes the activity of H⁺ ions, and is dimensionless. In solutions containing other ions, activity and concentration will not generally be the same. Activity is a measure of the effective concentration of hydrogen ions, rather than the actual concentration; it includes the fact that other ions surrounding hydrogen ions will shield them and affect their ability to participate in chemical reactions. These other ions change the effective amount of hydrogen ion concentration in any process that involves H⁺.

In dilute solutions such as tap water, activity is approximately equal to the numeric value of the concentration of the H⁺ ion, denoted as [H⁺] ([H₃O⁺]), measured in moles per litre (also known as molarity). Therefore, it is often convenient to define pH as:The pH scale is an inverse logarithmic representation of hydrogen proton (H⁺) concentration. Unlike linear scales, which have a constant relationship between the item being measured (H⁺ concentration in this case) and the value reported, each individual pH unit is a factor of 10 different than the next higher or lower unit. For example, a change in pH from 2 to 3 represents a 10-fold decrease in H⁺ concentration, and a shift from 2 to 4 represents a one-hundred (10 × 10)-fold decrease in H⁺ concentration. The formula for calculating pH is:

Where α_H⁺ denotes the activity of H⁺ ions, and is dimensionless. In solutions containing other ions, activity and concentration will not generally be the same. Activity is a measure of the effective concentration of hydrogen ions, rather than the actual concentration; it includes the fact that other ions surrounding hydrogen ions will shield them and affect their ability to participate in chemical reactions. These other ions change the effective amount of hydrogen ion concentration in any process that involves H⁺.

In dilute solutions such as tap water, activity is approximately equal to the numeric value of the concentration of the H⁺ ion, denoted as [H⁺] ([H₃O⁺]), measured in moles per litre (also known as molarity). Therefore, it is often convenient to define pH as:

For both definitions, log₁₀ denotes the base-10 logarithm, therefore pH defines a logarithmic scale of acidity. For example, if one makes a lemonade with a H⁺ concentration of 0.0050 moles per litre, its pH would be:

A solution of pH = 8.2 will have an [H⁺] concentration of 10^−8.2 mol/L, or about 6.31 × 10⁻⁹ mol/L. Thus, its hydrogen activity α_H⁺ is around 6.31 × 10⁻⁹. A solution with an [H⁺] concentration of 4.5 × 10⁻⁴ mol/L will have a pH value of 3.35.

In solution at 25 °C, a pH of 7 indicates neutrality (i.e. the pH of pure water) because water naturally dissociates into H⁺ and OH⁻ ions with equal concentrations of 1×10⁻⁷ mol/L. A lower pH value (for example pH 3) indicates increasing strength of acidity, and a higher pH value (for example pH 11) indicates increasing strength of basicity. Note, however, that pure water, when exposed to the atmosphere, will take in carbon dioxide, some of which reacts with water to form carbonic acid and H⁺, thereby lowering the pH to about 5.7.