In the vast landscape of chemistry, the pH scale stands as a crucial tool. Join us on this Class 10 journey as we unravel the concept of pH, delving into its significance, applications, and the impact it has on our daily lives.

Mastering Chemistry: CBSE NCERT Download for a Comprehensive Understanding of the Concept of pH Scale

pH Scale Properties

• The pH scale is very convenient to use. In this scale, odd expressions such as 1.24 × 10^-4 are converted into a single number of 3.89.
• This scale covers a very large range of [H+][H+].
• The division between zero and 1 is expanded to a linear scale. The pH scale also expands the compact scale into a large scale for comparison purposes.
• As there is a use of a negative log of [H+][H+] in the scale, the pH scale actually has positive values. It should be noted that if the pH is large, the [H+][H+] will be smaller.
• The pH range does not have any lower or upper bound. It is because the pH is an indication of the concentration of H⁺.

Unified Absolute pH Scale

In the year 2010, a new unified absolute pH scale was proposed. As per the details mentioned, this scale would allow various pH ranges across different solutions to be measured based on a common proton reference standard. This was to be developed on the basis of the absolute chemical potential of the proton. Additionally, this model used the Lewis acid-base definition and has been applied to liquids, gases and even solids.

pH Scale and Acidity

Acid solutions usually have protons, and basic solutions have hydroxide ions. The concentrations of the ions are low (negative power of ten). pH scale is a convenient way of expressing these low concentrations in simple numbers between 1 and 14.

pH is the negative logarithm to the base ten of hydrogen ion concentration in moles per litre.

pH = – log [H⁺]

p(OH) is the negative logarithm to the base ten of hydroxide ion concentration in moles per litre.

p(OH) = – log [OH^–]

In aqueous solutions, pH + p(OH) = 14.

pH scale is based on neutral water, where [H⁺] = [OH^–] = 10^-7

For a neutral solution pH = = – log [H+] = – log [10-7] = +7

pH of strong acid decreases with a limit of 1, and the pH of a base increases up to 14.

Generally, acids and bases will have a pH between

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Understanding pH:

pH is a measure of acidity or alkalinity in a solution, ranging from 0 to 14. Acids have a pH below 7, bases above 7, and 7 denotes neutrality. This numerical scale provides a quantitative insight into the strength of acidic or basic properties.

Applications in Everyday Life:

Explore how the pH scale influences daily experiences, from the tartness of citrus fruits to the soothing effects of antacids. Uncover its role in maintaining the balance of swimming pools, the fermentation of foods, and even the skincare products we use.

pH in Environmental Science:

Delve into the environmental implications of pH, understanding its impact on aquatic ecosystems. Learn how changes in pH levels can affect the health of rivers, lakes, and oceans, showcasing the broader relevance of this fundamental concept.

Hands-On Experimentation:

Engage in simple experiments to understand the pH of common substances around you. Witness the color-changing magic of universal indicators and gain practical insights into the acidic and alkaline nature of household items.

Periodic Variation of Acidic and Basic Properties

(a)  Hydracids of the Elements of the Same Periods

Along the period, acidic strength increases. Hydrides become increasingly acidic from CH4, NH3, H2O and HF. The increase in acidic properties is due to the fact that the stability of their conjugate bases increases in the order

CH–3< NH–2  < OH– < F–

(b)  Hydracids of the Elements of the Same Group

1. The acidic nature increases down the column. Hydrides of V group elements (NH3, PH3, AsH3, SbH3, BiH3) show basic character, which decreases due to an increase in size and a decrease in electronegativity from N to Bi. There is a decrease in electron density in the sp3 -hybrid orbital, and thus electron donor capacity decreases.
2. Hydracids of VI group elements (H20, H2S, H2Se, H2Te) act as weak acids. The strength increases in the order H20 < H2S < H2Se < H2Te. The increasing acidic properties reflect the decreasing trend in the electron donor capacity of OH–, HS–, HSe– or HTe– ions.
3. Hydracids of VII group elements (HF, HCI, HBr, HI) show acidic properties, which increase from HF to HI. This is explained by the fact that bond energies decrease.

(H-F = 135 kcal/mol, HCI = 103, HBr = 88 and HI = 71 kcal/mol).

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SAMPLE PRACTICE QUESTION

Q:1  What does pH stand for, and what does it measure?

Ans: pH stands for "potential of hydrogen." It measures the acidity or alkalinity of a solution on a scale from 0 to 14.

Q:2  How is the pH scale structured, and what does each range signify?

Ans: The pH scale ranges from 0 to 14. Values below 7 indicate acidity, 7 is neutral, and values above 7 indicate alkalinity.

Q:3  What are some common substances with a neutral pH of 7?

Ans: Water has a neutral pH of 7. Pure water contains an equal concentration of H⁺ and OH⁻ ions.

Q:4  How do acids and bases impact the pH scale?

Ans: Acids lower the pH, moving towards 0, while bases raise it, moving towards 14. The strength is determined by the concentration of H⁺ or OH⁻ ions.

Q:5  Can you provide examples of everyday items with varying pH levels?

Ans: Lemon juice and vinegar are acidic (pH < 7), while baking soda and soap are basic (pH > 7).

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