The dark blue colour in the pH scale refers to __________.

Question

Examoogle · Accepted Answer

The correct answer is Option B: weak basic solution. On the universal pH scale, which ranges from 0 to 14, a dark blue colour corresponds to a weak basic solution. Highly alkaline or strong basic solutions typically appear violet or dark purple, whereas neutral solutions are green and acidic solutions range from red to yellow.

Step-by-Step Solution & Concepts:
On the universal pH scale, which ranges from 0 to 14, a dark blue colour corresponds to a weak basic solution. Highly alkaline or strong basic solutions typically appear violet or dark purple, whereas neutral solutions are green and acidic solutions range from red to yellow.

Working Principle: Universal indicators are mixtures of several indicators (such as phenolphthalein, methyl red, and bromothymol blue) that undergo distinct structural changes and protonation states across varying hydronium ion concentrations ($[H^+]$). In weakly basic environments (pH approximately $8.5$ to $11.0$), specific components like thymol blue and phenolphthalein transition through turquoise to dark blue as hydroxyl ions ($[OH^-]$) moderately dominate over hydronium ions.

Key Formulas: $pH = - \log_{10} [H^+]$ — definition of potential of hydrogen; $pH + pOH = 14$ — ionic product relationship in aqueous solutions at $25^\circ C$

Key Points: The pH scale is logarithmic, meaning each whole unit change represents a tenfold change in hydronium ion concentration.; A neutral solution has a pH of exactly $7$ at $25^\circ C$, where $[H^+] = [OH^-]$.; Weak bases turn universal indicator blue to dark blue, while strong bases turn purple or violet.; Acids turn universal indicator red, orange, or yellow depending on their strength.

Advantages: Universal indicators provide a quick visual estimation of acidity or alkalinity across a broad pH range.

Disadvantages: Colour matching can be subjective and difficult to read accurately in intensely colored or turbid solutions.

Applications: Testing soil pH in agriculture; Monitoring swimming pool water chemistry; Clinical diagnostics in biological fluids

Additional Info: Option A is incorrect because strong acidic solutions appear red or pink.. Option C is incorrect because neutral solutions appear green.. Option D is incorrect because strong basic solutions appear violet or purple.

Answer: B is correct — The dark blue colour on the universal pH scale specifically indicates a weak basic solution.

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Option A: strong acidic solution

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Option C: neutral solution

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Option D: strong basic solution

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