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Science · 2025 · 4 marks
CBSE 2025 · Region 2 · Set 1 · Q37
Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different…
Acid-base indicators can be used to distinguish between an acid and a base. Universal indicator, a mixture of several indicators, shows different colours at different concentrations of acids and bases, thereby indicating their pH on the pH scale of $\displaystyle 0-14$. The pH of a solution is measured by pH paper, which is a paper impregnated with a universal indicator.
Answer the following questions :
(a)
Solution P is a strong acid while solution Q is a strong base. On the pH scale, where would you place the solutions P and Q ?
(b)
A solution has a pH of 7. Name a compound you would use to
(i)
increase its pH , and (ii) decrease its pH.
(c)(i)
When the pH of a solution is decreased from $\displaystyle 4$ to $\displaystyle 2$, what effect does it produce on its hydronium ion concentration ? State the colour change shown by the pH paper.
OR
(c)
(ii)
A person is feeling pain and irritation in the stomach due to indigestion. What could be the pH of the fluid in the stomach ? Write the common name of the medicines people use for remedy. Give the chemical name of "milk of magnesia" often used for this purpose.
Marking-scheme solution
(a) P - pH $\displaystyle 0$ to $\displaystyle 4$ \(\displaystyle \mathrm{Q}-\mathrm{pH} 12\) to $\displaystyle 14$
(b) (i) By adding sodium hydroxide
(ii) By adding hydrochloric acid
(c)
(i) • Hydronium ion \(\displaystyle \left(\mathrm{H}_{3} \mathrm{O}^{+} / \mathrm{H}^{+}\right)\)ion concentration increases. - Colour will change from yellow/orange to red/pink
OR
(c) (ii) • low pH/ between $\displaystyle 1$ to $\displaystyle 3$ - by the use of antacids/milk of magnesia/sodium hydrogen carbonate - Magnesium hydroxide/Mg(OH) \(\displaystyle { }_{2}\)
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CBSE Class 10 Science past-paper question from the 2025 board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.