A student was given two test-tubes A and B each containing a colourless solution. When a few drops of universal indicator were added to the two test-tubes, one by one, then the colour of solution in test-tube A changed to blue whereas the colour of solution in test-tube B changed to orange.
(a) Which test-tube has a solution of pH higher than 7?
(b) Which test-tube has a solution of pH lower than 7?
(c) Which test-tube has an acidic solution?
(d) Which test-tube has an alkaline solution?

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Solution

(a) The universal indicator changes its color towards violet in basic or alkaline medium. This, the pH of the solution in test tube A is greater than 7, that's why the universal indicator shows blue colour.

(b) The universal indicator changes its color towards red in acidic medium. So, the pH of the solution in test tube B is less than 7, that's why the universal indicator is giving an orange colour.

(c) The pH of the solution in the test tube B is less than 7, so the test tube "B" has an acidic solution.

(d) The pH of the solution in test tube A is greater than 7, so test tube "A" has an alkaline solution.

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