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We are apt to think we know what time is because we can measure it, but no sooner do we reflect upon it that illusion goes. So it appears that the range of the measurable is not the range of the knowable. There are things we can measure, like time, but yet our minds do not grasp their meaning. There are things we cannot measure, like happiness or pain, and yet their meaning is perfectly clear to us.

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  • Geoffrey

    Site author
    5 Oct 2026

    Hello Harprit, R2.3.4 gives X(g) ⇌ X(aq) as an example so I guess students could be asked to explain why a bottle of carbonated water fizzes when the top is removed from the bottle and then eventually goes flat if the top is left off.

    R2.3 The extent of chemical change

  • Harprit

    5 Oct 2026

    Hello Geoff, In R2.3.4, the IB guide talks about Le Chatelier's Principle to physical equilibria and gives a (g) / (aq) as an example. A very old IB question came up looking at a (l) / (g), and what happens when you increase the volume of liquid....

    R2.3 The extent of chemical change

  • Paul

    Site author
    1 Oct 2026

    Me too! As far as I know, there are no other issues with sodium hydroxide in the burette. My students frequently do it that way round. We just make sure that we don't leave the sodium hydroxide in the burette, and we rinse it out afterwards. The etching...

    Acid-alkali titration

  • Austin

    30 Sep 2026

    In general, I find it easier to identify the colourless to pink colour change rather than pink to colourless.  Is there any major issue with switching the acid and base other than that the sodium hydroxide may cause etching or other damage to...

    Acid-alkali titration

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