Q1.
I wonder how the level of a coloured liquid is measured?
Answer
For a coloured liquid you read the mark that coincides with the top of the meniscus — the opposite of what you do for water.
| Liquid | Which part of the meniscus to read | Reason |
|---|---|---|
| Water and other colourless liquids | Bottom of the meniscus | The curve is clearly visible through the transparent liquid |
| Coloured liquids (ink, potassium permanganate solution, milk) | Top of the meniscus | The colour hides the bottom of the curve; only the upper edge can be seen sharply |
In both cases the rule that never changes is: keep your eye exactly level with the surface you are reading.
Why it happens: A liquid in a narrow tube curves at the surface because it is attracted to the glass. In a colourless liquid you can look right through and see the lowest point of that curve. In a coloured liquid, light cannot pass through, so all you see is a solid band of colour whose sharp upper edge is the only line you can fix on.
Tip: If your eye is above the surface you will read too high, and if it is below you will read too low. This error, called parallax, is far bigger than the least count of the cylinder — so bend down and bring your eye to the level of the liquid.