How many spoons of salt were you able to dissolve before some of it remained undissolved?
Answer
In half a glass tumbler of water — about 100 mL — you will usually manage six to eight level teaspoons before salt starts staying at the bottom. Write down your own count, because it is the count for your volume of water, your spoon and your room temperature.
Why it happens: Common salt has a solubility of roughly 36 g in 100 mL of water at room temperature, and a level teaspoon holds about 5 g. So about seven spoons is what you should expect — and this is why the number is not a matter of opinion: it is a fixed property of salt and water at that temperature.
Try this: Repeat with only a quarter tumbler of water. You will find you can dissolve about half as many spoons — showing that the limit is set by how much water you took, not by the salt.
Q2.
What does this indicate about the capacity of water to dissolve salt?
Answer
It shows that a fixed amount of water has a definite limit — it cannot dissolve salt endlessly. Up to that limit the solution is unsaturated; at the limit it is saturated at that temperature, and any extra salt stays undissolved at the bottom.
Three things follow from this one observation:
The limit depends on how much solvent there is. Twice the water dissolves about twice the salt.
The limit depends on the temperature. Warm the water and it will take more salt (Activity 9.2 shows this for baking soda).
The limit is a property of that solute in that solvent. Sugar, baking soda and salt each have their own limit in water — this maximum amount that dissolves in a fixed quantity of solvent is called the solubility.
Why it happens: A water particle can hold apart only a limited number of salt particles. Once every water particle is already busy surrounding dissolved particles, there is nothing left to pull the next crystal apart — so it stays a crystal.