Chemistry
Solutions & concentration
What really happens when something dissolves and how to calculate concentrations in g/l.
What you need first
Sugar in tea, salt in pasta water, sherbet powder in lemonade: you dissolve substances in water all the time. The substance seems to disappear, but you can still taste it. Let's look at what happens there and how to state how much is really in it.
Dissolving in the particle model
Nothing disappears when something dissolves. The water particles surround the sugar particles, pull them out of the crystal, and spread them evenly through the whole glass. That is why the sugar is invisible, yet every sip tastes sweet. Mass is conserved: 10 g of sugar is still 10 g of sugar when dissolved.
Table salt dissolves up to about 360 g/l, anything more settles at the bottom.
Concentration in g/l
The concentration tells you how much substance is dissolved in a certain amount of liquid: dissolved mass divided by volume, measured in grams per litre (g/l). If you dissolve 10 g of salt in 1 l of water, the solution has 10 g/l. Dissolve the same 10 g in 2 l and it is only 5 g/l: the solution is more dilute. Lots of substance in little water means concentrated, little substance in lots of water means dilute. It works backwards just as well: if you know the concentration and the volume, multiply the two and you know how many grams are dissolved. 5 g/l times 2 l gives 10 g.
When nothing more fits in
Water cannot take up an unlimited amount. At some point the rest stays undissolved at the bottom: the solution is saturated, nothing more fits in. In 1 l of water at room temperature, meaning around 20 °C, about 360 g of table salt dissolves. Heat helps with dissolving: in warm water the particles move faster and most solids dissolve better and more quickly. That is why sugar vanishes much faster in hot tea than in cold.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
What happens to the sugar particles when sugar dissolves in water?
You dissolve 10 g of salt in 1 l of water. What is the concentration of the solution in g/l?
You dissolve 30 g of sugar in 2 l of water. What concentration in g/l does that give?
You work out the concentration in g/l by dividing the dissolved mass by the ….
Which solution is the most concentrated?
Match each term to the description that fits it.
Where this leads