Salt in water: where does the mass go?
Mass in a physical change.
Stirring salt into water
Ria stirs 20 g of salt into 200 g of water until the salt seems to disappear. Nothing is spilt.
What do you think is the mass of the salty water (the solution)?
What this lesson covers
The idea
Mass remains practically unchanged in all physical changes; the mass of a solution equals the sum of the masses of the water and the substance dissolved in it.
Stirring salt into water
Ria stirs 20 g of salt into 200 g of water until the salt seems to disappear. Nothing is spilt.
What do you think is the mass of the salty water (the solution)?
- 220 g
- Less than 220 g, because the salt has vanished
- More than 220 g, because stirring adds mass
Make a solution
Change the mass of water and of salt. The numbers are only examples. Watch the mass of the solution.
The salt is still there
Dissolving salt in water is a physical change. The salt is still in the solution, spread through the water, so its mass is still counted.
Mass remains practically unchanged in all physical changes. The mass of a solution equals the sum of the masses of the water and the substance dissolved in it.
Notes
In physical changes mass stays practically unchanged. The mass of a solution = mass of water + mass of the substance dissolved.
Check yourself
A solution is made with 150 g of water and 25 g of salt (an example). What is the mass of the solution, in g?
Answer: 175 g
Mass of solution = 150 g + 25 g = 175 g.
What happens to the total mass when salt dissolves in water?
A solution has a mass of 230 g. It was made with 200 g of water and some salt. How many g of salt were dissolved?
Answer: 30 g
Salt = 230 g − 200 g = 30 g.
Dissolving salt in water is a physical change. What does the book say about mass in physical changes?
- It becomes less, because the salt disappears. The salt does not disappear. It is still in the solution, so its mass is still part of the total.
- It stays practically unchanged — correct. Yes!
- It becomes more, because the water takes in extra mass. Nothing is added. The solution has the mass of the water plus the salt, and no more.
- Mass increases in a physical change. The book says that there is practically no change in mass when a solution is formed, and this is true for all physical changes.
- Mass is lost in a physical change. The book says that mass remains practically unchanged in physical changes.
- Mass remains practically unchanged — correct. Yes!