Solutions and Mixtures
Introduced| English |
|---|
| solute/ˈsɒljuːt/ |
| solvent/ˈsɒlvənt/ |
| solution/səˈluːʃn/ |
| molarity/məʊˈlærɪti/ |
When one thing vanishes into another
- Stir sugar into tea and it disappears -- but it is still there.
- The sweet, clear liquid tastes uniform everywhere.
- One substance has spread evenly through another.
- We measure exactly how crowded that spread is.
Solute and solvent
- A solution 溶液 is a uniform mixture of two or more substances.
- The solute 溶质 is what dissolves; the solvent 溶剂 does the dissolving.
- In salt water, salt is the solute and water is the solvent.
In salt water, the salt is the...
The dissolved substance is the solute; water is the solvent.
Measuring concentration
- Molarity 摩尔浓度 measures concentration:
- More solute per litre means a more concentrated solution.
Dissolve $2\ \text{mol}$ of solute to make $4\ \text{L}$ of solution. The molarity (in M)?
$M = \text{moles}/\text{liters} = 2/4 = 0.5\ \text{M}$.
Dilute and concentrated
- Adding solvent dilutes a solution, lowering $M$.
- Removing solvent or adding solute concentrates it, raising $M$.
- Dilution keeps the moles of solute the same: $M_1V_1 = M_2V_2$.
Concentration links moles and volume
Molarity is moles of solute per litre of solution: at fixed concentration, moles scale with volume.
Dilute $100\ \text{mL}$ of $2\ \text{M}$ solution to $200\ \text{mL}$. The new molarity (in M)?
From $M_1V_1 = M_2V_2$, $2\times100 = M_2\times200$, so $M_2 = 1\ \text{M}$.
Diluting a solution with more solvent keeps the number of moles of solute the same.
You only add solvent, so the solute moles are unchanged.
Removing solvent from a solution makes it more ____.
Less solvent for the same solute raises the molarity.
Dissolve $0.5\ \text{mol}$ of salt in enough water to make $2\ \text{L}$.
- $M = \dfrac{\text{moles}}{\text{liters}} = \dfrac{0.5}{2} = 0.25\ \text{M}$.
- Add water to $4\ \text{L}$ and the molarity halves to $0.125\ \text{M}$.
Molarity is moles of solute divided by litres of...
It uses the total volume of solution, not just the solvent.
Molarity uses litres of solution, not litres of solvent -- the final total volume. Diluting changes the concentration but not the moles of solute, which is why $M_1V_1 = M_2V_2$. And "concentrated" just means high molarity, not necessarily saturated.
A solution is a uniform mixture where a solute dissolves in a solvent. Its concentration is the molarity $M = \text{moles} / \text{litres of solution}$. Adding solvent dilutes it (moles unchanged, so $M_1V_1 = M_2V_2$); adding solute concentrates it.