Exercises: Solutions and concentration

Emma adds two spoonfuls of sugar to a glass of water and stirs until the sugar disappears, as shown in the figure.

What is sugar: the solute or the solvent? And water? What is the result of the mixture called?

The IV drip used in hospitals is water with dissolved salts and glucose. Which is the solvent? How many solutes does this solution have?

The alcohol used to disinfect a wound is alcohol dissolved in water.

1. a) Which is the solute and which is the solvent?
2. b) Can a liquid be a solute? What about a gas? Give an example of a dissolved gas.

Strawberry syrup tastes too sweet to you, so you decide to add more water to the glass.

1. a) Does the total amount of sugar in the glass change?
2. b) What about the taste? Explain what "diluting" a solution means.

Why does sugar dissolve faster in hot tea than in cold tea? What happens to the solubility of most solids when the temperature increases?

Complete the sentence with the missing words: "In a solution, the $\dots$ is the substance present in the largest amount and is the one that dissolves; the $\dots$ is the substance that dissolves; and the $\dots$ tells us how many grams of solute there are per liter of solution."

The solubility of salt in water at room temperature is about $36\ \text{g}$ per $100\ \text{mL}$. You pour $40\ \text{g}$ of salt into a glass with $100\ \text{mL}$ of water and stir well, as shown in the figure.

1. a) Does all the salt dissolve?
2. b) How many grams remain undissolved at the bottom?
3. c) What is a solution that can no longer dissolve any more solute called?

The solubility of sugar increases significantly with temperature: at $60\ \celsius$ up to $290\ \text{g}$ can be dissolved in $100\ \text{mL}$ of water, but at $20\ \celsius$ only up to $200\ \text{g}$, as shown in the graph.

You prepare a syrup with $250\ \text{g}$ of sugar in $100\ \text{mL}$ of hot water at $60\ \celsius$.

1. a) At $60\ \celsius$, does all the sugar dissolve?
2. b) You allow the syrup to cool to $20\ \celsius$. What will happen? How many grams of sugar will crystallize?

Gases dissolve less well in hot water than in cold water: unlike solids, their solubility decreases as the temperature rises.

1. a) Use this fact to explain why fish in a pond come up to the surface to gulp air in summer.
2. b) If you heat a saucepan of water to cook pasta, tiny bubbles appear long before boiling. What are these bubbles?

The solubility of potassium nitrate is $85\ \text{g}$ per $100\ \text{mL}$ of water at $50\ \celsius$, but only $32\ \text{g}$ per $100\ \text{mL}$ at $20\ \celsius$. In the lab, you dissolve $80\ \text{g}$ of potassium nitrate in $100\ \text{mL}$ of hot water at $50\ \celsius$.

1. a) Does everything dissolve at that temperature? Justify your answer using the data.
2. b) You allow the glass to cool to $20\ \celsius$. How much potassium nitrate can remain dissolved?
3. c) How many grams will appear as crystals at the bottom? Explain what happened upon cooling.