Solutions and concentration

Lucía 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 drip used in hospital 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.

Supermarket tiger nut milk (horchata) contains about $100\ \text{g}$ of sugar per litre. To make it taste milder, you mix $1\ \text{L}$ of horchata with $1\ \text{L}$ of water.

1. a) How many grams of sugar are now in the jug?
2. b) What is the new concentration in $\text{g/L}$?

To preserve olives, you make brine by dissolving $30\ \text{g}$ of salt in $270\ \text{g}$ of water.

1. a) What is the mass percentage of salt?
2. b) You leave the container in the sun and $100\ \text{g}$ of water evaporate. What mass of solution remains?
3. c) What is the mass percentage now?
4. d) Explain why it has increased even though you have not added more salt.

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

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?

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 laboratory, 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.

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 litre of solution."

To make syrup, you dissolve $25\ \text{g}$ of sugar in $175\ \text{g}$ of water.

1. a) How many grams of solution do you get in total?
2. b) Calculate the mass percentage of sugar in the syrup.

After training, you make a homemade drink to recover: you dissolve $10\ \text{g}$ of mineral salts in $490\ \text{g}$ of water.

1. a) What total mass of solution do you obtain?
2. b) Calculate the mass percentage of salts.
3. c) If you added $500\ \text{g}$ more of water, would that percentage increase, decrease, or stay the same? Explain your answer without performing any calculations.

You prepare two glasses of salt water: in glass A you dissolve $5\ \text{g}$ of salt to make up $100\ \text{mL}$; in glass B, $12\ \text{g}$ of salt to make up $300\ \text{mL}$, as shown in the figure.

1. a) Calculate the concentration of each glass in $\text{g/L}$.
2. b) Which one is saltier? Explain why comparing the grams of salt in each glass is not enough.