Solutions and concentration

Aoife 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.

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."

You dissolve $10\ \text{g}$ of baking soda in water to make $2\ \text{L}$ of solution. The concentration in grams per litre is calculated by dividing the mass of solute by the volume of solution. What is the concentration in $\text{g/L}$?

Aoife dissolves $15\ \text{g}$ of glucose in water to make $500\ \text{mL}$ of solution. Calculate the concentration in $\text{g/L}$. Note: first convert the volume to litres.

The lab teacher asks you to prepare $2\ \text{L}$ of a salt solution with a concentration of $8\ \text{g/L}$. How many grams of salt should you weigh on the balance?

A chocolate milkshake has a sugar concentration of $40\ \text{g/L}$. If you drink half a litre of milkshake, how many grams of sugar have you consumed?

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.

From a bottle containing $1\ \text{L}$ of salt solution with a concentration of $60\ \text{g/L}$, you pour half a litre into a jug, as shown in the figure.

1. a) How many grams of salt are in the jug?
2. b) What is the concentration of the solution remaining in the bottle? Many classmates would say half: explain which is the correct answer.

The label of a mineral water indicates that it contains $460\ \text{mg}$ of dissolved minerals per litre.

1. a) Express this concentration in $\text{g/L}$.
2. b) If you drink half a litre of this water, how many milligrams of minerals have you consumed?
3. c) Comparing it with a laboratory solution of $30\ \text{g/L}$, explain why we say that mineral water is a very dilute solution.

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 $\text{g/L}$?

A mint syrup has $200\ \text{g}$ of sugar per litre. You want to prepare $0.5\ \text{L}$ of drink with a sugar concentration of $40\ \text{g/L}$.

1. a) How many grams of sugar should your drink contain?
2. b) How many millilitres of syrup contain those grams?
3. c) How much water should you add?

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?

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 crystallise?

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.

The teacher prepares $200\ \text{g}$ of solution containing $20\ \text{g}$ of salt in the laboratory. The mass percentage is calculated by dividing the mass of solute by the total mass of solution and multiplying the result by $100$. What is the mass percentage of salt?

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.

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.

The water of the Dead Sea contains about $300\ \text{g}$ of dissolved salt per litre, about nine times more than normal seawater.

1. a) How many grams of salt are there in $2\ \text{L}$ of Dead Sea water?
2. b) How many litres would be needed to obtain $1.2\ \text{kg}$ of salt?
3. c) Using the concentration data, explain why hardly any fish live in that sea.