Exploring Mixtures and their Separation · Lesson 2 of 13
Solutions
“The solute disappears from sight, but it certainly has not left the beaker.”
• Identify the solute, solvent and solution in familiar examples. • Explain why a solution is homogeneous. • Describe concentration as a statement of proportion. • Explain why correct proportions matter in daily life. • Relate careful solution preparation to health, agriculture and food.
A spoonful of sugar seems to disappear when stirred into water. Nothing has been destroyed: the sugar particles have spread throughout the water so completely that they can no longer be distinguished by eye. Every small portion now contains both substances in the same general proportion.
A solution is a homogeneous mixture formed when one or more solutes are distributed uniformly in a solvent.
A solute is the substance that gets dissolved while a solution is being formed.
A solvent is the substance that dissolves the solute and usually forms the larger part of the solution.
In sugar water, sugar is the solute and water is the solvent. In vinegar, acetic acid is mixed uniformly with water. In an aerated drink, carbon dioxide gas is dissolved in water. These examples show that a solute need not always be a solid, although the familiar solid-in-liquid case is a useful starting point.
Problem
A small quantity of common salt is completely dissolved in a beaker of water. Identify the solute, solvent and solution.
- 1.Ask which substance gets dissolved: common salt is the solute.
- 2.Ask which substance does the dissolving: water is the solvent.
- 3.The uniform mixture produced is the salt solution.
- 4.No salt particles are visible because they are distributed at a scale too small to see directly.
Concentration of a Solution
Two glasses can both contain sugar solution yet taste very different. The difference lies in proportion. One may contain a small amount of sugar in much water, while the other contains more sugar in the same amount of water. To compare them meaningfully, we state how much solute is present in a specified amount of solvent or solution.
The concentration of a solution is the amount of solute dissolved in a given amount of solvent or solution.
Concentration is not merely a laboratory idea. Oral rehydration solution requires specified amounts of salt and sugar in a fixed amount of water. Changing those proportions may produce a sweet or salty drink, but it will not have the intended composition. The name of a solution does not guarantee that its concentration is suitable for a particular purpose.
Note
Not every sugary drink is an oral rehydration solution. A useful solution must contain the correct substances in the correct proportions. This is why preparation instructions should be followed exactly rather than adjusted by taste.
Farmers also prepare pesticide sprays by mixing a measured amount of pesticide with a specified amount of water. Too little may fail to protect the crop; too much may damage plants, soil and the surrounding environment. The same principle applies to medicines, cosmetics, food products and even a carefully prepared cup of tea.
Problem
Glass A contains 10 g sugar in 90 g water. Glass B contains 20 g sugar in 90 g water. Which is more concentrated?
- 1.Both glasses contain the same mass of solvent.
- 2.Glass B contains the greater mass of solute.
- 3.With the solvent amount fixed, the solution containing more solute is more concentrated.
- 4.Therefore, Glass B is more concentrated.
Problem
Two identical masses of salt are dissolved separately in 100 mL and 200 mL of water. Which solution is more concentrated?
- 1.The solute amount is the same in both solutions.
- 2.The first solution spreads that solute through less water.
- 3.The second solution spreads it through more water and is therefore more dilute.
- 4.The solution prepared with 100 mL water is more concentrated.
Meet a Scientist
Dilip Mahalanabis developed and implemented oral rehydration treatment for severe dehydration caused by illnesses such as diarrhoea and cholera. The carefully formulated solution became a simple, practical treatment that has saved millions of lives. Its success demonstrates how a precise mixture can have an enormous effect on human well-being.
Never prepare medical or agricultural solutions by guesswork. Use the stated quantity of each component and the stated final amount of solution.
Quiz
Which description best matches Solution?
Which description best matches Solute?
Which term matches this description: A solution is a homogeneous mixture formed when one or more solutes are distributed uniformly in a solvent.
Which term matches this description: A solute is the substance that gets dissolved while a solution is being formed.
Which statement is a key takeaway from this lesson?
Practice Problems
- Identify the solute and solvent in sugar water, vinegar and an aerated drink.
- Explain why two solutions containing the same substances can have different concentrations.
- Describe what changes when more solvent is added without adding more solute.
- Give two situations in which an incorrect concentration could cause harm.
Key Takeaways
• A solution is a homogeneous mixture. • The solute gets dissolved and the solvent does the dissolving. • Concentration expresses a proportion, not merely the presence of a solute. • Adding solute generally raises concentration, while adding solvent generally lowers it. • Correct concentration is essential wherever a solution has a specific purpose.