Chemistry Solutions

The practical takeaway is simple: whenever one substance is evenly distributed in another, look for the solution’s two parts - the material being dissolved and the material doing the dissolving.

Solutions, Solutes, Solvents, and Related Mixtures

Solution - a homogeneous mixture in which one or more substances are evenly distributed in another substance. “Homogeneous” means that the composition is uniform throughout. A salt solution taken from the top or bottom of a well-mixed glass has the same salt and water composition.

Solute - the substance that dissolves in a solvent. In salt water, salt is the solute. In a sugar drink, sugar is the solute. A solution can contain more than one solute, such as a sports drink containing sugar, salts, colours, and flavouring substances.

Solvent - the substance that dissolves the solute. It is usually present in the larger amount. Water is called a common solvent because it dissolves many substances, including salt, sugar, and some acids. However, the solvent is not always water. In iodine tincture, alcohol acts as the solvent for iodine.

When a solute dissolves, its particles separate and spread among the particles of the solvent. They do not simply vanish. For example, when sodium chloride dissolves in water, the sodium and chloride particles become distributed throughout the water. The mixture may look like a single substance, but it still contains both salt and water.

The following table compares three important types of mixtures:

| Mixture type | Particle behaviour | Settles on standing? | Can ordinary filtration separate it? | Example | |---|---|---|---|---| | Homogeneous solution | Particles are extremely small and evenly spread | No | No | Salt water | | Suspension | Large particles are spread through a fluid | Yes | Usually yes | Muddy water | | Colloid | Medium-sized particles remain dispersed | Usually no | Not by ordinary filter paper | Milk |

A homogeneous mixture has the same composition in every portion. Salt water, copper sulfate solution, and air are examples. Air is a mixture of gases, mainly nitrogen and oxygen, and appears uniform even though it contains several substances. Brass is a solid solution of copper and zinc; its composition is uniform in a properly mixed sample.

A suspension - a heterogeneous mixture containing large insoluble particles that are spread through a fluid but settle when the mixture is left undisturbed. Muddy water is a clear example. At first, mud particles may be spread throughout the water. After some time, they collect at the bottom. A filter can trap many of these particles.

A colloid - a mixture in which particles are larger than solution particles but small enough to remain dispersed for a long time. Milk, fog, shaving foam, and jelly are common colloids. Milk looks uniform to the eye, but it contains tiny fat droplets dispersed in water. These droplets do not quickly settle like mud particles.

Colloids often show the Tyndall effect, which is the scattering of light by colloidal particles. If a beam of light passes through fog, its path can become visible because the droplets scatter the light. A true salt solution usually does not show this effect because its particles are too small to scatter ordinary light strongly.

Ask yourself: if a mixture looks clear, does that always prove it is a solution? Not necessarily. Some colloids can look quite uniform, and some suspensions may seem uniform immediately after shaking. Use more than one test: observe the mixture after standing, try filtration, and check whether a light beam becomes visible through it.

Another useful distinction is between miscible and immiscible liquids. Miscible liquids mix completely, as alcohol and water do in suitable amounts. Immiscible liquids do not mix uniformly; oil and water form separate layers. Oil and water are therefore not a solution, even when shaken temporarily.

The practical takeaway is to connect appearance with behaviour: a solution stays uniform, a suspension settles, and a colloid stays dispersed while often scattering light.

Worked Example: Identifying Salt Water, Muddy Water, and Milk

Three labelled samples - A, B, and C - are available. Sample A is clear and remains clear after one hour. Sample B looks cloudy, and particles collect at the bottom after ten minutes. Sample C looks cloudy, does not form a visible layer after one hour, and makes the path of a torch beam visible. Classify each sample and identify the most likely components.

1. Observe sample A. Sample A is clear and remains uniform after one hour. This suggests that its particles are very small and do not settle. It is likely a true solution.

2. Check whether sample A can be separated by ordinary filtration. If sample A is salt water, filter paper will not trap the dissolved salt. The water and dissolved salt pass through together. Therefore, sample A is classified as a homogeneous solution.

3. Identify the components of sample A. In salt water, salt is the solute because it dissolves. Water is the solvent because it does the dissolving. The result is a salt solution.

4. Observe sample B after ten minutes. Particles have collected at the bottom. This settling behaviour is the main sign of a suspension. The particles are too large to remain evenly distributed.

5. Consider filtration for sample B. A filter can trap much of the solid material from muddy water. The solid mud is the dispersed material, and water is the continuous liquid. Sample B is therefore muddy water, a suspension.

6. Observe sample C with the torch beam. The visible beam means that particles are scattering light. Since the particles do not quickly settle, sample C shows the behaviour of a colloid.

7. Identify a likely example for sample C. Milk fits these observations. Tiny fat droplets are dispersed in water. They are larger than dissolved particles but small enough to remain spread out for a long time.

8. State the final classification. Sample A is a homogeneous solution, sample B is a suspension, and sample C is a colloid.

Final result: A = salt solution; B = muddy water suspension; C = milk colloid.

Notice the reasoning pattern: do not classify only by colour or cloudiness. First observe whether particles settle, then consider filtration and light scattering. A cloudy sample can be either a suspension or a colloid, so its behaviour provides the evidence.

As a practical check, shake a small amount of muddy water and milk separately. Muddy water gradually clears at the top as solid particles settle. Milk generally remains dispersed. This difference is more useful than simply saying that both samples look cloudy.

Guided Practice and Answer Key

1. In a sugar-water mixture, which substance is the solute and which is the solvent? Hint: Ask which substance dissolves and which substance is usually present in the greater amount.

2. A mixture remains uniform after standing, and its particles pass through ordinary filter paper. Is it more likely to be a solution, suspension, or colloid? Hint: A true solution contains particles too small for ordinary filtration to trap.

3. Why does muddy water become clearer at the top after standing? Hint: Think about the size and weight of the solid particles.

4. Milk does not quickly settle, but it can scatter a beam of light. How should it be classified? Hint: Light scattering is a characteristic clue for a colloid.

5. A student says that oil and water form a solution after vigorous shaking. Is the statement correct? Hint: Observe whether the liquids remain as one uniform phase or separate into layers.

Answer Key: In question 1, sugar is the solute and water is the solvent. Question 2 describes a homogeneous solution. In question 3, the relatively large mud particles settle under gravity, leaving clearer water above. Question 4 describes a colloid. Question 5 is incorrect because oil and water are immiscible and separate into layers after standing.

Keep the central comparison in mind: dissolved particles stay uniformly distributed, suspension particles settle, and colloidal particles remain dispersed but can scatter light. With these observations, the difference between a clear solution, muddy suspension, and milk-like colloid becomes easy to recognise.

End of chapter one. 5 more chapters in the full book.