Soluble and Insoluble Substances

Introduction

Salt vanishes when stirred into water, but sand just swirls around and settles again. Nail varnish will not wash off with water, yet it wipes away easily with nail-varnish remover. Whether something dissolves depends on the substance and on the liquid you try to dissolve it in. Knowing which substances are soluble and which are insoluble is one of the most useful tools for separating mixtures, and it explains many everyday cleaning tricks.

Core explanation

Soluble and insoluble. A substance that dissolves in a solvent is soluble in that solvent. A substance that does not dissolve is insoluble . Sugar, salt and copper(II) sulfate are soluble in water; sand, chalk (calcium carbonate), iron filings and most plastics are insoluble in water.

It always depends on the solvent. Solubility is a partnership between solute and solvent, so you should always say "soluble in …". Iodine is only very slightly soluble in water but dissolves well in ethanol. Grease and oil are insoluble in water but dissolve in solvents such as white spirit. Nail varnish is insoluble in water but soluble in propanone.

"Like dissolves like." A useful rule of thumb is that substances tend to dissolve in solvents whose particles attract them in a similar way. Water is good at dissolving salts and sugars; oily solvents are good at dissolving oils, fats and waxes.

Degrees of solubility. "Insoluble" in everyday chemistry usually means the amount that dissolves is too small to notice. Many substances are sparingly soluble : calcium hydroxide dissolves only a little in water, giving limewater. Chemists measure solubility as the maximum mass that dissolves in 100 g of solvent at a stated temperature, a topic explored further with solubility curves.

Liquids and gases. For two liquids, chemists use the words miscible (they mix completely, like ethanol and water) and immiscible (they form separate layers, like oil and water). Gases can dissolve in liquids too: oxygen dissolves slightly in water, which is what fish breathe, while carbon dioxide is more soluble.

Recognising the difference. When a soluble solid dissolves, the mixture becomes clear (it may be coloured). An insoluble solid leaves the liquid cloudy while stirred and then settles as a layer at the bottom.

Using solubility to separate. Differences in solubility let us separate mixtures. Stirring a mixture of sand and salt with water dissolves the salt but not the sand; filtration then removes the sand, and evaporation recovers the salt.

Step-by-step reasoning

To decide whether a solid is soluble in water:

1. Add a small amount of the solid to water and stir well. 2. If the solid disappears and the liquid is clear, it is soluble. 3. If the liquid stays cloudy or the solid settles unchanged, it is insoluble. 4. Record the solvent used, because the result may differ in another solvent.

Visual explanation

Picture two test tubes side by side after shaking. The first, containing copper(II) sulfate and water, is clear blue with no solid visible. The second, containing chalk and water, is cloudy white at first, then shows a clear liquid above a white layer of settled powder.

Real-world analogy

Think of solvents as keys and solutes as locks. Water is the right key for many locks — salt and sugar — but it will not open the grease lock. For that you need a different key, such as a detergent or an oily solvent. No single key opens everything.

Real-world example

Dry cleaning uses non-water solvents to remove greasy stains from delicate fabrics such as wool and silk. The grease is insoluble in water but soluble in the cleaning solvent, and avoiding water stops the fabric from shrinking or losing its shape.

Why?

Why does salt dissolve in water but sand does not? Water particles are strongly attracted to the particles in salt and can pull them away from the crystal. The particles in sand are held together by very strong bonds in a giant structure and are not pulled away by water, so the sand stays solid.

Common misconception

"If something does not dissolve in water, it is insoluble." Solubility depends on the solvent. Iodine, grease and nail varnish do not dissolve well in water, but they are soluble in other solvents. Always name the solvent.

Worked example

Question: A mixture contains sand and sugar. Suggest how to obtain dry sand, explaining the role of solubility.

Reasoning: Sugar is soluble in water; sand is insoluble. Adding water dissolves only the sugar, so the two can be separated by filtering.

Answer: Add water and stir to dissolve the sugar. Filter: the sand stays on the filter paper while sugar solution passes through. Rinse the sand with a little water and let it dry.

Quick check

1. Oil and water form two layers. Which word describes this pair of liquids? Answer: Immiscible.

Exam focus

Always state the solvent: "soluble in water" or "soluble in ethanol". Learn the pairs soluble/insoluble (solids and gases) and miscible/immiscible (two liquids). Separation questions often start by using a solvent that dissolves one component but not the other.

Advanced insight

Very few substances are completely insoluble; even glass dissolves in water to a tiny extent. In analysis, chemists deliberately make insoluble products called precipitates from two solutions — for example, adding silver nitrate solution to a chloride solution gives a white precipitate of silver chloride, a test for chloride ions.

Summary

A soluble substance dissolves in a particular solvent; an insoluble one does not. Solubility depends on both the solute and the solvent, so always name the solvent. Miscible liquids mix completely; immiscible liquids form layers. Differences in solubility are the basis of many separation methods, such as removing sand from salt.

Practice questions

1. Classify each as soluble or insoluble in water: sugar, sand, salt, chalk. Answer: Sugar: soluble. Sand: insoluble. Salt: soluble. Chalk: insoluble. 2. Why is it important to name the solvent when describing solubility? Answer: Because a substance insoluble in one solvent may be soluble in another, such as grease in white spirit. 3. Ethanol and water mix to form a single layer. What word describes them? Answer: Miscible. 4. Describe how you could tell, by observation, whether a white powder is soluble in water. Answer: Stir it into water: if it disappears and the liquid becomes clear, it is soluble; if the liquid stays cloudy or the powder settles, it is insoluble.