Question
What is solubility? Explain why sugar dissolves in water but sand does not, using the concept of molecular interactions.
Solution — Step by Step
Solubility is the maximum amount of a substance (solute) that can dissolve in a given amount of solvent at a specific temperature to form a stable, homogeneous solution.
A substance is described as soluble if it dissolves readily, sparingly soluble if it dissolves only a little, and insoluble if it does not dissolve under normal conditions.
Solubility is typically expressed in grams of solute per 100 g of solvent (or per 100 mL of solvent) at a specific temperature.
Whether a substance dissolves depends on the interactions between solute particles and solvent molecules. The guiding principle is “like dissolves like”:
- Polar solvents (like water) dissolve polar and ionic solutes well.
- Non-polar solvents (like petrol, benzene) dissolve non-polar solutes well.
Water is a polar molecule — it has a partial positive charge near the H atoms and a partial negative charge near the O atom (due to electronegativity difference).
Sugar (sucrose, C₁₂H₂₂O₁₁) molecules have many –OH (hydroxyl) groups on their surface. These –OH groups are polar, just like water molecules.
When sugar is added to water, water molecules cluster around the sugar molecules. The –OH groups of sugar form hydrogen bonds with water molecules. These attractions are strong enough to pull individual sugar molecules away from the solid sugar crystal.
Energy is released when new water–sugar attractions form. This released energy compensates for the energy needed to break apart the sugar crystal. Net result: sugar breaks apart and distributes uniformly throughout the water — it dissolves.
Sand is mainly silicon dioxide (SiO₂), a covalent network solid. Silicon and oxygen form very strong, directional covalent bonds in a three-dimensional network. There are no polar groups or ions on the surface of SiO₂ that water molecules can strongly interact with.
The energy required to break the Si–O covalent bonds in the SiO₂ network is far greater than the energy released by forming any Si…water or O…water attractions. The “energy balance” is strongly unfavorable.
So water molecules cannot pull SiO₂ units apart — sand remains as particles in water, settling to the bottom when stirred. Sand in water is a suspension, not a solution.
For most solids (including sugar), solubility increases with temperature because heating provides energy to break more solute–solute interactions and more solvent–solute interactions are formed at the higher temperature.
This is why hot tea dissolves more sugar than cold tea — and why a saturated hot solution becomes supersaturated when cooled, causing crystallisation (this is how rock candy is made!).
Why This Works
Dissolution is fundamentally an energy process. A substance dissolves when the energy released by forming solute–solvent interactions is large enough to compensate for:
- The energy needed to separate solute particles from each other
- The energy needed to create a “hole” in the solvent for the solute
Sugar and water are both polar with –OH groups → strong hydrogen bonding → energy balance favorable → dissolves. SiO₂ and water have mismatched interaction types → energy balance very unfavorable → does not dissolve.
Alternative Method — Everyday Observations
You can see this principle at work all around:
- Salt (NaCl, ionic) dissolves in water (polar) — ions attract polar water dipoles
- Oil does not dissolve in water — oil is non-polar, water is polar (you need to shake them constantly; left alone, they separate)
- Turmeric powder does not dissolve in water but dissolves in oil — turmeric pigments (curcumin) are non-polar
Common Mistake
Students sometimes say “sugar dissolves because it is small and sand does not because particles are large.” Particle size affects the rate of dissolving (smaller particles dissolve faster — more surface area) but not the ultimate solubility. Very fine sand still will not dissolve in water, no matter how small the grains. Solubility is determined by molecular interactions, not particle size.
For Class 6 board exams, the simple answer: “Sugar has –OH groups that attract water molecules; sand has strong covalent bonds that water cannot break.” For Class 9–10, add the concept of hydrogen bonding and “like dissolves like.”