Dissolving in the Particle Model

Introduction

What actually happens to a sugar crystal when it dissolves? You cannot see the individual particles, but the particle model lets you picture the process clearly. Dissolving is not magic and it is not melting: it is the story of solvent particles surrounding and pulling away solute particles, which then spread out among them. This picture explains why mass stays the same, why stirring helps and why hot tea dissolves sugar faster than iced tea.

Core explanation

A solid solute before dissolving. In a crystal of sugar or salt, the particles are held tightly in a regular arrangement by attractive forces. They vibrate but do not move from place to place.

Solvent particles attack the surface. Water particles are constantly moving and colliding with the surface of the crystal. Water particles are attracted to the solute particles. When these attractions are strong enough, particles at the surface are pulled away from the crystal and become surrounded by a shell of water particles.

The solute particles spread out. Once free, the solute particles move randomly among the solvent particles and gradually spread through the whole liquid by diffusion . Eventually they are evenly distributed and the solution is uniform.

Mass is conserved. Every solute particle is still present; none is destroyed. So the mass of the solution equals the mass of solute plus the mass of solvent.

Volume may not simply add up. Particles of one substance can fit into gaps between particles of the other. For example, mixing 50 cm³ of ethanol with 50 cm³ of water gives slightly less than 100 cm³ of solution (roughly 96–97 cm³). Mass is conserved; volume is not always additive.

Why some substances dissolve and others do not. Dissolving happens when the attraction between solvent and solute particles is strong enough to compete with the forces holding the solute together. Sand's particles are held too strongly and are not attracted enough to water, so sand does not dissolve.

Making dissolving faster. Three factors increase the rate of dissolving:

- Stirring carries dissolved particles away from the crystal surface and brings fresh solvent into contact with it. - Heating makes solvent particles move faster, so they hit the surface more often and with more energy. - Smaller pieces (for example, caster sugar instead of sugar cubes) have a larger total surface area, so more solute particles are exposed at once.

None of these changes what the substance is — they only change how fast it dissolves.

Step-by-step reasoning

Describing dissolving in particle terms:

1. Solvent particles collide with the solid's surface. 2. Attractions between solvent and solute particles pull surface particles away. 3. Freed solute particles are surrounded by solvent particles. 4. They spread out by random movement until the mixture is uniform.

Visual explanation

Draw a square block of closely packed red circles (solute) surrounded by blue circles (water). In the next frame, red circles at the corners and edges have come away, each ringed by blue circles. In the final frame, red circles are scattered evenly among the blue ones and the block has gone.

Real-world analogy

Think of a tightly packed group of people standing in the middle of a busy station hall. Travellers keep bumping into the edge of the group, and one by one people at the edge are drawn away into the moving crowd. Eventually the group has broken up completely and its members are scattered evenly through the hall.

Real-world example

A dishwasher tablet dissolves over several minutes during the wash. Manufacturers control its size, compression and coating to set the rate of dissolving, so some ingredients are released early in the cycle and others later, when hot water and spraying (stirring) speed things up.

Why?

Why do edges and corners of crystals dissolve first? Particles there have fewer neighbours holding them in place and are exposed to solvent on more sides, so they are pulled away more easily. That is why crystals become rounded as they dissolve.

Common misconception

"Dissolving is the same as melting." Melting is a single substance changing from solid to liquid when heated. Dissolving involves two substances, and the solute particles separate because of attractions to the solvent — sugar dissolves in cold water without being heated to its melting point.

Worked example

Question: A student dissolves 10 g of salt in 90 g of water. She predicts the solution will have a mass of 100 g and a volume of exactly 90 cm³ plus the volume of the salt. Comment on her predictions.

Reasoning: No particles are lost, so mass is conserved. However, solute particles can occupy gaps between water particles, so volumes do not always add exactly.

Answer: The mass prediction (100 g) is correct. The volume prediction is not reliable: the final volume is usually slightly less than the sum of the separate volumes.

Quick check

1. Why does stirring make sugar dissolve faster? Answer: It moves dissolved particles away from the crystal and brings fresh water to its surface.

Exam focus

Describe dissolving using particles: solvent particles surround and separate the solute particles, which then spread out. Always distinguish rate (how fast) from solubility (how much). Grinding, stirring and heating increase the rate; they do not change the substance.

Advanced insight

For ionic solids such as sodium chloride, water molecules cluster around each ion with their oppositely charged ends pointing towards it. This is called hydration , and the energy released when ions are hydrated helps pay for the energy needed to break up the crystal. Whether a solid dissolves depends on the balance of these energy changes.

Summary

In dissolving, solvent particles collide with a solid, pull surface particles away and surround them; the freed solute particles then spread evenly by random movement. Mass is conserved, but volumes may not add exactly. Stirring, heating and using smaller pieces increase the rate of dissolving but do not change what the substance is.

Practice questions

1. Explain, using particles, why powdered sugar dissolves faster than a sugar cube. Answer: The powder has a much larger surface area, so more sugar particles are in contact with water particles at once. 2. Explain why sugar dissolves faster in hot tea than in cold tea. Answer: In hot tea the water particles move faster, colliding with the sugar more often and with more energy, so particles are pulled away sooner. 3. 50 cm³ of ethanol and 50 cm³ of water are mixed. Predict whether the volume will be more than, equal to or less than 100 cm³, and explain. Answer: Slightly less than 100 cm³, because some particles fit into the gaps between particles of the other liquid. 4. State one difference between dissolving and melting. Answer: Melting involves one substance being heated to its melting point; dissolving involves a solute separating into a solvent without needing to reach its melting point.