Solubility

What "dissolving" means in particle terms. When sugar or salt is added to water and dissolves, the attraction holding the solid's own particles together is weak enough for water's particles to pull them apart. The separated particles do not float as a lump — they spread into the interparticle spaces that already exist between the water's particles, the way sand grains slip into the gaps between larger pebbles without visibly raising the pile.

Why the volume comes out short. Because the dissolved particles occupy gaps that were already there, rather than carving out brand-new space, the total volume of the solution is less than the sum of the solid's volume and the water's volume measured separately. This "missing" volume is not lost matter — every particle is still present — it is simply evidence that particles are not packed edge to edge, so empty space existed to fill.

Why insoluble solids behave the opposite way. Sand and chalk powder do not dissolve: the interparticle attraction gluing their particles together is too strong for water's particles to break apart. The solid stays intact as grains, settles at the bottom, and — since it cannot spread into existing gaps — adds its volume roughly on top of the water's volume instead of replacing empty space within it. The measured volume therefore rises by close to the volume added, unlike the shrinkage seen with a true solute.

What this chapter leaves out. There is no numerical formula here — dissolving is treated qualitatively, as a volume-based experiment supporting the existence of interparticle space. How much of a solid can dissolve, saturation, concentration, and the effect of temperature on dissolving belong to a later chapter and are not tested alongside this topic.

There is no formula to derive here — this chapter's treatment of dissolving is a reasoning chain from particle properties to a measurable outcome, and this section walks that chain in full, from what particles are made of, through the two possible outcomes of adding a solid to water, to what a measuring cylinder should show in each case, so the volume evidence can be read correctly rather than treated as a coincidence.

Step 1. Start from two particle properties established earlier in the chapter: particles of matter have interparticle space between them, and particles are held together by interparticle attraction, whose strength differs from substance to substance. Both are needed to explain dissolving — space alone does not tell you whether a solid will dissolve, and attraction alone does not tell you what happens to the volume.

Step 2. Ask what "dissolving" requires at the particle level. For a solid to dissolve, water's particles must pull the solid's own particles apart from each other. This is possible only when the interparticle attraction inside the solid is weak enough to be overcome — true for sugar and salt, but not for sand or chalk powder.

Step 3. Follow a soluble solid through this process. Once pulled apart, its particles do not stay as a cluster; they disperse into the interparticle spaces already present between the water's particles, because those spaces are simply empty room the solid's particles can occupy. Since existing space is being filled rather than new space created, the combined volume comes out less than the sum of the two separate volumes.

Step 4. Follow an insoluble solid through the same process. Here the interparticle attraction inside the solid is too strong to be broken by water's particles, so its particles remain locked together as tiny grains. They cannot enter the water's interparticle spaces, so they settle at the bottom, occupying volume of their own that sits in addition to the water's volume, not instead of empty gaps within it.

Step 5. Read off the testable prediction. Total volume measured before and after should show a decrease relative to the simple sum whenever a solid dissolves, and an increase roughly equal to the solid's own volume whenever it does not — exactly the pattern used to argue, from a measuring cylinder alone, that interparticle space genuinely exists.

Question: A student measures exactly $100\ \text{mL}$ of water in a measuring cylinder, then adds a level teaspoon of salt that, if it kept its own separate volume, would add about $6\ \text{mL}$. After stirring until it fully dissolves, the reading is about $104\ \text{mL}$, not the expected $106\ \text{mL}$. The student then repeats the experiment with $6\ \text{mL}$ of fine chalk powder instead, which does not dissolve. Explain both readings using interparticle space and interparticle attraction, and predict the chalk reading.

Step 1. For the salt: before mixing, the water already has interparticle spaces between its particles, and the salt exists as a separate solid with its own particles held together.

Step 2. Salt's interparticle attraction is weak enough for water's particles to pull its particles apart, so once stirred, the salt's particles disperse and spread into the water's already-existing interparticle spaces rather than sitting as a separate lump.

Step 3. Because these particles occupy gaps that were already there, the final volume is less than the expected sum of $100\ \text{mL} + 6\ \text{mL} = 106\ \text{mL}$. The measured $104\ \text{mL}$, about $2\ \text{mL}$ short, is exactly this evidence: salt particles have filled existing spaces instead of adding entirely new volume.

Step 4. For the chalk powder: its interparticle attraction is too strong for water's particles to break its particles apart, so it cannot disperse into the water's interparticle spaces. It settles at the bottom as intact solid grains, contributing its own volume on top of the water's volume rather than replacing empty space within it.

Step 5. Predicting the chalk reading: since none of its volume is absorbed into existing gaps, the total should come out close to the full expected sum. Answer: the salt reading of 104 mL (2 mL less than 106 mL) shows dissolved particles filling existing interparticle space, while the chalk powder should read close to 106 mL, since undissolved chalk grains simply add their own volume on top of the water.

• Thinking dissolved particles "disappear" — they are still present, only spread among the water's particles.
• Assuming every solid added to water gives a volume decrease — only true solutes show this; insoluble solids increase the total volume.
• Explaining insolubility by "too heavy" or "too big" rather than interparticle attraction being too strong to overcome.
• Mixing this volume evidence up with saturation, concentration, or how much solute a liquid can hold — a separate chapter's territory.

Ask the tutor: want to see exactly why salt "shrinks" the total volume but chalk powder doesn't — ask your AI tutor to walk through both measuring-cylinder experiments side by side.