 What happens when substances dissolve? In this video we'll consider solids dissolving in water When something dissolves in water it goes from the solid state where the molecules or ions are held in a regular lattice To the aqueous state where the molecules or ions are dispersed through the water Let's look at examples of a covalent molecular substance iodine and an ionic substance sodium chloride dissolving in water First let's consider them as solids At the lab level as you'd see them in the lab Idein crystals are shiny and gray almost, but not quite metallic in appearance Sodium chloride crystals are colorless transparent in fact, but when you have a lot of small crystals together They scatter light and so they appear white If we now zoom down to the molecular or ionic level a crystal of solid iodine is made of individual iodine molecules That's diatomic I2 molecules. That's these ones here Arranged regularly in a lattice and held together by weak attractions between the molecules Idein is a covalent molecular substance. So within each molecule the two iodine atoms are held together by a covalent bond Sodium chloride an ionic compound is composed of alternating sodium ions and chloride ions in a lattice The sodium ions are positive. They're the cations and the chloride ions are negative They're the anions and these oppositely charged ions attract each other And this means the lattice is strongly held together by ionic bonds between those alternating positive and negative ions Okay, now let's dissolve them in water At the lab level when iodine dissolves in water It produces a red-brown solution which used to be used to disinfect cuts or grazes and things like that also for purifying water Sodium chloride dissolved in water makes a colorless solution At the molecular ionic level again when a molecular compound like iodine dissolves in water The individual molecules separate from each other and spread out through the water Notice that I'm not showing the individual water molecules here. We're just assuming that they're there Now this is really important the covalent bonds within each molecule don't break when they dissolve each I2 Molecule remains unchanged rather it's the weak attractions which held the molecules together in a crystal that break and allow the molecules to move freely about At a symbolic level we could represent this process using an equation like this Where the subscripts here indicate the change in state from solid to aqueous notice that the formula of the Substance doesn't change. It's still I2 all that has changed is essentially the surroundings of each molecule Now just to remind you of some terminology here in this situation the iodine is the solute That's the solid being dissolved the water is the solvent That's the liquid into which the solute dissolves and the combination of those two is called the solution That's the mixture of the solute and the solvent The more iodine is dissolved in a certain volume of water the higher the concentration of the iodine solution will be Okay, now let's look at the sodium chloride dissolving on a molecular ionic level When this crystal dissolves the ionic bonds that hold the ions in the lattice break and The individual ions are able to move freely through the water As with the iodine we can represent this on a symbolic level using an ordinary chemical equation Like this However in this case the formula NACL no longer accurately represents the physical form of the aqueous Compound since it implies that the sodium ions and the chloride ions are still bonded together Instead we can see that when they're in the water when they're dissolved in the water the ions are now separate from each other So instead we can rewrite the equation and on the right hand side We're going to write the ions separately to show how they really physically are This is what's called an ionic equation It's a chemical equation that shows aqueous ions as separate from each other to represent their actual physical state more accurately Now why should ionic compounds dissolve into separate ions, but molecular compounds remain as molecules? Well to understand this we need to talk about intermolecular forces and that will come in another video So hold that thought and we'll dive into that later