Solutions

Lesson #1VocabularyDissolving

What is a Solution?

• It is a homogeneous mixture built from two or more components.

• The components may be elements and/or compounds and nature controls many of the factors associated with the solution itself.

Starting with Vocabulary

• Solute – this is the component that is dissolved in something else.

• Solvent – this is the component in which the solute is dissolved.

• An example: In a “salt-water” solution, the salt is the solute and the water is the solvent.

Solute and Solvent

Notes on Components

• It is possible to find examples of solutions that have components in any combination of the three phases of matter (solid, liquid, gas).

• The most common solutions have a solid dissolved in a liquid (like salt-water).

• Solutions in which the solute and the solvent are metals are prepared by melting the components first, then mixing, then allowing the mixture to solidify. The resulting solution is called an alloy.

• An example of a solution in the gas phase is simply “air”.

More Vocabulary

• Solubility:– This is a complicated

term because it is used in several contexts in the discussions of solutions.

– We will start with solubility describing whether (or not) a given solute will dissolve in a specific solvent.

• We learned in the structure and properties unit that solubility can be predicted using the general rule “likes dissolve likes”.– Polar dissolves in polar– Non-polar dissolves in

non-polar

Terms Connected to Solubility

• Soluble – this term is used to describe a solute that will dissolve in a specific solvent.

• Example – we know from experience that salt will dissolve in water. We can say that “salt is soluble in water”.

• Insoluble – this term will be used when describing a solute that will not dissolve in a specific solvent.

• Example – again, experience tells us that oil will not dissolve in water. We can say that “oil is insoluble in water”.

Another Take on Solubility

• Solubility can also be defined as the maximum amount of a given solute that can be dissolved in a specific amount of a solvent.

• Think about dissolving sugar in a beaker of water.

• From experience, you know that sugar dissolves easily in water.

• But, if you keep adding sugar, you eventually reach a point where the sugar will not dissolve, no matter how much you stir.

• You have reached the solubility (or “solubility limit”) of sugar in water.

Related Terms to this Definition of Solubility

• Dilute: a solution in which the amount of dissolved solute is much lower than the amount that would be the solubility limit.

• Concentrated: a solution in which the amount of dissolved solute is close to the amount that would be the solubility limit.

Saturation• A solution is said to be saturated when the

amount of dissolved solute is equal to the amount that is defined by the solubility-limit.

Lesson #1

Part IIThe Dissolving Process

Dissolving

• When a solute dissolves (we will assume in water), it’s solid structure is broken down into the representative particles that made up that solid solute.

• If the solute was an ionic compound, the representative particles are the ions from which the solute was built (think SOCCR).

• If the solute was a covalent compound, the representative particles are complete molecules.

The Two Steps of Dissolving• In the 1st step, the water

molecules (remember, water is the solvent in these examples) simply attach themselves to the crystal structure of the solute.

• This step is called solvation or more specifically “hydration” when the solvent is water.

• In the 2nd step of the process, the attached water molecules pull the solute mass apart into the representative particles described in the previous slide.

• This step is called “dissociation”.

Dissolving a Covalent Solute

Since sugar is a covalent (also called “molecular”) compound, when it dissolves, it is dissociated into sugar molecules.

Dissolving an Ionic Compound

Since salt (sodium chloride) is an ionic compound, when it dissociates, we get sodium ions and chloride ions – the ions from

which salt is originally built.

Ionic Dissociations

• So, sodium chloride will dissociate into sodium ions and chloride ions.

• You should also be able to think that potassium bromide will dissociate into potassium ions and bromide ions.

• And… that aluminum sulfate will dissociate into aluminum ions and sulfate ions.

• An ionic compound will dissociate into the ions from which it was built – think of the “SOCCR” process for writing formulas.

Dissociation Equations

• We need to be able to write chemical equations to illustrate dissociations.

• Keep in mind what occurs as you look through the following examples.– Since the solute is a solid, you will write its chemical

formula and indicate it as being a solid.– But, the “representative particles” are now dissolved in

water, so we have to indicate their charges (if they are ions) and show them as aqueous (dissolved in water).

Examples of Dissociation Equations

1. Sodium chloride dissolves in water.NaCl (s) Na+1 (aq) + Cl-1 (aq)

2. Potassium bromide dissolves in water.KBr (s) K+1 (aq) + Br-1 (aq)

May Need to Balance

1. Magnesium nitrate dissolves in waterMg(NO3)2 (s) Mg+2 (aq) + 2 (NO3)-1 (aq)

2. Sodium carbonate dissolves in waterNa2(CO3) (s) 2 Na+1 (aq) + (CO3)-2 (aq)

3. Aluminum sulfate dissolves in waterAl2(SO4)3 (s) 2 Al+3 (aq) + 3 (SO4)-2 (aq)

Have to Watch for Covalent

1. Sugar (as C6H12O6) dissolves in waterC6H12O6 (s) C6H12O6 (aq)

Remember that a covalent compound is “molecular”. It’s representative particles are complete molecules. Basically all that happens is that the solid changes to aqueous.