Solutions (HSC SSCE Chemistry): Revision Notes

Nature, Concentration, and Volume of Solutions

Chemistry happens in solutions everywhere around us. The chemical reactions in our bodies, in plants and animals, and throughout our environment mostly occur in solutions. Many household products we use daily are solutions, and industrial chemistry relies heavily on solutions for manufacturing processes like producing caustic soda, extracting sugar, and purifying copper.

Understanding solutions means being able to measure quantities of substances in terms of volumes rather than just masses. This allows chemists to work with liquids more practically in laboratories and industry.

Nature of solutions

What is a solution?

A solution is a homogeneous mixture where one substance (the solute) dissolves completely in another substance (the solvent).

Solutions appear uniform throughout because the solute particles are evenly distributed at the molecular or ionic level. This uniformity distinguishes solutions from suspensions, where particles are larger and will eventually settle out.

How molecular substances dissolve

When molecular substances like sugar (sucrose) or urea dissolve in water, an interesting process occurs:

1. The solid crystal structure breaks apart
2. Individual molecules separate from each other
3. These molecules spread uniformly throughout the water
4. The molecules remain dispersed and don't clump back together

This happens because the interactions between solute molecules and water molecules are stronger than the forces holding the solute molecules together in the crystal. For example, when you dissolve sugar in tea, the sugar molecules don't just float around as tiny crystals - they completely separate into individual molecules that mix uniformly with the water molecules.

Similarly, when iodine dissolves in hexane, the iodine molecules disperse as individual molecules throughout the hexane solvent. Because the particles are so small (molecular level), they don't settle out. Solutions remain stable indefinitely unless you evaporate the solvent or cool the solution significantly.

How ionic substances dissolve

Ionic substances dissolve differently from molecular substances. When ionic compounds like sodium chloride (NaCl), baking soda (NaHCO₃), or Epsom salts (MgSO₄) dissolve in water:

1. The ionic crystal structure breaks apart
2. The compound separates into individual positive and negative ions
3. These ions move freely and independently through the water
4. The ions remain separated and don't recombine

For example, when sodium chloride dissolves in water, you get sodium ions ($\text{Na}^+$) and chloride ions ($\text{Cl}^-$) moving independently throughout the solution. The ions don't exist as paired NaCl units anymore - they're completely separate.

Different measures of concentration

Understanding concentration

Concentration tells us how much solute is present in a specific amount of solvent or solution. Think of it like the strength of cordial - more cordial in the same amount of water means higher concentration (stronger taste).

There are many ways to express concentration, each useful for different situations. We can group them into volume-based and mass-based measures.

Volume-based concentration measures

These measures express concentration using volumes, which is practical for making solutions in the laboratory:

• Mass per volume:
  • $\text{g}/100\text{ mL}$ or $\text{g L}^{-1}$ of solution
  • Example: $5\text{ g}/100\text{ mL}$ means $5\text{ g}$ of solute in every $100\text{ mL}$ of solution
• Volume per volume (for liquid solutes):
  • $\text{mL}/100\text{ mL}$ or $\text{mL L}^{-1}$ of solution
  • Example: $10\text{ mL}/100\text{ mL}$ means $10\text{ mL}$ of liquid solute in every $100\text{ mL}$ of solution
• Percent weight per volume ($\%(\text{w}/\text{v})$):
  • Mass of solute per $100\text{ mL}$ of solution
  • Example: $2.5\%(\text{w}/\text{v})$ means $2.5\text{ g}$ of solute in $100\text{ mL}$ of solution
  • Convenient for solid solutes like sodium chloride
• Percent volume per volume ($\%(\text{v}/\text{v})$):
  • Volume of solute per $100\text{ mL}$ of solution
  • Example: $14\%(\text{v}/\text{v})$ means $14\text{ mL}$ of solute in $100\text{ mL}$ of solution
  • Convenient for liquid solutes like ethanol

Mass-based concentration measures

These measures express concentration using masses:

• Percent by weight ($\%(\text{w}/\text{w})$):
  • Mass of solute per $100\text{ g}$ of solution
  • Example: $5\%(\text{w}/\text{w})$ means $5\text{ g}$ of solute in $100\text{ g}$ of solution
  • When "%" is used alone, it usually means $\%(\text{w}/\text{w})$
• Parts per million (ppm):
  • Grams of solute per million grams of solution
  • For gases, it means molecules per million molecules
  • Example: $50\text{ ppm}$ means $50\text{ g}$ of solute in $1\,000\,000\text{ g}$ (or $1000\text{ kg}$) of solution
  • Useful for very dilute solutions

Dilute versus concentrated solutions

We use descriptive terms to indicate the general concentration level:

• Dilute solution: Contains relatively low concentration of solute
  • Typically less than $10-20\text{ g}/100\text{ mL}$ or less than $10-20\%(\text{w}/\text{w})$
  • Example: Weak cordial, dilute acid solutions
• Concentrated solution: Contains relatively high concentration of solute
  • Typically greater than $50\text{ g}/100\text{ mL}$
  • Example: Strong cordial, concentrated acid solutions
• Moderately dilute/concentrated: Used for intermediate concentrations

Why use different concentration measures?

Each concentration measure has practical advantages for specific situations:

Liquid solutes: Volume per unit volume ($\%(\text{v}/\text{v})$) is preferred because:

• We naturally think of liquids in terms of volume rather than mass
• It's more intuitive (e.g., alcohol content in wine)

Environmental contexts: Parts per million (ppm) is most useful because:

• Environmental concentrations are usually very low
• Using $\%$ or $\text{g L}^{-1}$ would give very small, awkward numbers
• For example, 1.5 ppm is more convenient than 0.000 15% or 0.0015 g L⁻¹

Worked examples with concentration calculations

Measuring volumes of solutions

Laboratory glassware for volume measurement

Different types of laboratory equipment provide different levels of accuracy. Choosing the right equipment depends on how precise your measurement needs to be.

Measuring cylinders:

• Used for approximate volume measurements
• Accuracy: ±5% (relatively low precision)
• Common sizes: $10\text{ mL}$, $100\text{ mL}$, $250\text{ mL}$, $500\text{ mL}$
• Best for: Quick measurements where exact volume isn't critical

Pipettes:

• Deliver fixed, accurate volumes
• Accuracy: ±0.2% to ±0.5% (high precision)
• Common sizes: $5\text{ mL}$, $10\text{ mL}$, $25\text{ mL}$ (also $1\text{ mL}$, $2\text{ mL}$, $50\text{ mL}$ available)
• Best for: Transferring precise volumes from one container to another

Burettes:

• Deliver variable accurate volumes
• Accuracy: ±0.2% to ±0.5% (high precision)
• Typical range: $0$ to $50\text{ mL}$
• Best for: Adding precise volumes gradually, especially in titrations

Volumetric flasks:

• Contain fixed, accurate volumes when filled to the mark
• Accuracy: ±0.2% to ±0.5% (high precision)
• Common sizes: $100\text{ mL}$, $250\text{ mL}$, $500\text{ mL}$, $1\text{ L}$ (also $50\text{ mL}$ and $2\text{ L}$ available)
• Best for: Making solutions of known concentration

Reading the meniscus

The meniscus is the curved surface of a liquid in a container. When reading volumes in glassware, always read from the bottom of the meniscus at eye level.

Making accurate solutions

To prepare an aqueous solution with accurately known concentration (better than $\pm 1\%$):

Step 1: Measure the required amount of solute

• For solid solutes: Use a sensitive balance to weigh accurately
• For liquid solutes: Use a pipette to measure volume accurately

Step 2: Transfer to volumetric flask

• Add the solute to an appropriately sized volumetric flask
• Ensure all solute is transferred (rinse any container used)

Step 3: Dissolve in water

• Add water to the flask (about half full)
• Swirl to dissolve the solute completely
• Let the solution cool if needed (dissolution can generate heat)

Step 4: Fill to the mark

• Carefully add more water until the meniscus bottom sits on the graduation mark
• Use a wash bottle for final drops to control addition
• Mix thoroughly by inverting the flask several times

Accuracy and significant figures

The type of glassware you use indicates the accuracy of your measurement:

• Using a pipette: Volume like $25\text{ mL}$ implies $25.0\text{ mL}$ (3 significant figures)
• Using a burette: Volume like $18\text{ mL}$ implies $18.0\text{ mL}$ (3 significant figures)
• Using a volumetric flask: Volume like $500\text{ mL}$ implies $500.0\text{ mL}$ (4 significant figures)
• Using a measuring cylinder: Volume like $50\text{ mL}$ might only be $50\text{ mL}$ (2 significant figures)