Purity and separating mixtures

Welcome to the World of Purity!

In this chapter, we are going to explore what "pure" actually means in science (hint: it’s different from what you see on a bottle of orange juice!) and how chemists use clever tricks to pull mixtures apart. Whether it's making clean drinking water or creating life-saving medicines, separating mixtures is one of the most important jobs a chemist has. Don't worry if some of the math or long words seem tricky at first—we'll break them down step-by-step!

1. What is "Pure"?

In everyday life, we might say milk is "pure," but a chemist would disagree! To a scientist, a pure substance contains only one type of element or one type of compound. If there is anything else mixed in, it is impure.

Scientific vs. Everyday Use

• Everyday "Pure": Usually means nothing has been added to it, like "pure honey."
• Scientific "Pure": Means it is 100% one substance. Honey is actually a mixture of different sugars and water, so it’s not scientifically pure!

Testing for Purity

How do we know if something is pure? We look at its melting point.
• Pure substances have a "sharp" melting point. For example, pure ice melts exactly at \(0^\circ C\).
• Impure substances (mixtures) will melt over a range of temperatures and usually at a lower temperature than the pure version.

Quick Review: If a sample of sulfur melts slowly between \(110^\circ C\) and \(115^\circ C\), it is impure. If it melts exactly at \(113^\circ C\), it is pure.

Key Takeaway: Purity in chemistry means "only one thing present." We use melting point data to prove it.

2. Useful Mixtures: Formulations and Alloys

Not all mixtures are "dirty" or unwanted. Many things we use are formulations—mixtures that have been specially designed to have useful properties.

• Formulations: Medicines, paints, and fuels are mixtures where every ingredient is measured exactly to do a specific job.
• Alloys: These are mixtures of metals with other elements. For example, steel is a mixture of iron and carbon. Alloys are often much stronger than pure metals.

Did you know? Pure gold is actually very soft! Most gold jewelry is an alloy mixed with silver or copper to make it tough enough to wear.

3. The Math of Chemistry: Relative Masses

To understand mixtures, we need to know how much atoms weigh. We use the Relative Atomic Mass (\(A_r\)) from the Periodic Table.

Relative Formula Mass (\(M_r\))

The \(M_r\) is just the total mass of all the atoms in a chemical formula added together.
Example: Find the \(M_r\) of water (\(H_2O\)).
1. Look up the masses: \(H = 1\), \(O = 16\).
2. Count the atoms: There are two \(H\) atoms and one \(O\) atom.
3. Add them up: \(1 + 1 + 16 = 18\).
So, the \(M_r\) of \(H_2O\) is 18.

Common Mistake: When calculating mass in a balanced equation, like \(2H_2\), only calculate the \(M_r\) for the molecule (\(H_2 = 2\)). The big number "2" at the front tells you how many molecules there are, but it doesn't change the \(M_r\) of the substance itself!

Key Takeaway: \(M_r\) is just the "sum of the parts." Add the atomic masses together to get the total.

4. Empirical Formula

The empirical formula is the simplest whole-number ratio of atoms in a compound.
Example: A molecule of glucose is \(C_6H_{12}O_6\). Its empirical formula is \(CH_2O\) because we can divide all the numbers by 6.

How to calculate it (The "Moles" Method):

If you are given masses in a question, follow these steps:
1. Divide the mass of each element by its \(A_r\) (this gives you the number of moles).
2. Divide all the answers by the smallest number you just found.
3. Round to the nearest whole number to get your ratio.

Memory Aid: Mass over Atomic mass, then Smallest Share (MASS!).

5. Separation Techniques

When we have a mixture, we use different physical properties (like size or boiling point) to pull the substances apart. Here are the big four you need to know:

Filtration

Used to separate an insoluble solid from a liquid.
Example: Separating sand from water.
The liquid passes through the holes in the paper (the filtrate), but the solid is too big and gets stuck (the residue).

Crystallisation

Used to separate a soluble solid from a liquid.
Example: Getting salt crystals from salty water.
We gently heat the solution to evaporate the water. As the solution gets "crowded," the solid starts to form crystals.

Simple Distillation

Used to separate a liquid from a solution (when we want to keep the liquid).
Example: Getting pure water from sea water.
1. Evaporation: Heat the mixture until the liquid turns to gas.
2. Condensation: The gas travels through a cooled tube (condenser) and turns back into a liquid in a separate beaker.

Fractional Distillation

Used to separate a mixture of different liquids that have different boiling points.
Example: Separating the different parts of crude oil.
The mixture is heated. The liquid with the lowest boiling point evaporates first and is collected at the top of a tall column.

Key Takeaway: Choose your method based on what you are trying to separate! Solid from liquid? Filtration. Liquid from liquid? Distillation.

6. Chromatography

Chromatography is a brilliant way to separate mixtures of colored substances, like inks or food dyes. It can also help us tell if a substance is pure.

How it works

There are always two "phases":
• Stationary Phase: This part doesn't move (like the paper).
• Mobile Phase: This is the solvent (like water or ethanol) that moves up the paper, carrying the substances with it.

Analogy: Imagine walking through a busy shopping mall. If you love shopping, you’ll stop at every store and move slowly (like a chemical attracted to the stationary phase). If you hate shopping, you’ll race through to the exit (like a chemical attracted to the mobile phase).

Interpreting a Chromatogram

• Pure substances show up as one single spot.
• Impure substances (mixtures) separate into multiple spots.

Calculating \(R_f\) Values

The \(R_f\) value is a ratio that helps us identify a substance. It is always a number between 0 and 1. Use this formula:
\(R_f = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent}}\)

Quick Review: If the solvent moved 10cm and the ink spot moved 5cm, the \(R_f\) is \(5 / 10 = 0.5\).

Common Mistake: Always measure from the baseline (where the spots started), not from the bottom of the paper!

Key Takeaway: Chromatography separates based on how much a substance "likes" the mobile phase versus the stationary phase.

Summary Checklist

• Can you explain why a pure substance has a "sharp" melting point?
• Do you know how to calculate \(M_r\) and Empirical Formulas?
• Can you pick the best separation technique for a given mixture?
• Do you remember that the mobile phase moves and the stationary phase stays?

Final Tip: Don't let the big words scare you. Chemistry is just a giant puzzle, and these techniques are the tools you use to solve it. You've got this!