Purity and separating mixtures

Welcome to Purity and Separating Mixtures!

In this chapter, we are going to explore what "pure" really means in a lab, how we can calculate the "fingerprint" of a chemical using its mass, and the clever tricks scientists use to pull mixtures apart. Whether it’s making sure your drinking water is safe or helping a forensic scientist identify an unknown ink, these techniques are used every single day. Don’t worry if this seems like a lot of information at first—we’ll take it one step at a time!

1. What is "Pure"? (Scientific vs. Everyday)

In the supermarket, a bottle of orange juice might say it’s "100% pure." To a chemist, that’s actually very impure because it contains water, sugar, citric acid, and vitamins!

Scientific Purity

In Chemistry, a pure substance is made of only one type of element or compound. If there is anything else mixed in, even a tiny amount, it is considered impure.

Testing for Purity: The Melting Point Test

How do we know if a white powder is pure salt or salt mixed with sugar? We check its melting point.

• Pure substances have a sharp, specific melting point. For example, pure ice melts exactly at \( 0^\circ C \).
• Impure substances (mixtures) will:
  1. Melt over a range of temperatures rather than one specific point.
  2. Usually melt at a lower temperature than the pure substance.

Formulations and Alloys

Sometimes, we want to mix things on purpose to make them more useful. These are called formulations.
Example: An alloy is a mixture of a metal with other elements. Pure gold is very soft, so we mix it with silver or copper to make it harder for jewelry. This "formulation" is more useful than the pure metal!

Quick Review:
- Pure: Only one substance.
- Impure: Melts over a range and at a lower temperature.
- Formulation: A mixture designed for a specific use.

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

2. The "Mass" of Chemicals

To understand mixtures, we need to know the mass of the bits inside them. We use two main values:

Relative Formula Mass \( (M_r) \)

This is the total mass of a compound. To find it, you just add up the Relative Atomic Masses \( (A_r) \) of all the atoms in the formula (you get these numbers from the Periodic Table).

How to calculate \( M_r \):
Example: Find the \( M_r \) of water \( (H_2O) \).
1. Look up the mass of Hydrogen \( (H = 1) \) and Oxygen \( (O = 16) \).
2. There are 2 Hydrogens: \( 2 \times 1 = 2 \).
3. There is 1 Oxygen: \( 1 \times 16 = 16 \).
4. Add them together: \( 2 + 16 = 18 \). So, the \( M_r \) of \( H_2O = 18 \).

Empirical Formula

The empirical formula is the simplest whole-number ratio of atoms in a compound.

Memory Aid: "Percent to Mass, Mass to Mole, Divide by Small, Multiply 'til Whole!"
1. Write down the mass (or %) of each element.
2. Divide by the Relative Atomic Mass \( (A_r) \) to find the moles.
3. Divide both numbers by the smallest answer you just got.
4. This gives you the ratio for the formula.

Key Takeaway: \( M_r \) is the total mass; Empirical Formula is the simplest ratio.

3. Separating Insoluble Mixtures

If you have a solid that does not dissolve (like sand in water), we call it insoluble. We use Filtration to separate it.

Filtration Step-by-Step:

1. Put filter paper into a funnel.
2. Pour the mixture through.
3. The liquid that passes through is the filtrate.
4. The solid left behind is the residue.

Did you know? Your kidneys are giant biological filters! They filter your blood to remove waste while keeping the "good stuff" in.

4. Separating Soluble Mixtures

If a solid does dissolve (like salt in water), it is soluble. We use Crystallisation or Evaporation.

Crystallisation:

1. Heat the solution in an evaporating dish to remove some water.
2. Once crystals start to form, stop heating.
3. Leave it to cool. As it cools, the solid becomes less soluble and forms large, pure crystals.
4. Filter the crystals out and pat them dry.

Key Takeaway: Use filtration for sand-like solids; use crystallisation for salt-like solids.

5. Distillation: Catching the Liquid

Sometimes, we don't want to just evaporate the liquid into the air; we want to keep the liquid.

Simple Distillation

Used to separate a liquid from a solid (e.g., getting pure water from sea water).

• Heat the mixture. The liquid evaporates into a gas.
• The gas travels into a condenser (a tube cooled by cold water).
• The gas cools and turns back into a liquid, which is collected in a separate beaker.

Fractional Distillation

Used to separate two or more liquids that are mixed together (like alcohol and water, or crude oil).
It works because different liquids have different boiling points.

Common Mistake: Students often forget the fractionating column. This is a tall column filled with glass beads. It ensures that only the liquid with the lowest boiling point reaches the top and evaporates into the condenser first.

Key Takeaway: Simple distillation = 1 liquid + 1 solid. Fractional = 2+ liquids.

6. Chromatography: The Color Detective

Chromatography is used to separate mixtures of soluble substances, like dyes in ink or food colorings.

How it Works

Every chromatography experiment has two "phases":
1. Stationary Phase: The thing that doesn't move (e.g., the paper).
2. Mobile Phase: The thing that moves (the solvent, like water or ethanol).

The chemicals in the ink move up the paper. If a chemical is more attracted to the mobile phase, it moves further. If it’s more attracted to the paper, it stays lower down.

Paper vs. Thin Layer Chromatography (TLC)

• Paper Chromatography: Uses specialized paper.
• TLC: Uses a glass or plastic plate coated with silica or alumina. It’s faster and more sensitive than paper.

Calculating \( R_f \) Values

An \( R_f \) value is a ratio that helps us identify a substance. It is always a number between 0 and 1.

Formula: \( R_f = \frac{\text{Distance moved by substance}}{\text{Distance moved by solvent}} \)

Gas Chromatography (GC)

Used for very complex mixtures. The mobile phase is an inert gas (like Helium) and the stationary phase is a solid or thick liquid inside a long tube. It is extremely accurate!

Important Practical Tips:
- Always draw the baseline in pencil (ink would dissolve and ruin the experiment!).
- Make sure the solvent level is below the spots of ink.

Quick Review Box:
- 1 spot on the paper = Pure substance.
- Multiple spots = Mixture.
- Higher spot = More soluble in the solvent.

Key Takeaway: Chromatography separates based on how substances distribute themselves between a moving phase and a still phase.

Summary: Which Method Should I Use?

Don't worry if you get confused—just ask yourself: "What am I trying to get out?"

• Insoluble solid from a liquid? Filtration.
• Soluble solid from a liquid? Crystallisation.
• Liquid from a solid (keeping the liquid)? Simple Distillation.
• Two liquids mixed together? Fractional Distillation.
• A mixture of colored dyes? Chromatography.

You’ve got this! Keep practicing those \( M_r \) calculations and you'll be a pro in no time!