Chemistry | Elements, compounds and mixtures

👋 Welcome to Elements, Compounds, and Mixtures!

Hello future chemist! This chapter is absolutely fundamental. It’s like learning the alphabet before writing a novel—you need to know the basic building blocks of all matter around you.
In this unit, we will learn how to categorise everything, from the air we breathe to the devices we use, into three basic groups: elements, compounds, and mixtures. Understanding these differences will unlock the rest of your chemistry course!

🧱 Section 1: Elements – The Simplest Building Blocks

What is an Element?

An element is the simplest pure substance that cannot be broken down into anything simpler by chemical means. Think of elements as the primary colours of the universe.

• Every element is made up of only one type of atom.
• We organise all known elements in the Periodic Table (like Hydrogen, Oxygen, Gold, Carbon).
• Elements can exist as single atoms (like Helium, He) or as molecules of the same type of atom bonded together (like Oxygen gas, O₂).

🔑 Key Term: The Atom

The atom is the smallest unit of an element that retains the chemical properties of that element. If you have a lump of gold, every single atom in that lump is a Gold atom.

Analogy: Imagine a huge box of Lego. If you only pull out the small, red, square pieces, and there are no other colours or shapes, then your collection of red pieces represents a single element.

🧪 Section 2: Compounds – Chemically Joined Together

How are Compounds Formed?

A compound is a pure substance formed when two or more different elements are chemically joined together.
This joining process is called a chemical reaction, and it involves forming chemical bonds.

Key Characteristics of a Compound:

• Fixed Ratio: The elements are always present in the same, fixed proportions. For example, water (H₂O) always has exactly two hydrogen atoms for every one oxygen atom.
• New Properties: When elements form a compound, they lose their original properties. The new compound has entirely different properties.

  Example: Hydrogen (H₂) is a flammable gas, and Oxygen (O₂) is a gas that supports combustion. When they combine to form water (H₂O), the result is a liquid that puts out fire!

• Difficult Separation: Compounds can only be separated back into their elements using difficult chemical methods (like heating strongly or passing an electric current through them – electrolysis).

Analogy: If the red and blue Lego bricks stick together with powerful glue in a fixed pattern (always two reds attached to one blue), they are now a compound. They have become a new object (a 'Lego molecule') and are very hard to pull apart.

⚠️ Common Mistake Alert!

Do not confuse an element molecule (like O₂) with a compound molecule (like H₂O). Both are molecules, but O₂ is still just one type of atom (Oxygen), so it is an element.

⚗️ Section 3: Mixtures – Simply Stirred Together

What is a Mixture?

A mixture is formed when two or more substances (elements, compounds, or both) are combined physically, but not chemically joined.

Key Characteristics of a Mixture:

• Variable Ratio: The components can be mixed in any proportion. You can have slightly sweet tea or very sweet tea.
• Retained Properties: The components keep their individual properties.

  Example: Salt water is a mixture. The salt still tastes salty, and the water is still wet.

• Easy Separation: Mixtures can be separated using relatively simple physical methods (no chemical reactions needed). This is a crucial distinction!

Analogy: A fruit salad is the perfect mixture. You have apples, grapes, and strawberries all together, but they haven't reacted. The apple still tastes like apple, and you can easily pick out the grapes if you wanted to.

💡 Memory Trick: C & M

Compounds = Chemically joined (fixed)
Mixtures = Merely mixed (variable)

📊 Section 4: Comparing Elements, Compounds, and Mixtures

To make sure you've got this, here is a breakdown of the differences you must know:

⚗️ Section 5: Separation Techniques for Mixtures

Since mixtures retain their individual properties, we can separate them by exploiting these differences (like boiling point, particle size, or solubility).

1. Filtration

This technique separates an insoluble solid from a liquid.

Example: Separating sand (solid) from water (liquid).

Process:

1. Pour the mixture through filter paper in a funnel.
2. The solid (the residue) is too large to pass through the tiny pores and stays behind on the filter paper.
3. The liquid (the filtrate) passes through the filter paper into the flask.

2. Crystallisation

This technique separates a soluble solid (the solute) from a solvent (the liquid).

Example: Obtaining pure salt crystals from salt water.

Process:

1. Heat the solution gently to evaporate most of the solvent, creating a saturated solution (a solution holding the maximum amount of solute).
2. Stop heating and allow the concentrated solution to cool down slowly.
3. As it cools, the solubility decreases, and pure solid crystals will form.
4. Filter the crystals and dry them (this is much safer than evaporating all the liquid, which might decompose the solid).

3. Distillation (Simple Distillation)

Simple distillation is used to separate a solvent from a solution, or to separate liquids with very different boiling points. The goal is often to obtain the pure liquid (the solvent).

Example: Getting pure water from salt water.

Process:

1. Heat the mixture in a flask. The liquid with the lower boiling point (usually the solvent, e.g., water) evaporates first, turning into a gas (vapour).
2. The vapour travels into a condenser, which is kept cool by running cold water around it.
3. The cooling causes the vapour to turn back into a liquid (condensation).
4. This pure liquid (the distillate) is collected in a separate beaker. The solid solute remains in the original flask.

4. Fractional Distillation

Fractional distillation is used to separate miscible liquids (liquids that mix fully) that have close but different boiling points.

Example: Separating ethanol from water, or separating different products from crude oil.

What’s the difference? Simple distillation has one flask leading directly to the condenser. Fractional distillation adds a fractionating column between the flask and the condenser.

Process:

1. The mixture is heated. Vapours rise up the fractionating column.
2. A temperature gradient exists in the column (hotter at the bottom, cooler at the top).
3. The vapour of the substance with the lowest boiling point travels all the way to the top and into the condenser, where it collects as the first distillate.
4. Substances with higher boiling points condense further down the column and fall back into the flask, achieving multiple evaporation/condensation cycles to ensure better separation.

5. Chromatography

This technique is used to separate substances that are dissolved in a liquid or gas, often used for separating coloured dyes or checking for purity.

Principle: Separation occurs because different components travel at different speeds through a medium (like filter paper).

• Stationary Phase: The material that stays put (e.g., the chromatography paper).
• Mobile Phase: The solvent that moves up the paper (e.g., water or ethanol).

Components that are more soluble in the mobile phase (and less attracted to the stationary phase) travel further up the paper. Components that are less soluble travel shorter distances.

Understanding the \(R_f\) Value (Retention Factor)

The \(R_f\) value helps identify components separated by chromatography. It is calculated by:

\(R_f = \frac{\text{Distance travelled by substance}}{\text{Distance travelled by solvent front}}\)

Did you know? The \(R_f\) value for a specific substance is always the same under the same conditions, making it a powerful identification tool!

You’ve got this! Now try applying these methods to real-life examples!