TOPIC SIX: MIXTURES, ELEMENTS AND COMPOUNDS

Introduction to Mixtures, Elements, and Compounds

This topic introduces learners to the basic building blocks of matter: elements, compounds, and mixtures. These are fundamental concepts in chemistry that explain how substances combine and interact with each other. Understanding the differences between these three helps learners grasp how materials are formed and how they can be separated.

ELEMENTS

An element is a pure substance that cannot be split or divided into two or more simpler forms by any known chemical means. An element is made up of atoms.

Elements are the simplest forms of matter and are represented by chemical symbols. For example, hydrogen is represented by "H", and sodium is represented by "Na". The periodic table lists all known elements, each with unique properties.

Examples:

- Oxygen (O₂): A gas essential for breathing.
- Iron (Fe): A metal used in construction.
- Gold (Au): A metal used in jewelry.
- Carbon (C): A non-metal found in all living organisms.

Explanation:

COMPOUND
A compound is a substance made from two or more different types of elements chemically bonded together in a fixed ratio.

In compounds, elements combine chemically, forming substances with new properties that are different from the elements that make them up. For example, sodium (a reactive metal) and chlorine (a toxic gas) combine to form sodium chloride (salt), which is safe to consume. Compounds cannot be separated by physical means; they require chemical reactions to break them apart

Examples:
- Water (H₂O): A compound made of hydrogen and oxygen atoms.
- Carbon dioxide (CO₂): A gas made of carbon and oxygen atoms.
- Sodium chloride (NaCl): Common table salt made of sodium and chlorine.

Pure Substances and Mixtures

Mixtures

What is a mixture? What about a pure substance? How can you know that something is a mixture and not a pure substance?

Pure substances

Many substances around us are mixtures. Examples of mixtures are air, tap water, and milk. Can you name other mixtures? In everyday life, we often need to separate mixtures to obtain pure substances. This applies in cases where only pure substances are required

A pure substance is one that possesses identical components.

Properties of pure substances

1. Pure substances have the same composition that is, they are composed of only one type of particles. 2. Pure substances melt and boil at definite temperatures.

Study questions

1. You are provided with a cup of tea and a dish of sugar. Add a spoonful of sugar to the tea and do not stir.

(a) How does the tea taste?

(b) Why do you think it tastes that way?

2. Now stir the tea and taste it again.

(a) How does it taste? (b) Why do you think it tastes that way?

Expected response

Well-stirred tea tastes sweeter than the unstirred tea. Stiring tea helps to dissolve sugar evenly within the hot water. However, in the unstirred tea, sugar tends to settle at the bottom of the cup. This leaves the upper part of the tea without sugar.

Facts

Methods of separating mixtures

Separation of materials is important in our everyday life. This is because sometimes we can only get the desired results when using the materials in their non-combined states. Methods used to separate mixtures depend on the physical properties of the components. Such properties include solubility, density, boiling point and miscibility. Some of the methods of separating mixtures include:

• Manual sorting

• Filtration

• Decantation

• Simple and fractional distillation

• Paper chromatography

• Magnetic separation

• Centrifugation

• Crystallisation

• Evaporation

• Sublimation

(a) Filtration

Questions

1. Describe the contents left on the filter paper and those in on the conical flask?

2. What name do we give to the content on the filter paper and those in the conical flask?

Expected response…

Wet filter paper sticks on the funnel. When the mixture is added, it acts as a fine sieve allowing the liquid to pass through but retaining the solid particles.

The Facts

After filtration, the liquid which passes through the filter paper is called the filtrate while the solid that remains on it is called the residue.

(b) Decantation