Elements, Compounds and Mixtures

BASIC SCIENCE 

Jss 2.

Third Term

Week 11

Topic: Elements, Compounds and Mixtures

Previous Knowledge: The pupils have previous knowledge of

Crude Oil and Petrochemicals

that was taught in their previous lesson.

Behavioural  Objectives : At the end of the lesson, learners will be able to

• Define elements, mixtures and compounds
• Write out the chemical symbols of the twenty elements
• List methods of separating mixtures.

Introduction

In our environment, we see iron nails, sulphur roll, copper wire, aluminium sheets, etc. we also breathe in oxygen (air) to stay alive. These substances: iron, sulphur, copper and oxygen, are referred to as elements.

Elements

Elements are substances that cannot be separated into simpler substances.

Any substance that contains only one kind of an atom is known as an element. Salt is made up of the elements sodium and chloride. Water is made up of the elements hydrogen and oxygen.

Each element is represented by a unique symbol. The notation for each element can be found on the periodic table of elements.

The elements can be divided into three categories that have characteristic properties: metals, nonmetals, and semimetals. Most elements are metals, which are found on the left and toward the bottom of the periodic table. A handful of nonmetals are clustered in the upper right corner of the periodic table. The semimetals can be found along the dividing line between the metals and the nonmetals.

Elements are made up of atoms, the smallest particle that has any of the properties of the element. John Dalton, in 1803, proposed a modern theory of the atom based on the following assumptions.

These are the first 20 elements and their symbols, listed in order:

1 – H – Hydrogen
2 – He – Helium
3 – Li – Lithium
4 – Be – Beryllium
5 – B – Boron
6 – C – Carbon
7 – N – Nitrogen
8 – O – Oxygen
9 – F – Fluorine
10 – Ne – Neon
11 – Na – Sodium
12 – Mg – Magnesium
13 – Al – Aluminum
14 – Si – Silicon
15 – P – Phosphorus
16 – S – Sulfur
17 – Cl – Chlorine
18 – Ar – Argon
19 – K – Potassium
20 – Ca – Calcium

Chemists use one or two letters to represent elements. The symbol for aluminum is Al. The symbol for oxygen is O. The symbol for oxygen is O. “O” stands for one atom of oxygen. Oxygen atoms are joined in pairs. To write a pair of oxygen atoms using symbols, we use the symbol O and the number 2. Oxygen would be (O₂). A pair of oxygen atoms is a molecule of oxygen. A molecule is the smallest particle of a substance that exists independently. Molecules of most elements are made up of only one of atom of that element. Oxygen, along with nitrogen, hydrogen, and chlorine are made up of two atoms. The two balls represents the two oxygen molecules.

Compound

A compound is a substance formed when two or more elements are chemically joined. Water, salt, and sugar are examples of compounds. When the elements are joined, the atoms lose their individual properties and have different properties from the elements they are composed of. A chemical formula is used a quick way to show the composition of compounds. Letters, numbers, and symbols are used to represent elements and the number of elements in each compound.

Elements combine to form chemical compounds that are often divided into two categories.

• Metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules.
• Nonmetals combine with each other to form covalent compounds, which exist as neutral molecules.

By convention, no subscript is written when a molecule contains only one atom of an element. Thus, water is H₂O and carbon dioxide is CO₂.

Mixtures

Mixtures are two or more substances that are mixed together but not chemically joined. In other word, a mixture contains two or more substances which can easily be separated by physical means. The constituents can be elements or compounds or both. In a given mixture, the constituents which may be present in different proportions retain their individual identities (properties). This is because their physical and chemical properties are not changed by simple mixing. Examples of mixtures are air, urine, blood, milk, coca-cola, petroleum, etc. forming a mixture does not involve chemical change. For instance, a mixture of common salt and water can easily be separated, with each other’s component retaining its properties.

Methods of Separating mixtures

The different substances in mixtures are usually easily separated from one another. The method you use depends upon the type of mixture you have.

Chromatography

This is good for separating dissolved substances that have different colours, such as inks and plant dyes. It works because some of the coloured substances dissolve in the liquid better than others, so they travel further up the paper.

Separating dissolved substances

Filtration

Filtration is good for separating an insoluble solid from a liquid. (An insoluble substance is one that does not dissolve).

Sand, for example, can be separated from a mixture of sand and water using filtration. That’s because sand does not dissolve in water.

Separating insoluble solids

Evaporation

This is good for separating a soluble solid from a liquid (a soluble substance does dissolve, to form a solution).

For example copper sulphate crystals can be separated from copper sulphate solution using evaporation. Remember that it is the water that evaporates away, not the solution.

Separating a soluble solid

Simple distillation

This is good for separating a liquid from a solution. For example, water can be separated from salty water by simple distillation. This method works because the water evaporates from the solution, but is then cooled and condensed into a separate container. The salt does not evaporate and so it stays behind.

Separating a liquid from a solution

Fractional distillation

This is good for separating two or more liquids from each other. For example, ethanol (alcohol) can be separated from a mixture of ethanol and water by fractional distillation. This method works because the two liquids have different boiling points.

Separating funnel

If two liquids are immiscible, then a separating funnel is used.

For example: If you pour a mixture of oil and water into the funnel, the oil floats on top of the water. All that is left to do is for the tap to be opened to allow the water to pour through. The tap is closed once all the water has passed.

Evaluation

1. Define Elements?
2. Elements can be divided into three categories, what are they?
3. Give 4 examples of elements
4. Mention three division or categories of elements
5. What is an atom
6. Write out the 20 elements and their chemical symbols
7. What is a compound?
8. Write out 2 differences between elements and compounds.
9. What are mixtures?
10. What is chromatography?