ELEMENTS, MIXTURES and COMPOUNDS at a glance

 

ELEMENTS: - An element is a substance that cannot be split into simpler units by any ordinary chemical process. 

examples of elements include sodium, oxygen, carbon, nitrogen e.t.c.

There about 118 elements that has been discovered so far and grouped into metals and non-metals in general. we also have elements that are referred to as metalloids, these are actually nonmetals that show or exhibit some metallic properties.

Examples of metals and non-metals 

Metals                 Non-metals

Sodium              Oxygen

Potassium         Carbon

Iron                     Silicon

Gold                    Lead

Tin                      Sulphur 

Calcium              Phosphorus 

 

METALS: - These are elements that ionize by the loss of electrons.

     Na → Na⁺ + e^-

     Ca → Ca²⁺ + 2e^-

      Al → Al³⁺ + 3e^-

They form positively charged ions, that is, Cations when the loss electrons (ionize)

Physical Properties of Metals

i.  Metals are malleable, that is they can be beaten or hammered into flat sheets. 

ii. Metals are ductile, that is, they can be drawn into wires

iii. Metals are Sonorous, i.e. they produce musical notes when struck

iv. Metals conduct heat and electricity

v. Metals have metallic luster, i.e.  they shine when polished.

vi. metals have high tensile strength

Chemical Properties of Metals 

i. Ionization: - Metals ionize by the loss of their valence (outermost) electrons to form Cations

      Na_(s) → Na⁺ + e^-

      Mg_(s) → Mg²⁺ + 2e^-

      Al_(s) → Al³⁺ + 3e^-

ii. Reaction with air: - most metals especially the group I metals like sodium, potassium when exposed to air reacts with the atmospheric gases and become tarnished. Example 

     4Na_(s) +O_2(g) → 2Na₂O_(s)

     Na₂O_(s) + H₂O_(l) → 2NaOH_(aq)

     2NaOH_(aq) +CO_2(g) → Na₂CO_3(s) +H₂O_(l)

iii. Reaction with Acids: - Most metals react with dilute acids to liberate hydrogen gas. Example

     I. Mg_(s) + H₂SO₄ → MgSO_4(aq) + H_2(g)

      ii. Ca_(s) + 2HCl_(aq) → CaCl_2(aq) + H_2(g)

Reaction with Bases: - Some metals react with Bases to form complex salts. These metals are said to be Amphoteric elements. Examples include Aluminium (Al), Tin (Sn), Zinc (Zn) and Lead (Pb).

Zn_(s) + 2NaOH_(aq) + 2H₂O_(l) → Na₂Zn(OH)_4(aq)

Reaction with non-metals: - Most metals combine directly with nonmetals to form compounds, example metals combine with oxygen to form oxides, with sulphur to form sulphides, with nitrogen to form nitrides.

   Ca_(s) + O_2(g) → CaO_(s)

   Fe_(s) + S_(s) → FeS_(s)

    2 Mg_(s) + O_2(g) →MgO_(s)

 

NON-METALS: - These are elements that ionize by gaining electrons.

They form negatively charged ions known as Anions, when they gain electrons

Example

Physical properties of nonmetals 

i. Non-metals do not conduct heat and electricity

ii. They are hard and brittle i.e. they break easily  

iii. They have low tensile strength 

iv. They do not shine when polished

v. They are not malleable 

vi. They are not ductile 

COMPOUNDS: - A compound is a substance formed when two or more elements are chemically combined together.

examples of compounds include

 i. sodium chloride (NaCl), 

ii. calcium trioxocarbonate (IV) (CaCO₃),

Iii. Water (H₂O)

Iv. Sugar (C₆H₁₂O₆)

MIXTURES: - Mixtures are substances that contain two or constituents that can be separated by physical methods.    

                 Differences between Compounds and Mixture 

	Compounds	Mixture
i.	Can be represented by a formula	Can not be represented by a formula
ii	Is always homogenous (every part of the compound is always the same)	May be homogenous (that is, every part of the mixture is the same) or heterogenous (every part of the mixture is not the same)
iii.	Composition is always constant	Composition is not constant
iv.	Property is different from the properties of the individual elements that make up the compound	Constituents still retain their individual properties
v.	The elements are chemically combined together	The constituents are not chemically combined

 

Example of mixtures include: - 

	Mixtures	Constituents
i.	Sodium chloride solution	Sodium chloride + water
ii.	Blood	Plasma + red and white blood cells
iii.	Urine	Water + salts + Urea
iv.
v.
vi

               

In order for substances to be analyzed they have to be separated into their pure forms. The various methods for separating substances into their various pure forms are discussed in a different post.

 Objective Questions 

1. A substance that cannot be broken down into simpler substances by ordinary chemical means is called: A. Compound
B. Mixture
C. Element
D. Solution

2. Which of the following is a compound? A. Iron
B. Oxygen
C. Water
D. Brass

3. Which of the following is a mixture? A. Carbon dioxide
B. Sodium chloride
C. Air
D. Ammonia

4. In a compound, the constituent elements are: A. Mixed in any proportion
B. Chemically combined in a fixed ratio
C. Easily separated by filtration
D. Physically combined only

5. Which of the following is an element? A. Copper
B. Water
C. Sugar
D. Salt

6. Which method can be used to separate the components of a mixture? A. Electrolysis only
B. Chemical decomposition
C. Physical methods such as filtration or distillation
D. Nuclear reaction

7. Which of the following contains only one type of atom? A. Compound
B. Mixture
C. Element
D. Solution

8. Brass is an example of: A. Element
B. Compound
C. Alloy (mixture)
D. Molecule

9. Which of the following is NOT a mixture? A. Crude oil
B. Sea water
C. Air
D. Carbon dioxide

10. A major difference between a compound and a mixture is that:

 A. A mixture has a fixed composition while a compound does not. 

B. A compound can be separated by physical means.

 C. A compound has a fixed chemical composition, while a mixture does not. 

D. Both have the same properties as their components.