A compound can be defined as a substance that is composed of two or more elements that are chemically bonded together. These elements combine through a chemical reaction to form a compound, resulting in a distinct substance with unique properties and characteristics.

Here are some examples of compounds and their constituent elements:

1. Water: Water is a compound composed of two elements, hydrogen and oxygen. Its chemical formula is H2O, indicating that two hydrogen atoms are bonded to one oxygen atom.

2. Sand: Sand is a compound predominantly made up of silicon and oxygen. It consists of small granules or particles of silicon dioxide (SiO2), where one atom of silicon is bonded to two atoms of oxygen.

3. Limestone: Limestone is a compound that primarily consists of calcium, carbon, and oxygen. It is formed from the accumulation of marine organisms’ skeletal remains over millions of years. The chemical formula for limestone is typically written as CaCO3, representing one calcium atom bonded to one carbon atom and three oxygen atoms.

4. Common salt: Commonly known as table salt, common salt is a compound composed of sodium and chlorine. Its chemical formula is NaCl, indicating that one sodium atom is bonded to one chlorine atom.

5. Ethanol: Ethanol is a compound found in alcoholic beverages and many other products. Its chemical formula is C2H5OH, representing two carbon atoms, six hydrogen atoms, and one oxygen atom. Ethanol is composed of carbon, hydrogen, and oxygen elements.

These examples illustrate the diversity of compounds and the various combinations of elements that can occur through chemical reactions.


1. Different Properties: A compound possesses properties that are distinct from those of its constituent elements. When elements chemically combine to form a compound, the resulting substance exhibits new physical and chemical characteristics. For example, while hydrogen and oxygen are gases at room temperature, water, a compound formed by their combination, is a liquid.

2. Heat Requirement: The formation of a compound often necessitates the input of a significant amount of heat. This heat energy is required to facilitate the chemical reaction between the elements, allowing them to bond together and form the compound. The amount of heat required can vary depending on the specific elements involved and the reaction conditions.

3. Separation by Physical Means: Unlike mixtures, compounds cannot be separated into their constituent elements through physical means such as filtration or distillation. The chemical bonds between the elements in a compound are strong, requiring specific chemical reactions or processes to break them apart. For example, water cannot be separated into hydrogen and oxygen simply by physical methods; electrolysis, a chemical process, is needed to break the water molecules into their elemental components.

4. Fixed Ratio by Mass: The components of a compound are present in a fixed ratio by mass. This means that the elements combine in specific proportions to form a compound, and these proportions are consistent regardless of the amount of compound formed. For example, water is always composed of two hydrogen atoms for every one oxygen atom, giving it a fixed ratio of 2:1 by mass.

5. Homogeneity: Compounds are homogeneous in nature, meaning that they have a uniform composition throughout. The atoms of different elements in a compound are uniformly distributed and intimately mixed at the molecular level. As a result, compounds have consistent properties and exhibit the same composition and behavior in any given sample.

Understanding the properties of compounds helps us comprehend their unique characteristics and behavior, facilitating the study of chemical reactions and the applications of compounds in various fields, including chemistry, materials science, and biology.


When representing a compound’s molecular formula, several conventions are followed to indicate the number and type of atoms present in the compound.

1. Symbols of Component Elements: The symbols of all the component elements in a compound are written close together as a group. This helps to visually represent the elements that combine to form the compound. For example, in hydrochloric acid (HCl), the symbols “H” and “Cl” are written together, indicating the presence of hydrogen and chlorine.

2. Subscripts for Atom Count: The number of atoms of each component element is written as a subscript after the symbol of that element. These subscripts indicate the ratio or count of atoms present in the compound. For instance, water (H2O) consists of two hydrogen (H) atoms and one oxygen (O) atom, as denoted by the subscripts “2” and “1” respectively.


1. Hydrochloric acid: The molecular formula for hydrochloric acid is HCl. This indicates that one hydrogen atom (H) is combined with one chlorine atom (Cl) to form the compound.

2. Water: The molecular formula for water is H2O. It represents the presence of two hydrogen atoms (H) and one oxygen atom (O) in each molecule of water.

3. Ammonia: The molecular formula for ammonia is NH3. This shows that one nitrogen atom (N) is bonded with three hydrogen atoms (H) in each ammonia molecule.

4. Carbon(IV)oxide: The molecular formula for carbon(IV)oxide, also known as carbon dioxide, is CO2. It reveals that one carbon atom (C) is combined with two oxygen atoms (O) in each molecule of carbon dioxide.

5. Lead II chloride: The molecular formula for lead II chloride is PbCl2. This signifies the presence of one lead atom (Pb) and two chlorine atoms (Cl) in each unit of lead II chloride.

6. Calcium trioxonitrate(V): The molecular formula for calcium trioxonitrate(V), commonly known as calcium nitrate, is Ca(NO3)2. This indicates that there is one calcium atom (Ca) bonded with two nitrate ions (NO3), where each nitrate ion consists of one nitrogen atom (N) and three oxygen atoms (O).

These molecular formulas provide valuable information about the composition and structure of compounds, allowing scientists to understand their chemical properties and reactions more effectively.


When deducing the formula of a compound based on the valencies of its component elements or radicals, the following rules are typically followed:

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1. Write the Symbols: Begin by writing the symbols of the elements or radicals that make up the compound.

2. Write Valencies: Below each symbol, write the valencies or charges associated with the respective element or radical.

3. Exchange Valencies: Swap or exchange the valencies between the symbols of the elements or radicals.

4. Write the Formula: Finally, write the formula of the compound by bringing the symbols of the elements or radicals together, indicating the appropriate subscripts based on the exchanged valencies.


1. Sodium Tetraoxosulphate(VI):

Rule 1: Na S04
Rule 2 & 3: 1 2
Rule 4: Na2S04

The formula of sodium tetraoxosulphate(VI) is Na2SO4.

2. Calcium Chloride:

Rule 1: Ca Cl
Rule 2 & 3: 2 1
Rule 4: CaCl2

The formula of calcium chloride is CaCl2.

By following these rules, one can determine the formula of a compound based on the valencies of the elements or radicals involved. This approach helps in understanding the composition and structure of compounds, enabling effective communication and study of chemical reactions and properties.


A mixture is a combination of two or more constituents that can be easily separated by physical methods due to the differences in their physical properties. The constituents in a mixture retain their individual identities and properties, and they can be present in varying amounts.

Here are some examples of mixtures and their constituents:

1. Air: Air is a mixture primarily composed of oxygen, carbon (IV) oxide, nitrogen, rare gases (such as argon, neon, helium, etc.), dust particles, and moisture. These constituents can be separated through techniques like fractional distillation, filtration, and condensation.

2. Soil: Soil is a complex mixture consisting of various components, including sand, clay, humus (decayed organic matter), water, air, and mineral salts. These constituents contribute to the fertility and structure of soil and can be separated through sieving, sedimentation, or other physical methods.

3. Urine: Urine is a mixture found in mammals and humans, comprising components such as urea, water, mineral salts, and other waste products. While it may contain various dissolved substances, the constituents of urine can be separated by techniques like filtration or distillation.

4. Palm wine: Palm wine is a traditional alcoholic beverage obtained from palm trees. It contains constituents such as water, sugar, alkanols (ethanol), mineral salts, vitamins, yeast, proteins, and fats. These constituents can be separated through distillation or filtration methods.

5. Coca-Cola: Coca-Cola is a popular soft drink composed of water, sugar, cola flavorings, and carbon dioxide (CO2). While it may contain other additives, these primary constituents can be separated by various processes such as filtration or evaporation.

6. Milk: Milk is a mixture of water, sugar (lactose), fats, proteins (such as casein), mineral salts, and vitamins. Through techniques like centrifugation or filtration, the components of milk can be separated.

7. Sea water: Sea water is a mixture primarily consisting of water, dissolved mineral salts (such as sodium chloride), bacteria, and remains of organic matter. Desalination processes like distillation or reverse osmosis can be used to separate the salt from the water.

8. Brass: Brass is an alloy composed of copper and zinc. It is a solid mixture where the constituents, copper and zinc, are combined in specific proportions. Although brass is a homogeneous mixture, it is considered a solid solution and can be separated from other materials by physical means like melting or mechanical separation.

Understanding the nature of mixtures and their constituents allows us to utilize various separation techniques to isolate and purify different components for specific purposes and applications.


A comparison between mixtures and compounds can be made based on several key characteristics:


1. Homogeneity: A mixture can be either homogeneous or heterogeneous. In a homogeneous mixture, the components are uniformly distributed, resulting in a consistent composition throughout the mixture. In contrast, a heterogeneous mixture has distinct regions with varying compositions.

2. Physical Separation: The constituents of a mixture can be separated by physical means, such as filtration, distillation, magnetism, or evaporation. These methods exploit the differences in physical properties, such as size, solubility, boiling point, or magnetic properties, to separate the components.

3. Variable Composition: In a mixture, the constituents can be added in any ratio by mass. The proportions of the components are not fixed and can be adjusted according to the desired composition or application.

4. Chemical Formula: A mixture cannot be represented by a chemical formula because it lacks a fixed ratio between its components. Instead, mixtures are typically described based on the identities and relative amounts of their constituents.

5. Properties: The properties of a mixture are the sum of the properties of its individual constituents. Each component retains its own distinct characteristics, and the overall properties of the mixture arise from the combined effects of the components.


1. Homogeneity: A compound is always homogeneous, meaning that its composition is uniform throughout. The atoms or ions of different elements are chemically bonded in a specific arrangement, resulting in a consistent distribution of the components.

2. Separation by Physical Means: Unlike mixtures, compounds cannot be separated into their constituent elements or ions by physical means. Breaking the chemical bonds within a compound requires a chemical reaction or process.

3. Fixed Composition: Compounds have a fixed ratio by mass between their constituent elements. Each compound has a specific and consistent arrangement of atoms, and the ratios are determined by the chemical formula. This fixed composition remains the same regardless of the amount of compound present.

4. Chemical Formula: Compounds are represented by chemical formulas, which indicate the type and number of atoms or ions in a compound. The chemical formula provides crucial information about the composition and structure of the compound.

5. Properties: The properties of a compound are entirely different from those of its component elements. Chemical bonding leads to new properties emerging in compounds, such as different melting points, boiling points, reactivity, and chemical behavior. The compound may exhibit characteristics that are distinct from the properties of the individual elements involved.

Understanding the distinctions between mixtures and compounds is crucial for various scientific fields, including chemistry, materials science, and biology, as it helps in predicting and understanding the behavior of substances and designing appropriate separation or synthesis methods.

Read also:

Structure of The Atom

Atomic Number, Relative Atomic Masses, Isotopes & Calculations

IUPAC Nomenclature of Chemical Compounds



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