Characteristic Properties of Covalent Compounds are an important part of Class 11 Chemistry, helping students understand the physical and chemical behavior of substances formed through covalent bonding. The Class 11 Chemistry notes of Anand Classes, written by Neeraj Anand, explain the properties of covalent compounds, including their melting and boiling points, solubility, electrical conductivity, volatility, and molecular nature with simple explanations and exam-oriented examples. Designed for CBSE, JEE Main, JEE Advanced, and NEET, these study materials build strong conceptual understanding and support effective preparation for board and competitive examinations.
What are the main characteristic properties of covalent compounds?
The main characteristic properties of covalent compounds are listed as follows :
- Exist as molecules (not ions).
- Generally have low melting and boiling points.
- Are generally insoluble or sparingly soluble in water but soluble in non-polar solvents.
- Are poor conductors of electricity (non-conductors).
- Undergo molecular reactions.
- Possess directional covalent bonds.
- Have low enthalpies of fusion and vaporization.
- Are generally more flammable than ionic compounds.
- Many solid covalent compounds are hard and brittle.
- Generally have high vapor pressure.
- May be colorless or exhibit a variety of colors.
The characteristic properties of covalent compounds mentioned above are discussed in the following sections.
State of existence of Covalent Compounds
The covalent compounds do not exist as ions but they exist as molecules. There are weak intermolecular forces between the molecules and hence, they exist as liquids or gases at room temperature. However, a few compounds also exist in the solid state e.g., urea, sugar, etc.
The following questions and answers explain the state of existence of covalent compounds in a simple and easy-to-understand manner.
In what form do covalent compounds exist?
Covalent compounds do not exist as ions. They exist as neutral molecules because their atoms are held together by covalent bonds formed through the sharing of electrons.
Examples: H₂, H₂O, CO₂, CH₄.
Why do covalent compounds exist as molecules instead of ions?
In covalent compounds, atoms share electrons to complete their valence shells. Since no electrons are completely transferred from one atom to another, ions are not formed. Therefore, covalent compounds exist as molecules.
Example:
In H₂O, oxygen shares electrons with two hydrogen atoms. No H⁺ or O²⁻ ions are formed, so water exists as H₂O molecules.
What are intermolecular forces?
Intermolecular forces are the weak forces of attraction between molecules. They are much weaker than the covalent bonds that hold atoms together within a molecule.
Example:
The weak attraction between one water (H₂O) molecule and another is an intermolecular force.
Why are most covalent compounds gases or liquids at room temperature?
Most covalent compounds have weak intermolecular forces. Only a small amount of energy is needed to separate their molecules. Therefore, they generally have low melting and boiling points and exist as gases or liquids at room temperature.
Examples:
- Hydrogen (H₂) – Gas
- Oxygen (O₂) – Gas
- Carbon dioxide (CO₂) – Gas
- Water (H₂O) – Liquid
Do all covalent compounds exist as gases or liquids?
No. Some covalent compounds exist as solids at room temperature. This is because their molecules experience stronger intermolecular forces or have larger molecular structures.
Examples:
- Urea (NH₂CONH₂) – Solid
- Sugar (Sucrose, C₁₂H₂₂O₁₁) – Solid
- Iodine (I₂) – Solid
Why are urea and sugar solids even though they are covalent compounds?
Urea and sugar are made of large covalent molecules. The attraction between their molecules is strong enough to keep them closely packed, so they exist as solids at room temperature.
Examples:
- Urea – White crystalline solid
- Sugar (Sucrose) – White crystalline solid
What is the state of existence of common covalent compounds?
| Covalent Compound | State at Room Temperature |
|---|---|
| Hydrogen (H₂) | Gas |
| Oxygen (O₂) | Gas |
| Nitrogen (N₂) | Gas |
| Carbon dioxide (CO₂) | Gas |
| Water (H₂O) | Liquid |
| Urea | Solid |
| Sugar (Sucrose) | Solid |
Why do ionic compounds and covalent compounds differ in their state of existence?
Ionic compounds contain strong electrostatic forces between oppositely charged ions, making them hard solids. Covalent compounds consist of neutral molecules held together by weak intermolecular forces, so many of them are gases or liquids.
Example:
- Sodium chloride (NaCl) – Ionic compound, solid.
- Carbon dioxide (CO₂) – Covalent compound, gas.
Learn more about Octet Rule Exceptions, Incomplete and Expanded Octet of central atom, Odd Electron molecules
Low Melting and Boiling points of Covalent Compounds
The melting and boiling points of covalent compounds are generally low. This is because of the fact that the forces between the molecules are weak and are easily overcome at low temperatures.
The following questions and answers explain why covalent compounds generally have low melting and boiling points.
What are intermolecular forces?
Intermolecular forces are weak forces of attraction between molecules. They are much weaker than the covalent bonds that hold atoms together within a molecule.
Why do covalent compounds generally have low melting and boiling points?
Most covalent compounds have weak intermolecular forces between their molecules. Therefore, only a small amount of heat is needed to separate the molecules, giving them low melting and boiling points.
Example:
Methane (CH₄) boils at −161.5°C, so it is a gas at room temperature.
What happens to the intermolecular forces when a covalent compound is heated?
When a covalent compound is heated, the weak intermolecular forces are easily overcome, allowing the molecules to move freely. As a result, the compound melts or boils at a relatively low temperature
Do covalent bonds break when a covalent compound melts or boils?
No. During melting or boiling, only the intermolecular forces are overcome. The covalent bonds within the molecules remain intact.
Example:
When ice (H₂O) melts into water, the water molecules remain H₂O. Only the intermolecular forces between the molecules are weakened.
Give examples of covalent compounds with low melting and boiling points.
Examples include:
- Hydrogen (H₂)
- Oxygen (O₂)
- Nitrogen (N₂)
- Methane (CH₄)
- Carbon dioxide (CO₂)
These compounds have weak intermolecular forces and therefore low melting and boiling points.
Why do ionic compounds generally have higher melting and boiling points than covalent compounds?
Ionic compounds have strong electrostatic forces of attraction between oppositely charged ions, which require a large amount of heat to overcome. In contrast, covalent compounds have weak intermolecular forces.
To understand this topic better, learn about Formal Charge Calculation Formula from Lewis structure
Solubility of Covalent Compounds
Covalent compounds are generally insoluble or less soluble in water and other polar solvents. However, these are soluble in non-polar solvents such as benzene, carbon-tetrachloride, etc.
The following questions and answers explain the solubility of covalent compounds in different types of solvents.
What is the solubility of covalent compounds?
Covalent compounds are generally insoluble or only slightly soluble in water and other polar solvents. However, they are soluble in non-polar solvents such as benzene and carbon tetrachloride.
Why are covalent compounds generally insoluble in water?
Water is a polar solvent, whereas most covalent compounds are non-polar. Since polar and non-polar substances do not mix well, most covalent compounds do not dissolve in water.
Example:
Oil does not dissolve in water because oil is a non-polar covalent substance.
Why are covalent compounds soluble in non-polar solvents?
Covalent compounds are usually non-polar, so they dissolve easily in non-polar solvents. This follows the principle “like dissolves like,” which means substances with similar polarity dissolve in each other.
Examples:
- Benzene (C₆H₆)
- Carbon tetrachloride (CCl₄)
What is meant by the phrase “like dissolves like”?
The phrase “like dissolves like” means that polar substances dissolve in polar solvents, while non-polar substances dissolve in non-polar solvents.
Examples:
- Sugar dissolves in water (polar in polar).
- Iodine dissolves in carbon tetrachloride (non-polar in non-polar).
Name two non-polar solvents in which covalent compounds are soluble.
Two common non-polar solvents are:
- Benzene (C₆H₆)
- Carbon tetrachloride (CCl₄)
Are all covalent compounds insoluble in water?
No. Some covalent compounds are soluble in water because they are polar molecules and can form strong interactions, such as hydrogen bonds, with water.
Examples:
- Water (H₂O)
- Ethanol (C₂H₅OH)
- Ammonia (NH₃)
Give examples of covalent compounds that are insoluble in water.
Examples include:
- Methane (CH₄)
- Carbon tetrachloride (CCl₄)
- Benzene (C₆H₆)
- Iodine (I₂)
These compounds are non-polar and therefore do not dissolve in water.
Why is water called a polar solvent?
Water is called a polar solvent because it has a positive end (hydrogen atoms) and a negative end (oxygen atom) due to the unequal sharing of electrons. This polarity allows water to dissolve other polar substances.
For better understanding, also read What is Chemical Bond ? Why do Atoms Combine ? How do Atoms Combine ?
Covalent Compounds are Non-Conductors
Since covalent compounds do not give ions in solution, these are poor conductors of electricity in the fused or dissolved state.
The following questions and answers explain why covalent compounds are poor conductors of electricity.
Why are covalent compounds called non-conductors of electricity?
Covalent compounds are called non-conductors of electricity because they do not produce free ions in the molten (fused) or dissolved state. Without free ions, electricity cannot flow through them.
Why do covalent compounds not conduct electricity in solution?
Covalent compounds do not conduct electricity in solution because they do not dissociate into ions when dissolved in water. They remain as neutral molecules.
Example:
A solution of sugar does not conduct electricity because sugar molecules do not form ions in water.
What is meant by the fused state of a substance?
The fused state is the molten or liquid state obtained when a solid substance is heated above its melting point.
Why do covalent compounds not conduct electricity in the fused (molten) state?
Even in the fused (molten) state, covalent compounds remain as neutral molecules and do not produce free-moving ions. Therefore, they do not conduct electricity.
Why are ionic compounds good conductors of electricity, whereas covalent compounds are not?
Ionic compounds produce free ions when molten or dissolved in water, allowing electricity to pass through them. Covalent compounds do not produce ions and therefore are poor conductors of electricity.
Example:
- Sodium chloride (NaCl) solution conducts electricity.
- Sugar solution does not conduct electricity.
Give examples of covalent compounds that do not conduct electricity.
Answer:
Examples include:
- Sugar (Sucrose)
- Urea
- Methane (CH₄)
- Carbon dioxide (CO₂)
These compounds remain as neutral molecules and do not produce ions.
Do covalent compounds form ions when dissolved in water?
No. Covalent compounds generally do not form ions when dissolved in water. They remain as neutral molecules, making them poor conductors of electricity.
Students should also study Lewis Electron Dot (Symbols) Structure Theory and its Significance
Molecular Reactions of Covalent Compounds
The reactions between covalent compounds occur between their molecules. These involve the breaking of covalent bonds in reacting molecules and forming new covalent bonds to give molecules of the products. These reactions are quite slow because energy is required to break covalent bonds.
The following questions and answers explain the molecular reactions of covalent compounds.
What are molecular reactions?
Molecular reactions are chemical reactions that occur between the molecules of covalent compounds. During these reactions, old covalent bonds break and new covalent bonds are formed to produce new molecules.
Why are the reactions of covalent compounds called molecular reactions?
The reactions of covalent compounds are called molecular reactions because they occur between neutral molecules, not between ions.
What happens during a molecular reaction?
During a molecular reaction, existing covalent bonds are broken in the reactant molecules, and new covalent bonds are formed to produce the product molecules.
Why do molecular reactions require energy?
Molecular reactions require energy because breaking covalent bonds between atoms needs an input of energy.
Why are the reactions of covalent compounds generally slow?
The reactions of covalent compounds are generally slow because a considerable amount of energy is required to break the strong covalent bonds in the reacting molecules.
Give an example of a molecular reaction.
The reaction between hydrogen and chlorine is a molecular reaction:
H₂ + Cl₂ → 2HCl
In this reaction, the H–H and Cl–Cl covalent bonds break, and new H–Cl covalent bonds are formed.
Do molecular reactions involve ions?
No. Molecular reactions do not involve ions. They occur between neutral covalent molecules.
What type of bonds are broken and formed during molecular reactions?
During molecular reactions, covalent bonds are broken in the reactants, and new covalent bonds are formed in the products.
Master related concepts such as Ionic or Electrovalent Bond Explanation, Examples, Factors Affecting Formation of Ionic Bonds
Directional Character of Covalent Bond and Covalent Compounds have definite molecular shapes
Since the covalent bond is localised in between the nuclei of atoms, it is directional in nature.
The following questions and answers explain the directional character of the covalent bond.
What is meant by the directional character of a covalent bond?
The directional character of a covalent bond means that the bond is formed in a specific direction between the nuclei of two bonded atoms.
Why is a covalent bond directional in nature?
A covalent bond is directional because the shared pair of electrons is localized between the nuclei of the bonded atoms, giving the bond a definite direction.
What is meant by the term “localized” in a covalent bond?
Localized means that the shared pair of electrons is concentrated between the nuclei of the bonded atoms and is not free to move throughout the molecule.
How does the directional nature of covalent bonds affect the shape of molecules?
The directional nature of covalent bonds causes atoms to arrange themselves in specific orientations, giving molecules definite shapes and bond angles.
Examples:
- Water (H₂O) has a bent (V-shaped) structure.
- Methane (CH₄) has a tetrahedral structure.
Why do covalent compounds have definite molecular shapes?
Covalent compounds have definite molecular shapes because their covalent bonds are directional and hold the atoms in fixed positions.
Are ionic bonds directional like covalent bonds?
No. Ionic bonds are non-directional because the electrostatic force of attraction acts equally in all directions, whereas covalent bonds are directional due to the localized sharing of electrons.
Give examples of molecules that show the directional nature of covalent bonds.
Examples include:
- Water (H₂O) – Bent (V-shaped)
- Methane (CH₄) – Tetrahedral
- Ammonia (NH₃) – Trigonal pyramidal
These molecules have definite shapes because of the directional nature of covalent bonds.
Low Enthalpies of Fusion and Vaporization of Covalent Compounds
Covalent Compounds have lower enthalpies of fusion and vaporization compared to ionic compounds. The following questions and answers explain why covalent compounds have low enthalpies of fusion and vaporization.
What is meant by the enthalpy of fusion?
The enthalpy of fusion is the amount of heat required to convert one mole of a solid into a liquid at its melting point without any change in temperature.
What is meant by the enthalpy of vaporization?
The enthalpy of vaporization is the amount of heat required to convert one mole of a liquid into a gas at its boiling point without any change in temperature.
Why do covalent compounds have low enthalpies of fusion and vaporization?
Covalent compounds have weak intermolecular forces between their molecules. Therefore, only a small amount of heat is needed to separate the molecules during melting or boiling, giving them low enthalpies of fusion and vaporization.
Why do ionic compounds have higher enthalpies of fusion and vaporization than covalent compounds?
Ionic compounds have strong electrostatic forces of attraction between oppositely charged ions. A large amount of heat is required to overcome these forces, so they have higher enthalpies of fusion and vaporization than covalent compounds.
How are low enthalpies of fusion and vaporization related to the melting and boiling points of covalent compounds?
Since only a small amount of heat is needed to separate the molecules, covalent compounds generally have low melting and boiling points as well as low enthalpies of fusion and vaporization.
Give examples of covalent compounds with low enthalpies of fusion and vaporization.
Examples include:
- Methane (CH₄)
- Carbon dioxide (CO₂)
- Oxygen (O₂)
- Nitrogen (N₂)
These compounds require relatively little heat to melt or vaporize because of their weak intermolecular forces.
What type of forces are overcome during the fusion and vaporization of covalent compounds?
During fusion and vaporization, only the weak intermolecular forces between the molecules are overcome. The covalent bonds within the molecules remain intact.
Q8. State one important characteristic of covalent compounds related to enthalpy.
Answer:
Covalent compounds have low enthalpies of fusion and vaporization because weak intermolecular forces between their molecules require only a small amount of heat to overcome.
Flammability of Covalent Compounds
Covalent compounds are generally more flammable than ionic compounds because many of them contain carbon and hydrogen, which burn readily in oxygen. The following questions and answers explain the flammability of covalent compounds.
What is meant by the flammability of a substance?
Flammability is the ability of a substance to catch fire and burn in the presence of oxygen, releasing heat and light.
Why are covalent compounds generally more flammable than ionic compounds?
Covalent compounds are generally more flammable because many of them contain carbon and hydrogen, which readily react with oxygen during combustion to produce heat and light.
Why are ionic compounds generally non-flammable?
Ionic compounds are generally non-flammable because they are already in a stable, oxidized state and do not burn easily in air.
Give examples of flammable covalent compounds.
Examples include:
- Methane (CH₄)
- Ethane (C₂H₆)
- Propane (C₃H₈)
- Ethanol (C₂H₅OH)
These compounds burn readily in oxygen.
What are the products formed when covalent compounds containing carbon and hydrogen burn completely?
When carbon- and hydrogen-containing covalent compounds burn completely in oxygen, they produce carbon dioxide (CO₂) and water (H₂O) along with heat and light.
Example:
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
Are all covalent compounds flammable?
No. Not all covalent compounds are flammable. Some, such as carbon dioxide (CO₂) and carbon tetrachloride (CCl₄), do not burn under normal conditions and they are generallu use fire extinguishers.
What type of reaction occurs when a flammable covalent compound burns?
When a flammable covalent compound burns, it undergoes a combustion reaction, in which it reacts with oxygen to release heat and light.
Flexible but Network solids (Covalent Compounds) can be Hard and Brittle
Many solid covalent compounds are hard and brittle because they contain strong covalent bonds, but they break easily when force is applied rather than bending. The following questions and answers explain the hardness and brittleness of covalent compounds.
Are covalent compounds hard and brittle?
Many solid covalent compounds are hard and brittle. They are hard because of the strong covalent bonds holding the atoms together, but they break easily when force is applied.
Note: This property mainly applies to solid covalent compounds. Many covalent compounds are gases or liquids and therefore are neither hard nor brittle.
Why are some covalent compounds hard?
Some covalent compounds are hard because their atoms are held together by strong covalent bonds, which require a large amount of energy to break.
Example:
Diamond is extremely hard because each carbon atom is strongly bonded to four other carbon atoms.
Why are solid covalent compounds brittle?
Solid covalent compounds are brittle because they break or shatter when subjected to force instead of bending or deforming.
Example:
Diamond is hard but brittle and can break if struck with sufficient force.
Are all covalent compounds hard and brittle?
No. Not all covalent compounds are hard and brittle. Most covalent compounds are gases or liquids at room temperature. Only some solid covalent compounds show hardness and brittleness.
Examples:
- Diamond – Hard and brittle
- Sugar – Brittle solid
- Methane (CH₄) – Gas
- Water (H₂O) – Liquid
Give examples of hard covalent substances.
Examples include:
- Diamond – Extremely hard
- Silicon dioxide (SiO₂ or quartz) – Hard
- Silicon carbide (SiC) – Very hard
Why is diamond much harder than most other covalent compounds?
Diamond has a giant covalent network structure in which each carbon atom is covalently bonded to four other carbon atoms. This three-dimensional network makes diamond extremely hard.
High Vapor Pressure of Covalent Compounds
Covalent compounds generally have high vapor pressure because their weak intermolecular forces allow molecules to escape easily into the vapor phase. The following questions and answers explain why covalent compounds generally have high vapor pressure.
What is meant by vapor pressure?
Vapor pressure is the pressure exerted by the vapor of a liquid when it is in equilibrium with its liquid at a given temperature.
Why do covalent compounds generally have high vapor pressure?
Covalent compounds generally have high vapor pressure because the intermolecular forces between their molecules are weak. As a result, the molecules escape easily from the liquid surface into the vapor state.
How do weak intermolecular forces affect the vapor pressure of covalent compounds?
Weak intermolecular forces allow molecules to evaporate easily, increasing the number of molecules in the vapor phase. Therefore, covalent compounds have high vapor pressure.
Why do ionic compounds have lower vapor pressure than covalent compounds?
Ionic compounds have strong electrostatic forces of attraction between ions. These strong forces prevent the particles from escaping into the vapor state, resulting in very low vapor pressure.
How is vapor pressure related to the boiling point of covalent compounds?
Covalent compounds with high vapor pressure usually have low boiling points because their molecules escape into the vapor state more easily.
Give examples of covalent compounds with high vapor pressure.
Examples include:
- Ether (Diethyl ether)
- Acetone
- Benzene
- Methane (CH₄)
These compounds evaporate easily because of weak intermolecular forces.
What is the relationship between intermolecular forces and vapor pressure?
The weaker the intermolecular forces, the higher the vapor pressure. Conversely, stronger intermolecular forces result in lower vapor pressure.
Covalent compounds have varied colors to colorless
Covalent compounds may be colorless or exhibit a variety of colors, depending on their molecular structure and interaction with visible light. The following questions and answers explain why covalent compounds may be colorless or exhibit different colors.
What is the colour of covalent compounds?
Covalent compounds may be colorless or exhibit a variety of colors, depending on their molecular structure and the way they interact with light.
Why are many covalent compounds colorless?
Many covalent compounds are colorless because they do not absorb visible light. Instead, they transmit visible light, making them appear colorless.
Examples:
- Water (H₂O)
- Methane (CH₄)
- Ethanol (C₂H₅OH)
Why do some covalent compounds have different colours?
Some covalent compounds appear colored because their molecules absorb certain wavelengths of visible light and reflect or transmit the remaining wavelengths.
Examples:
- Iodine (I₂) – Violet
- Sulfur – Yellow
Do all covalent compounds have the same colour?
No. Covalent compounds do not all have the same colour. They may be colorless or colored, depending on their molecular structure.
Give examples of colorless covalent compounds.
Examples include:
- Water (H₂O)
- Carbon dioxide (CO₂)
- Methane (CH₄)
- Ammonia (NH₃)
Give examples of colored covalent compounds.
Examples include:
- Iodine (I₂) – Violet
- Sulfur – Yellow
- Nitrogen dioxide (NO₂) – Brown
What determines the colour of a covalent compound?
The molecular structure and the compound’s ability to absorb visible light determine whether a covalent compound is colorless or colored.
Volatility of Covalent Compounds
Covalent compounds are generally volatile because their weak intermolecular forces allow their molecules to evaporate easily. The following questions and answers explain the volatility of covalent compounds.
What is meant by the volatility of a substance?
Volatility is the tendency of a substance to evaporate easily at ordinary temperatures.
Why are covalent compounds generally volatile?
Covalent compounds are generally volatile because they have weak intermolecular forces between their molecules. These weak forces allow the molecules to escape easily into the vapor state.
How do weak intermolecular forces affect the volatility of covalent compounds?
Weak intermolecular forces require only a small amount of energy to overcome. Therefore, the molecules evaporate easily, making covalent compounds highly volatile.
Why are ionic compounds less volatile than covalent compounds?
Ionic compounds have strong electrostatic forces of attraction between oppositely charged ions. These strong forces prevent the particles from escaping easily into the vapor state, making ionic compounds less volatile.
How is volatility related to the boiling point of covalent compounds?
Covalent compounds with high volatility generally have low boiling points because their molecules escape into the vapor state easily.
Give examples of volatile covalent compounds.
Examples include:
- Ether (Diethyl ether)
- Acetone
- Benzene
- Methane (CH₄)
These compounds evaporate readily due to weak intermolecular forces.
What is the relationship between volatility and intermolecular forces?
The weaker the intermolecular forces, the greater the volatility of the substance. Stronger intermolecular forces result in lower volatility.