Features/Characteristics of a Compound

Key Characteristics of Chemical Compounds

1. Definite Chemical Formula

Every compound can be represented by a specific chemical formula that shows the exact ratio of atoms present. For example:

• Water (H₂O) – always contains 2 hydrogen atoms and 1 oxygen atom
• Table salt (NaCl) – always contains 1 sodium atom and 1 chlorine atom
• Calcium carbonate (CaCO₃) – always contains 1 calcium, 1 carbon, and 3 oxygen atoms
• Sulfuric acid (H₂SO₄) – always contains 2 hydrogen, 1 sulfur, and 4 oxygen atoms

This formula remains constant whether the compound is prepared in Lagos, London, or any laboratory worldwide.

2. Fixed and Definite Composition

The Law of Definite Proportions states that a compound always contains the same elements in the same proportion by mass. Water is always 88.9% oxygen and 11.1% hydrogen by mass, regardless of whether it comes from a well in Kano, the Atlantic Ocean, or melted ice from Antarctica.

If you decompose 18 grams of water, you will always get 16 grams of oxygen and 2 grams of hydrogen – never a different ratio. This fixed composition distinguishes compounds from mixtures, which can have varying proportions.

3. Homogeneous Nature

Compounds are completely uniform throughout. Every sample of a pure compound has identical composition and properties. A spoonful of sugar (C₁₂H₂₂O₁₁) from the top of a container has exactly the same sweetness and chemical composition as sugar from the bottom.

Similarly, sodium chloride crystals from Kaduna salt mines have the same properties as laboratory-prepared NaCl. You cannot visually distinguish different parts of a compound because it is uniform at the molecular level.

4. Chemical Methods Required for Separation

Unlike mixtures that can be separated by physical methods (filtration, evaporation, magnetic separation), compounds require chemical reactions to break apart their constituent elements. The bonds holding atoms together in compounds are strong and need significant energy input to break.

Examples of chemical separation methods:

• Electrolysis: Passing electricity through water breaks it into hydrogen and oxygen gases
• Thermal decomposition: Heating calcium carbonate (limestone) produces calcium oxide and carbon dioxide
• Chemical displacement: Adding carbon to copper oxide produces copper metal and carbon dioxide

You cannot separate water into hydrogen and oxygen by boiling, filtering, or any physical process. Only chemical methods work.

5. Formation Involves Energy Changes

When elements combine to form compounds, energy is either released (exothermic) or absorbed (endothermic). This energy change is always the same amount for a given compound.

• When hydrogen burns in oxygen to form water, 286 kJ of heat energy is always released per mole
• When sodium reacts with chlorine to form table salt, heat and light energy are released
• Breaking compounds apart requires the same amount of energy that was released during formation

6. Properties Differ from Constituent Elements

A compound has completely different physical and chemical properties from the elements that make it up. This is a crucial characteristic that helps identify compounds.

Classic examples:

• Water (H₂O): Hydrogen is a flammable gas, oxygen supports burning, but water extinguishes fires
• Sodium chloride (NaCl): Sodium is a soft, reactive metal that explodes in water; chlorine is a poisonous yellow-green gas; but their compound is safe table salt we eat daily
• Carbon dioxide (CO₂): Carbon is a black solid, oxygen is a colorless gas, but CO₂ is a colorless gas that turns limewater milky

7. Constant Melting and Boiling Points

Pure compounds always melt and boil at specific, fixed temperatures. Pure water always boils at exactly 100°C at sea level pressure and freezes at exactly 0°C. This constancy helps identify and verify the purity of compounds.

If your water sample boils at 102°C, it contains impurities and is no longer a pure compound. WAEC examiners often test this concept in practical chemistry.

Comparison: Compounds vs Elements vs Mixtures

Property	Compound	Element	Mixture
Composition	Two or more elements chemically combined	Only one type of atom	Two or more substances physically mixed
Formula	Has definite formula (H₂O, NaCl)	Represented by symbol (O, Na)	No specific formula
Separation method	Chemical methods only	Cannot be separated	Physical/mechanical methods
Properties	Different from elements	Unique to that element	Shows combined properties
Ratio of components	Fixed ratio by mass	Not applicable	Variable ratio
Energy change	Yes, during formation/decomposition	No	Usually no energy change
Nature	Homogeneous	Pure substance	Can be homogeneous or heterogeneous

Common Exam Mistakes

WAEC Chief Examiners report these frequent errors:

1. Confusing compounds with mixtures: Students write that compounds can be separated by physical methods or have varying composition. Remember: compounds need CHEMICAL separation and have FIXED composition.
2. Saying compounds have similar properties to their elements: Wrong! Sodium chloride (table salt) properties are completely different from sodium metal and chlorine gas. Always state that compound properties differ from constituent elements.
3. Not providing specific examples: When asked to “state” characteristics, students give vague answers. Use specific chemical formulas: H₂O, NaCl, CaCO₃, not just “compounds have formulas.”
4. Mixing up homogeneous and heterogeneous: Compounds are HOMOGENEOUS (uniform throughout). Don’t confuse this with heterogeneous mixtures.
5. Incomplete explanations: Writing “compounds have fixed composition” without explaining what this means. WAEC wants you to explain that the ratio of elements never changes.

Practice Questions

Multiple Choice Questions

1. Which of the following is NOT a characteristic of a compound?
   • It has a definite chemical formula
   • Its composition varies depending on the source
   • It is homogeneous in nature
   • Its constituent elements can only be separated by chemical means

   Answer: (b) ✓ – Compounds have FIXED composition that never varies.

2. Water can be represented by the formula H₂O. This shows that water is:
   • An element
   • A mixture
   • A compound
   • A solution

   Answer: (c) ✓ – Only compounds have definite chemical formulas.

3. Which statement is correct about the properties of sodium chloride (NaCl)?
   • It has the same properties as sodium metal
   • It has the same properties as chlorine gas
   • Its properties are completely different from both sodium and chlorine
   • Its properties are a mixture of sodium and chlorine properties

   Answer: (c) ✓ – Compound properties differ completely from constituent elements.

4. The fact that pure water always contains 88.9% oxygen and 11.1% hydrogen by mass illustrates which characteristic of compounds?
   • Homogeneous nature
   • Fixed and definite composition
   • Ability to be represented by a formula
   • Chemical separation requirement

   Answer: (b) ✓ – This is the Law of Definite Proportions.

Essay Questions

1. State FIVE characteristics of chemical compounds. (5 marks)

   Examiner’s tip: Use clear points. Don’t just list – briefly explain each characteristic. Give examples where possible.

   Sample answer:

   • Compounds have definite chemical formulas (e.g., H₂O for water)
   • They have fixed composition that does not vary
   • They are homogeneous in nature
   • Their constituent elements can only be separated by chemical means
   • Their properties are completely different from the constituent elements
2. (a) Explain why sodium chloride is classified as a compound and not a mixture. (3 marks)
   (b) Describe how you would separate sodium chloride into its constituent elements. (3 marks)
   (c) Compare the properties of sodium chloride with those of sodium and chlorine. (4 marks)

   Sample answer:

   (a) Sodium chloride is a compound because:

   • It has a definite chemical formula (NaCl)
   • Its composition is always fixed (equal parts sodium and chlorine atoms)
   • It can only be separated into sodium and chlorine by chemical means, not physical methods

   (b) Separation method:

   • Melt the sodium chloride at high temperature
   • Pass electricity through the molten salt (electrolysis)
   • Sodium metal forms at the negative electrode (cathode)
   • Chlorine gas is released at the positive electrode (anode)

   (c) Property comparison:

   • Sodium is a soft, silvery metal; chlorine is a yellow-green poisonous gas; sodium chloride is a white crystalline solid
   • Sodium reacts violently with water; chlorine dissolves in water; sodium chloride dissolves safely in water
   • Sodium is stored in kerosene; chlorine is stored in sealed containers; sodium chloride can be stored in open containers
   • The compound’s properties are completely different from both elements
3. A student claims that sugar solution is a compound because it is uniform throughout. Explain why this claim is incorrect. (4 marks)

   Examiner’s tip: Address both why the student might be confused and why the claim is wrong. Mention the key differences.

   Sample answer:

   • Sugar solution is uniform (homogeneous) but this alone does not make it a compound
   • It is a mixture of sugar and water that can be separated by physical means (evaporation)
   • The ratio of sugar to water can vary – you can make weak or strong solutions
   • It cannot be represented by a single chemical formula
   • Compounds must have fixed composition and require chemical methods for separation

Memory Aids

Acronym: CHEF-D

• Chemical separation needed (not physical)
• Homogeneous nature (uniform throughout)
• Energy changes during formation/decomposition
• Fixed composition (Law of Definite Proportions)
• Definite formula (H₂O, NaCl, etc.)

Remember: “Compounds are FIXED like marriage – same formula, same ratio, need chemistry to separate!”

Related Topics

• Definition of a Compound
• Definition of a Mixture
• Differences Between Compound and Mixture
• Definition of Element
• Methods of Separating Mixtures