Chemical Composition of Matter - Study Notes

Key Concepts

Elements, Compounds, and Mixtures

Elements:

The simplest form of matter that cannot be broken down into simpler substances by chemical methods
Made up of only one type of atom
Examples: oxygen (O), carbon ©, iron (Fe), gold (Au), hydrogen (H)
There are 118 known elements, organized in the Periodic Table
Each element has a unique chemical symbol (1 or 2 letters)
Elements can be metals (e.g., copper, aluminium), non-metals (e.g., oxygen, sulfur), or metalloids (e.g., silicon)

Compounds:

Pure substances formed when two or more different elements chemically combine in a fixed ratio
Have completely different properties from the elements that form them
Can only be separated into elements by chemical methods (not physical methods)
Represented by chemical formulae showing the elements and their ratios
Examples:
- Water (H₂O) - 2 hydrogen atoms + 1 oxygen atom
- Carbon dioxide (CO₂) - 1 carbon atom + 2 oxygen atoms
- Sodium chloride (NaCl) - 1 sodium atom + 1 chlorine atom
- Glucose (C₆H₁₂O₆) - 6 carbon + 12 hydrogen + 6 oxygen atoms

Mixtures:

Two or more substances (elements or compounds) mixed together but not chemically combined
Components retain their individual properties
No fixed composition - can be mixed in any proportion
Can be separated by physical methods (filtration, evaporation, distillation, chromatography, use of magnets)
Types of mixtures:
- Homogeneous mixtures (uniform composition throughout): salt solution, air, brass
- Heterogeneous mixtures (non-uniform composition): sand and water, oil and water, salad

Atoms and Molecules

Atoms:

The smallest particle of an element that can exist
Cannot be divided into smaller particles by chemical means
All matter is made up of atoms
Atoms of the same element are identical
Atoms of different elements have different properties
Structure: Contains a nucleus (protons + neutrons) surrounded by electrons
Represented by element symbols: H (hydrogen atom), O (oxygen atom), Fe (iron atom)

Molecules:

Two or more atoms chemically bonded together
Can be atoms of the same element or different elements
The smallest particle of a substance that can exist independently and retain its chemical properties
Types:
- Element molecules: Atoms of the same element bonded together
  - O₂ (oxygen molecule - 2 oxygen atoms)
  - H₂ (hydrogen molecule - 2 hydrogen atoms)
  - N₂ (nitrogen molecule - 2 nitrogen atoms)
  - O₃ (ozone - 3 oxygen atoms)
- Compound molecules: Atoms of different elements bonded together
  - H₂O (water molecule)
  - CO₂ (carbon dioxide molecule)
  - CH₄ (methane molecule)

Particle Theory:

All matter is made up of tiny particles (atoms, molecules, or ions)
These particles are in constant motion
Particles in solids vibrate in fixed positions
Particles in liquids move freely but stay close together
Particles in gases move rapidly and are far apart

Physical vs Chemical Changes

Physical Changes:

Changes in the physical properties of a substance (size, shape, state)
No new substance is formed
The chemical composition remains the same
Usually easily reversible
Examples:
- Changes of state: melting, freezing, boiling, condensation, sublimation
- Dissolving sugar in water
- Breaking glass
- Cutting paper
- Mixing sand and salt
- Magnetizing iron
Energy changes are usually small

Chemical Changes:

One or more new substances with different properties are formed
The chemical composition changes
Usually difficult or impossible to reverse
Also called chemical reactions
Examples:
- Burning wood or paper
- Rusting of iron
- Cooking an egg
- Digestion of food
- Photosynthesis in plants
- Fermentation of sugar to alcohol
Energy changes are usually large (heat, light released or absorbed)

Evidence of Chemical Changes:

Formation of a new substance with different properties
Colour change
Gas produced (bubbles, fizzing)
Energy change (heat or light given out or absorbed)
Formation of a precipitate (solid forming in a liquid)
Permanent change that is difficult to reverse

Important Definitions

Element: A pure substance that contains only one type of atom and cannot be broken down into simpler substances by chemical methods.

Compound: A pure substance formed when two or more different elements are chemically combined in a fixed ratio.

Mixture: Two or more substances mixed together but not chemically combined, where each substance retains its own properties.

Atom: The smallest particle of an element that can exist and still retain the properties of that element.

Molecule: Two or more atoms chemically bonded together, which is the smallest particle of a substance that can exist independently.

Physical change: A change in which no new substance is formed; only the physical properties (such as size, shape, or state) change.

Chemical change: A change in which one or more new substances with different properties are formed; also called a chemical reaction.

Homogeneous mixture: A mixture that has uniform composition throughout, where the components cannot be easily distinguished.

Heterogeneous mixture: A mixture that has non-uniform composition, where the different components can be seen or easily distinguished.

Chemical formula: A representation of a compound using chemical symbols and numbers to show which elements are present and in what ratio.

Pure substance: A substance that contains only one type of element or compound, with no other substances mixed in.

Precipitate: A solid substance that forms in a liquid during a chemical reaction.

Diagrams and Structures

Diagram 1: Particle Arrangement in Elements, Compounds, and Mixtures

Element (e.g., oxygen gas O₂):

Draw several pairs of circles joined together
Each pair represents an oxygen molecule (O₂)
Label: “Element - same atoms bonded together”
All particles should look identical

Compound (e.g., water H₂O):

Draw molecules with one larger circle (O) bonded to two smaller circles (H)
Each molecule looks identical to the others
Label: “Compound - different atoms chemically bonded in fixed ratio”
Use different colours or shading for different atoms

Mixture (e.g., air):

Draw different types of molecules scattered randomly
Include pairs of circles (O₂), pairs of different circles (N₂), and single circles (Ar)
Label: “Mixture - different substances mixed but not chemically bonded”
Show variety in particle types

Diagram 2: Atom vs Molecule

Single Atom:

Draw one circle
Label: “Atom - smallest particle of an element”
Example: He (helium atom)

Molecule of Element:

Draw two identical circles joined together
Label: “Molecule - atoms bonded together”
Example: O₂ (oxygen molecule)

Molecule of Compound:

Draw different circles bonded together (e.g., 2 small + 1 large)
Label: “Compound molecule - different atoms bonded”
Example: H₂O (water molecule)

Diagram 3: Physical vs Chemical Change

Physical Change (e.g., ice melting):

Draw: Left side - solid arrangement (particles close, ordered)
Arrow labeled “HEAT”
Draw: Right side - liquid arrangement (particles close, disordered)
Label: “H₂O molecules unchanged - only arrangement changes”

Chemical Change (e.g., water forming from hydrogen and oxygen):

Draw: Left side - H₂ molecules (pairs) and O₂ molecules (pairs) separate
Arrow labeled “REACTION”
Draw: Right side - H₂O molecules (different structure)
Label: “New substance formed - chemical bonds broken and formed”

Diagram 4: Separation of Mixtures vs Compounds

Separating a Mixture (sand and salt):

Draw a flowchart:
1. Sand + salt mixture → Add water
2. Sand (doesn’t dissolve) / Salt solution → Filter
3. Sand on filter paper / Salt solution → Evaporate water
4. Sand (collected) / Salt (collected)
Label: “Physical methods separate mixtures”

Breaking Down a Compound (water):

Draw: Water (H₂O) → Electric current (electrolysis) → Hydrogen gas (H₂) + Oxygen gas (O₂)
Label: “Chemical method needed to separate compounds”

Worked Examples

Example 1: Classifying Substances

Question: Classify the following as element, compound, or mixture: a) Sodium chloride (NaCl) b) Iron (Fe) c) Salt water d) Carbon dioxide (CO₂) e) Air

Solution:

a) Sodium chloride (NaCl) = Compound

Contains two different elements: sodium (Na) and chlorine (Cl)
Chemically combined in fixed ratio (1:1)
Has chemical formula

b) Iron (Fe) = Element

Contains only one type of atom
Cannot be broken down into simpler substances
Has chemical symbol only (not formula)

c) Salt water = Mixture

Contains salt (compound) and water (compound) mixed together
Not chemically combined
No fixed ratio - can have more or less salt
Can be separated by physical method (evaporation)

d) Carbon dioxide (CO₂) = Compound

Contains two different elements: carbon © and oxygen (O)
Chemically combined in fixed ratio (1:2)
Has chemical formula

e) Air = Mixture

Contains several gases (nitrogen, oxygen, carbon dioxide, argon, etc.)
Not chemically combined
Variable composition
Can be separated by physical methods (fractional distillation)

Example 2: Atoms vs Molecules

Question: State whether each of the following represents an atom or molecule, and identify if it’s an element or compound: a) O b) O₂ c) H₂O d) Cu e) CO₂

Solution:

a) O = Atom of an element

Single oxygen atom
Element because it contains only one type of atom

b) O₂ = Molecule of an element

Two oxygen atoms bonded together
Element because all atoms are the same type

c) H₂O = Molecule of a compound

Three atoms bonded together (2 hydrogen + 1 oxygen)
Compound because it contains different types of atoms

d) Cu = Atom of an element

Single copper atom
Element because it contains only one type of atom

e) CO₂ = Molecule of a compound

Three atoms bonded together (1 carbon + 2 oxygen)
Compound because it contains different types of atoms

Example 3: Physical vs Chemical Change

Question: Identify whether each change is physical or chemical, and give a reason:

a) Ice melting to water b) Burning magnesium in air c) Iron rusting d) Sugar dissolving in tea e) Frying an egg

Solution:

a) Physical change

Reason: Ice (H₂O) changes to water (H₂O) - same substance, only state changes
No new substance formed
Reversible by freezing

b) Chemical change

Reason: Magnesium reacts with oxygen to form magnesium oxide (new substance)
Magnesium + oxygen → magnesium oxide
Properties change (shiny metal → white powder)
Heat and light produced
Irreversible

c) Chemical change

Reason: Iron reacts with oxygen and water to form rust (iron oxide)
New substance with different properties formed
Colour changes (grey → reddish-brown)
Irreversible

d) Physical change

Reason: Sugar molecules remain unchanged
No new substance formed
Can get sugar back by evaporating water
Reversible

e) Chemical change

Reason: Proteins in egg undergo permanent change
New substances formed
Colour and texture change permanently
Cannot be changed back to raw egg
Irreversible

Common Mistakes to Avoid

Confusing elements with compounds
- Mistake: Thinking O₂ is a compound because it has a subscript
- Correction: O₂ is still an element because it contains only oxygen atoms; compounds must have different types of atoms
Mixing up atoms and molecules
- Mistake: Calling O₂ an atom
- Correction: O₂ is a molecule (two atoms bonded together); O is an atom
Thinking mixtures have fixed compositions
- Mistake: Saying mixtures always have the same ratio of components
- Correction: Mixtures can have any proportion of components; compounds have fixed ratios
Confusing physical and chemical changes
- Mistake: Saying dissolving salt is a chemical change
- Correction: Dissolving is physical - you can get the salt back by evaporation; no new substance forms
Using incorrect chemical formulae
- Mistake: Writing water as HO or H2O2
- Correction: Water is H₂O (subscripts must be correct)
Thinking all mixtures are homogeneous
- Mistake: Not recognizing heterogeneous mixtures
- Correction: Mixtures can be homogeneous (uniform) or heterogeneous (non-uniform)
Forgetting that energy changes indicate chemical reactions
- Mistake: Not recognizing burning or cooking as chemical changes
- Correction: Large energy changes (heat, light) usually indicate chemical changes
Confusing separation methods
- Mistake: Saying you can separate compounds by filtration
- Correction: Compounds need chemical methods; mixtures use physical methods
Not understanding the difference between pure substances and mixtures
- Mistake: Thinking air is a pure substance
- Correction: Air is a mixture of gases; pure substances are elements or compounds only
Misidentifying changes of state
- Mistake: Thinking boiling water is a chemical change because bubbles form
- Correction: Boiling is physical - water vapour is still H₂O

Exam Tips

Keywords to Use in Answers

For Elements:

“Contains only one type of atom”
“Cannot be broken down by chemical methods”
“Pure substance”
“Found in the Periodic Table”

For Compounds:

“Two or more different elements”
“Chemically combined”
“Fixed ratio”
“Different properties from the elements”
“Separated by chemical methods only”

For Mixtures:

“Not chemically combined”
“Components retain their properties”
“No fixed composition/ratio”
“Separated by physical methods”

For Physical Changes:

“No new substance formed”
“Only physical properties change”
“Reversible”
“Small energy changes”
“Same chemical composition”

For Chemical Changes:

“New substance(s) formed”
“Different properties”
“Irreversible” or “difficult to reverse”
“Chemical composition changes”
“Large energy changes”

Mark-Earning Phrases

When explaining why something is an element: “It contains only one type of atom”
When explaining why something is a compound: “It contains two or more different elements chemically combined in a fixed ratio”
When explaining separation: “Physical methods for mixtures, chemical methods for compounds”
When identifying chemical changes: “Evidence: colour change/gas produced/heat released/precipitate formed/new substance formed”
When comparing atoms and molecules: “An atom is the smallest particle of an element, while a molecule is two or more atoms bonded together”

Answering Strategies

Classification Questions:
- Always give a reason with your classification
- Check if it’s pure (element/compound) or mixed (mixture)
- Look for chemical formulae to identify compounds
Physical vs Chemical Change Questions:
- Look for evidence of new substances
- Check if it’s reversible
- Mention specific observations (colour change, gas, heat, etc.)
Separation Methods:
- Identify if it’s a mixture or compound first
- For mixtures: suggest physical methods with reasons
- For compounds: state that chemical methods are needed
Formula Interpretation:
- Count the total number of atoms
- Identify how many different elements
- State the ratio of elements
Drawing Diagrams:
- Use different sizes or colours for different atoms
- Show bonding clearly for molecules
- Label all parts properly
- Include a key if needed

Quick Summary

Elements, Compounds, and Mixtures:

✓ Elements contain one type of atom only; cannot be broken down chemically
✓ Compounds contain two or more different elements chemically combined in fixed ratios
✓ Mixtures contain substances mixed but not chemically combined; variable composition
✓ Compounds can only be separated by chemical methods; mixtures by physical methods

Atoms and Molecules:

✓ Atoms are the smallest particles of elements
✓ Molecules are two or more atoms bonded together
✓ Element molecules contain same type of atoms (O₂, H₂, N₂)
✓ Compound molecules contain different types of atoms (H₂O, CO₂)

Physical Changes:

✓ No new substance formed; same chemical composition
✓ Usually reversible; small energy changes
✓ Examples: changes of state, dissolving, cutting, breaking

Chemical Changes:

✓ New substance(s) formed with different properties
✓ Usually irreversible; large energy changes
✓ Evidence: colour change, gas produced, heat/light released, precipitate formed
✓ Examples: burning, rusting, cooking, digestion, reactions

Key Skills:

✓ Classify substances as element, compound, or mixture with reasons
✓ Distinguish between atoms and molecules from chemical formulae
✓ Identify physical and chemical changes from observations
✓ Suggest appropriate separation methods for mixtures vs compounds