Class 7 – Science – Notes – Chapter 6 – Physical and Chemical Changes

Definition of Physical Change

Physical properties refer to characteristics like shape, size, color, texture, and physical state (solid, liquid, gas) of a substance.
A physical change is one where a substance undergoes a change in these properties without any alteration in its chemical composition.

• 🔍 Example: The process of crystallization involves forming solid crystals from a dissolved substance, which is a physical transformation, not chemical.

What is a Physical Change?

When a substance experiences a change in its appearance, dimensions, or state of matter—but its internal composition remains unchanged—it undergoes a physical change.

• No new substance is created
• Usually reversible

Example: Ice melting into water. Though the form changes, it remains H₂O.

Definition of Chemical Change

In a chemical change, the internal structure of the substance is altered, resulting in the formation of entirely new substances with distinct properties.

• Atoms rearrange to form new combinations
• Generally irreversible
• Cannot be brought back to the original substance easily

 Conditions Required for a Chemical Change

Certain conditions must be fulfilled for a chemical transformation to occur:

1. Minimum activation energy to begin the reaction
2. Molecular collisions must occur with enough force and proper orientation
3. Sometimes require light, heat, pressure, or catalysts to start

 Types of Chemical Changes

Combination Reaction

Two or more substances merge to form a single, new substance.

• Example: Rusting of iron, burning wax

Decomposition Reaction

One substance breaks down into two or more different substances.

•  Example: Electrolysis of water → Hydrogen + Oxygen

Chemical Change

A chemical change results in the formation of one or more entirely new substances with properties different from the original substances. This is often called a chemical reaction.

•  Examples: Combustion, corrosion, digestion, rusting of iron

 Energy Involved in Chemical Changes

During chemical reactions:

• Energy is required to break bonds in the original substances.
• Energy is released when new bonds form in the products.

 Endothermic Reaction:

• Takes in energy
• Mixture becomes cooler

 Exothermic Reaction:

• Releases energy
• Mixture becomes warmer

 Chemical Change Needs Close Contact of Particles

For a chemical reaction to happen effectively:

• Reactant particles must be in close contact.
• The chance of collision increases, allowing atoms to rearrange and form new bonds.

Chemical Changes Requiring Energy

Some reactions do not proceed on their own and need energy input to start:

•  Example: Heating calcium carbonate causes it to break into calcium oxide and carbon dioxide.

 Burning Magnesium in Air

• Strip of magnesium is cleaned and brought near a flame.
• Burns with a bright white flame, forming a white ash (magnesium oxide).

Reaction:
2Mg + O₂ → 2MgO

• New substance formed
• Heat and light released

Detailed Example of a Chemical Change: Iron + Sulphur

• Iron is metallic, conducts electricity, and is magnetic.
• Sulphur is yellow, non-metallic, and does not conduct electricity.

When iron and sulphur are heated together:

• A new substance called iron sulphide is formed.
• Iron sulphide is black, not magnetic, and behaves differently from both elements.
• This demonstrates a complete chemical change.

 What Happens During a Chemical Change?

Key features:

• Energy is either absorbed or released
• Atoms are not created or destroyed, only rearranged
• Signs of chemical change include:
  • Change in smell
  • Change in color (e.g., silver to brown when iron rusts)
  • Change in temperature (heat absorbed or released)

What is Rusting?

Rusting is a form of slow chemical change involving iron reacting with oxygen and moisture in the air.

• Chemical Formula:
  4Fe + 3O₂ → 2Fe₂O₃
  (Iron + Oxygen → Iron Oxide)
• Result: Red-brown flaky coating (rust)

Conditions Needed for Rusting

Three main factors that promote rusting:

1. Presence of water (moisture) and oxygen
2. Impurities in iron, like salt or acids
3. High temperature accelerates corrosion

 Rusting of Iron – Chemical Reaction

In humid conditions:

4Fe + 3O₂ + xH₂O → 2Fe₂O₃·xH₂O
(Hydrated Iron(III) Oxide = Rust)

This is a slow, natural chemical change that weakens iron over time.

 Crystallization – A Physical Change

Crystallization is a process where a dissolved solid separates from a solution and forms well-defined solid crystals.

• Usually done by heating and slowly evaporating the solvent.
• Leaves behind pure crystals of the solute.

 Understanding Crystallization

Crystallization is the process where atoms or molecules organize into a structured, solid form.

It can occur:

• Naturally (e.g., snowflakes, mineral crystals)
• Artificially (e.g., growing sugar crystals)

 Protein Crystallization:

• Proteins can crystallize when certain salts are added.
• This is known as salting out.
• Conditions like pH, temperature, and concentration affect the success.
• Used in biotechnology and pharmaceuticals to study protein structure.