CBSE Notes Class 7 Science Chapter 5 Physical and Chemical Changes

In our everyday lives, we encounter various changes that substances undergo. These changes can be categorised into two main types: physical and chemical. Physical changes modify the appearance or form of a substance while keeping its chemical composition intact. Chemical changes involve a transformation that alters the chemical composition of a substance, leading to the formation of new substances.

1.0Physical and Chemical Changes: Introduction

You notice many changes in your surroundings daily, often involving one or more substances. For instance, when your mother asks you to dissolve sugar in water to make a cold drink, that’s a change. Similarly, setting curd from milk is another example of a change. When milk becomes sour, it represents a change as well. Even stretching a rubber band shows a change in its form.

These changes can be categorized into two main types: Physical and Chemical.

CBSE Class 7 Science Notes Chapter 5, Physical and Chemical Changes, will help students identify physical and chemical changes in various materials and processes. Students will also be able to explain why a change is classified as physical or chemical, including the underlying properties and behaviours of substances involved.

2.0Physical change

A physical change is when a substance's state or appearance changes without altering its chemical composition. 

No chemical reactions occur, and the substance's identity remains the same. 

Physical changes can involve state changes (like melting or freezing) or actions such as cutting or dyeing. Energy may be absorbed or released during the change, but the substance does not transform into a new material.

Characteristics of Physical Changes:

1. No new substance is formed: Physical changes do not create a new material; the original substance remains the same in its chemical composition.
2. Temporary and Reversible: Physical changes are generally temporary and can be reversed. The substance can return to its original form after the change.
3. Change in Appearance: A temporary change in physical properties like colour, size, or shape does not alter the substance’s identity.
4. Minimal Energy Change: Physical changes involve little energy; only a tiny amount of heat or other energy may be absorbed or released during the process.

Examples of Physical Changes:

• Cutting: When a large object like a rope or log is cut into smaller pieces, its shape and size change, but its chemical composition remains unchanged.
• Pouring: Transferring a liquid into smaller containers changes its distribution but not its identity or composition.

Physical Changes Involving Energy:

• Gases: Gases consist of fast-moving particles with high kinetic energy. They can expand or contract to fit a container, but their chemical composition remains unchanged. For example, heating air in a balloon causes it to expand, but it stays as air.

3.0Chemical Changes

Chemical changes, or chemical reactions, occur when substances react to form a new substance with different chemical properties.

A common example is the rusting of iron. Rust is a brownish substance that differs chemically from iron. This process is commonly seen in iron gates, tools, and other objects exposed to the atmosphere.

Some Chemical Change Activities:

1. Baking Soda and Vinegar Reaction:

• Reactants: Baking soda (NaHCO₃) and vinegar (acetic acid).
• Product: Carbon dioxide (CO₂), which turns lime water milky.
• (CO₂) + [Ca(OH)₂]  (CaCO₃) + Water (H₂O)
• (CO₂) + [Ca(OH)₂] (CaCO₃) + Water (H₂O)

2. Burning Magnesium Ribbon:

• Reactant: Magnesium ribbon.
• Product: Magnesium oxide (ash), a new substance.
• Magnesium (Mg) + Oxygen (O₂)  Magnesium oxide (MgO)

3. The reaction of Copper Sulphate and Iron:

• Reactants: Copper sulphate (CuSO₄) and iron.
• Products: Iron sulphate (FeSO₄) and copper.
• Copper sulphate solution (blue) + Iron  Iron sulphate solution (green) + Copper (brown deposit)
• CuSO4(aq)+Fe(s)→FeSO4(aq)+Cu(s) 

New substances are formed in each case, making these chemical changes irreversible.

Indicators of Chemical Changes:

• Heat or Light: Heat, light, or other forms of radiation (e.g., ultraviolet) may be released or absorbed during the reaction.
• Sound Production: Sounds may be produced due to the chemical change.
• Change in Smell: A change in odour or the release of a new smell can occur.
• Colour Change: There may be a noticeable change in colour during the reaction.
• Gas Formation: The formation of gas is often observed, indicating a chemical change has taken place.

4.0Rusting

Rusting is a chemical reaction where iron reacts with oxygen and water, forming rust (iron oxide, Fe₂O₃). 

The reaction can be represented as:

Iron (Fe) + Oxygen (O₂) + Water (H₂O) → Rust (Fe₂O₃)

Moist air and seawater accelerate rusting.

Prevention of Rusting:

1. Coating: Apply paint or lubricant to shield iron from moisture and oxygen.
2. Metal Coating: Metals like chromium or zinc are used to cover iron.
3. Galvanization: Coat iron with zinc to prevent rust.

The ozone layer protects against harmful UV rays from natural chemical processes involving UV light.

5.0Crystallization

Crystallization is the process of obtaining large, pure crystals from a solution. When evaporating, seawater yields salt, but the salt obtained is often impure and consists of tiny crystals.

In contrast, crystallization involves cooling a hot, concentrated solution, allowing pure crystals to form.

For example, impure copper sulfate powder can be purified through crystallization, resulting in large, pure copper sulfate crystals. This method effectively separates pure solid substances from impurities and is considered a physical change.