التغيرات الكيميائية والفيزيائية

Study Notes

Introduction to Chemical and Physical Changes

Chemical changes involve a transformation of substances into new substances with different properties.
Physical changes alter the state or form of a substance without changing its chemical composition.

Identifying Physical Changes

Changes in state (e.g., melting, freezing, vaporization, condensation, sublimation):
- These changes involve a rearrangement of molecules but do not create new substances.
- Example: Ice melting into water.
Changes in shape or size (e.g., cutting, bending, crushing):
- These changes don't alter the chemical makeup of the substance.
- Example: Crushing a piece of chalk.
Dissolving:
- When a substance dissolves in a solvent, its particles disperse throughout the solvent.
- Example: Sugar dissolving in water.
Changes in physical properties (e.g., density, color, texture, odor):
- These changes only affect the physical appearance and do not change the chemical identity of the substance.
- Example: Iron rusting. Although the color changes, the chemical formula has changed creating a new substance. Important to note Physical changes can change a substance's appearance, but do not alter its chemical makeup.
Filtration, distillation, and chromatography are common physical separation techniques.

Identifying Chemical Changes

Formation of a precipitate (a solid formed from a solution):
- When two solutions are mixed, a solid may form and settle out.
- Example: Mixing silver nitrate and sodium chloride solutions forming silver chloride precipitate.
Evolution of gas:
- The production of bubbles or gases as a result of reaction.
- Example: Baking soda and vinegar reacting.
Color change:
- A change in colour often accompanied by a change in the chemical makeup of the substance.
- Example: Iron rusting.
Change in temperature:
- A change in temperature can indicate a chemical reaction, often accompanied by either heat loss or heat production.
- Example: Combustion (fire) releases heat energy.
Formation of a new substance with different properties:
- Chemical reactions create substances such as compounds or elements with new properties, different from that of the original substances.
- Example: Burning wood; the wood (cellulose) changes to new substances like ash and other byproducts/gases.

Examples of Physical Changes

Ice melting (water to liquid)
Water boiling (liquid to gaseous state)
Bending a wire
Cutting wood
Dissolving salt in water

Examples of Chemical Changes

Burning wood
Rusting of iron
Baking a cake (new substances are formed)
Digesting food in the stomach (enzymes alter food molecules to create new ones)
Formation of a cloud from water vapor
Souring milk
Fireworks exploding
Cooking an egg. The albumen in the egg changes structure.
Taking medication. The medicine changes how the body functions.
Photosynthesis: This is a chemical process. Plants use sunlight, carbon dioxide, and water to create glucose and oxygen. Glucose and oxygen are new substances formed from the initial reactants.

Distinguishing Between Physical and Chemical Changes

Chemical changes result in new substances with new properties.
Physical changes do not result in new substances.

Importance of Understanding Chemical and Physical Changes

Identifying chemical and physical changes is essential in various fields of science, engineering, and daily life.
Understanding these changes helps predict the outcome of different procedures and processes.
Everyday activities (cooking, cleaning, etc.) frequently involve both kinds of changes. You will also find these changes at play in processes at work like manufacturing and construction, and even on a larger scale like environmental effects.

Key Differences Summarized

Feature	Physical Change	Chemical Change
Substance	No new substance formed	New substance(s) formed
Composition	Composition remains same	Composition changes
Properties	Properties may change (color, shape, texture)	Properties change significantly (different odor, bubbling, colors, etc)
Reversibility	Often reversible	Often irreversible (some exceptions)

Description

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