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Introduction
In everyday life, substances undergo changes when exposed to different conditions such as heating, cooling, or mixing with other substances. These changes can be classified as temporary (reversible or physical) or permanent (irreversible or chemical). Understanding these changes helps us differentiate between materials that can return to their original state after a change and those that form new materials permanently.
Permanent and Temporary Changes to Substances
Permanent and Temporary Changes to Substances
Chemical or Permanent Changes (Irreversible Changes)
When a substance undergoes a permanent (irreversible) change, new materials or substances are formed, and the original substance cannot be recovered in its original form. These changes often occur when substances are heated or burned.
Examples of Permanent Changes:
Burning of Wood:
When wood is burned, it reacts with oxygen in the air to form new substances such as carbon dioxide, water vapor, and ash. The original wood cannot be recovered after burning.the main component of wood).Rusting of Iron:
When iron reacts with oxygen and moisture over time, it forms iron oxide, commonly known as rust. This process is irreversible, and the original iron cannot be restored easily.Cooking of an Egg:
When an egg is cooked, the heat causes the proteins in the egg white and yolk to undergo a chemical change, solidifying the egg. The change is permanent, and the egg cannot be returned to its raw state.Baking Bread:
When dough is baked, the heat causes it to rise and solidify, forming bread. The chemical reactions that occur during baking make it impossible to return the bread to its dough form.
Characteristics of Permanent Changes:
A new substance is formed.
The change cannot be easily reversed.
Energy is usually released or absorbed.
The properties of the material change permanently.
Physical changes or Temporary Changes (Reversible Changes)
A temporary (reversible) change occurs when a substance changes form or appearance, but no new substances are formed. The substance can return to its original form when the conditions causing the change are reversed.
Examples of Temporary Changes:
Melting of Ice:
When ice is heated, it melts and turns into water. However, this change is reversible because when water is cooled, it can freeze back into iceEvaporation of Water:
Water evaporates into water vapor when heated but can condense back into liquid water when cooled. This is a reversible change.Dissolving Salt in Water:
When salt is dissolved in water, it forms a salt solution. This change is reversible because the salt can be recovered by evaporating the water.Stretching a Rubber Band:
When a rubber band is stretched, it temporarily changes shape. However, when the force is released, the rubber band returns to its original shape. This is an example of a reversible physical change.
Characteristics of Temporary Changes:
No new substance is formed.
The change can be reversed.
Usually, there is no permanent change in the properties of the material.
Practical Examples of Permanent and Temporary Changes
Practical Examples of Permanent and Temporary Changes
Heating Sugar:
Temporary Change: When sugar is heated gently, it may dissolve in water, and the solution can be evaporated to recover the sugar.
Permanent Change: When sugar is heated strongly, it caramelizes and turns brown, forming new substances. This is irreversible.
Ironing a Shirt:
Temporary Change: When the fabric of a shirt is ironed, the wrinkles disappear due to the heat, but if the shirt is crumpled again, wrinkles return. No new substance is formed.
Burning a Candle:
Permanent Change: When a candle burns, the wax reacts with oxygen to produce carbon dioxide, water vapor, and heat. This is a permanent change because the wax is converted into new substances.
Temporary Change: When the candle melts due to heat, the wax turns from solid to liquid, but when cooled, the wax solidifies again, making this a reversible change.
Summary of the topic Key Concepts:
Physical Changes:
In a physical change, the substance’s form or state is altered without changing its chemical composition.
Reversible Nature: Most physical changes are temporary and reversible. For example, melting, freezing, condensation, and boiling are physical changes.
Example: Melting ice into water. When ice melts, it changes from a solid to a liquid, but both states are chemically the same (H₂O).
Chemical Changes:
A chemical change involves a chemical reaction that produces one or more new substances with different properties.
Irreversible Nature: Most chemical changes are permanent and irreversible. Examples include burning, rusting, and cooking.
Example: Burning wood to form ash, carbon dioxide, and water vapor.
Temporary (Reversible) Changes:
Some changes, especially physical changes, are reversible, meaning the substance can return to its original state without altering its chemical makeup.
Example: Dissolving salt in water. The water can be evaporated to recover the salt.
Permanent (Irreversible) Changes:
Permanent changes, particularly chemical reactions, cannot easily be undone. New substances are formed with different properties from the original substances.
Example: Rusting of iron. Once iron rusts, it forms a new compound (iron oxide), and the process cannot be reversed by simple means.
Effects of Heating on Substances:
Heating a substance can result in either a physical or chemical change.
Physical Change: Melting, evaporation, and sublimation (e.g., ice to water vapor).
Chemical Change: Decomposition or combustion (e.g., burning paper).
Revision Questions
Revision Questions
Question 1:
a) Define a physical change and give two examples.
b) Explain why freezing water is considered a reversible physical change.Question 2:
a) What is a chemical change?
b) List two examples of chemical changes that occur when substances are heated.Question 3:
a) Differentiate between reversible and irreversible changes.
b) Classify the following as either reversible or irreversible changes:
i) Melting of ice
ii) Burning of wood
iii) Rusting of iron
iv) Dissolving sugar in waterQuestion 4:
a) Explain what happens when iron is exposed to moisture and oxygen over time.
b) Is this a physical or chemical change? Justify your answer.Question 5:
a) Describe what happens when baking soda is heated with vinegar.
b) Is this an example of a physical or chemical change? Explain why.
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