Describe the concept of matter (physical and chemical changes of matter)

A physical change is a change in the state or appearance of matter that does not produce a new substance. The original substance keeps its same chemical properties even after the change.

Characteristics of Physical Change

• No new substance is formed — the chemical composition stays the same
• Generally reversible — the original substance can be recovered easily
• Mass remains the same — no atoms are created or destroyed
• No significant heat change or gas emission — except during simple state changes like boiling

Examples of Physical Change

1. Melting ice: When ice (solid water) melts, it becomes liquid water. Both are still H₂O — no new substance is created.
2. Boiling water: Water changes to steam when heated, but it remains water in gaseous form.
3. Dissolving sugar in water: Sugar dissolves, but the sugar and water can be separated again by evaporating the water.
4. Tearing paper: The paper changes shape and size, but it is still paper.

Practical Activity

You can observe physical changes in your school laboratory:

• Place a few ice cubes in a beaker and observe them melt at room temperature. This demonstrates a change in state that is reversible.
• Dissolve a teaspoon of table salt in a glass of water. The salt disappears but can be recovered by evaporating the water.

A chemical change is a process that produces one or more new substances with completely different properties from the original substances. The original substances react to form new substances through chemical reactions.

Characteristics of Chemical Change

• New substances are formed — the chemical composition changes completely
• Generally irreversible — the change cannot be easily undone
• Significant temperature change — the reaction may release or absorb heat
• Change in color, smell, or appearance — new properties emerge
• Gas may be formed — bubbles or fumes may appear

Examples of Chemical Change

1. Burning a candle: The wax reacts with oxygen in the air to produce carbon dioxide, water vapor, and ash — all new substances.
2. Rusting of iron: Iron reacts with oxygen and moisture in the air to form iron oxide (rust), which is a brownish-red substance different from shiny iron.
3. Baking a cake: The heat causes chemical reactions between the ingredients, producing new substances like carbon dioxide gas (which makes the cake rise) and changing the color and texture.
4. Baking soda and vinegar reaction: When mixed, they produce carbon dioxide gas, water, and sodium acetate — completely new substances.

Practical Activity

Try these experiments in your laboratory:

• Rusting experiment: Place some clean iron nails in a test tube. Add some water so that half the nails are submerged. Leave the tube open to the air for several days. Observe the brown rust forming on the nails — this is a chemical change.
• Sublimation: Place a few iodine crystals in an evaporating basin and gently heat them. Watch the purple vapor rise and deposit on a cold surface like a watch glass placed above. This demonstrates sublimation (solid to gas directly).

Matter can change in two fundamental ways. In a physical change, the appearance or state of a substance changes, but it remains the same substance with the same chemical properties. In a chemical change, new substances with different properties are formed through a chemical reaction. Being able to distinguish between these two types of changes is a foundational skill in chemistry that helps us understand how substances interact and transform in the world around us.

In everyday life in Tanzania, you can observe both types of changes. When you dry your clothes in the sun, the water evaporates — this is a physical change because the water simply changes from liquid to gas, and it can condense back to liquid. However, when food is cooked over a jiko (charcoal stove), chemical changes occur in the ingredients, creating new flavors and textures that cannot be reversed. Understanding these changes helps farmers know how to store harvested maize properly to prevent rotting, which is a chemical change caused by microorganisms.