Key Points

A physical change alters properties like shape, size, or state of a substance, but no new substance is formed. Examples include melting ice, chopping vegetables, and folding paper.

A chemical change results in the formation of one or more entirely new substances through a chemical reaction. These changes are often difficult to reverse.

Changes that can be returned to their original state are reversible, like melting ice and freezing it back. Changes that cannot be undone are irreversible, such as burning paper into ash.

Rusting is a chemical change where iron reacts with oxygen and moisture from the air to form a new flaky, brown substance called iron oxide (rust).

When vinegar (acetic acid) is mixed with baking soda (sodium hydrogen carbonate), a chemical reaction occurs that produces carbon dioxide gas ($\text{CO}_2$), causing fizzing and bubbling.

To test for carbon dioxide ($\text{CO}_2$), the gas is passed through lime water (a solution of calcium hydroxide). The lime water turns milky, indicating a chemical reaction has occurred.

The reaction is: Calcium hydroxide + Carbon dioxide $\rightarrow$ Calcium carbonate + Water. The milky appearance is due to the formation of insoluble calcium carbonate.

Combustion is a chemical process in which a substance reacts rapidly with oxygen to produce heat and/or light. Substances that undergo combustion are called combustible substances, like wood or paper.

For combustion to occur, three components are necessary: a combustible substance (fuel), a continuous supply of oxygen, and heat sufficient to reach the ignition temperature.

The ignition temperature is the lowest temperature at which a combustible substance catches fire and begins to burn. A substance will not burn below this temperature.

When a magnesium ribbon is burnt, it undergoes a chemical change, reacting with oxygen to form a new white powder called magnesium oxide ($\text{MgO}$) while producing bright light and heat.

The burning of a candle involves both types of changes. The melting of solid wax into liquid wax is a physical change, while the burning of the wax vapor is a chemical change.

Weathering is the breakdown of rocks into smaller pieces, which can be a physical change (from temperature or ice) or a chemical change (from reactions with water), leading to soil formation.

Changes can be classified based on their effects. Desirable changes are useful, such as the ripening of fruits, while undesirable changes are harmful, such as the spoiling of food or rusting of iron.