Physical and chemical changes of matter

Physical change

A physical change is a change in the state or appearance of matter that does not involve the formation of a new substance. The substance retains its original properties even after the change.

Characteristics of physical change

1. No new substance is formed: The substance undergoes a change, but its chemical composition remains the same. For example, ice melts into water, but both are still $H_2O$.
2. It is generally reversible: Most physical changes can be undone or reversed. For instance, when water freezes to form ice, it can be melted back into water without any chemical change.
3. The mass of a substance does not change: Physical changes do not affect the mass. For example, when a piece of paper is torn, the total mass remains the same, even though the size and shape have changed.
4. No significant heat change or emission of gas: Physical changes do not involve the release or absorption of large amounts of heat, nor do they usually produce gas. An example is dissolving salt in water, which doesn't involve any noticeable heat change or gas emission.

Examples of physical change

1. Melting of ice: Ice changes from solid to liquid water, but no new substance is formed.
2. Boiling water: Water changes from liquid to gas (steam), but it remains $H_2O$ in all forms.
3. Dissolving sugar in water: Sugar dissolves, but the composition of sugar and water remains unchanged. The sugar can be recovered by evaporating the water.

Chemical change

A chemical change is a process that results in the formation of one or more new substances with different properties. The original substances undergo a chemical reaction to create a new substance.

Characteristics of chemical change

1. New substances are formed: Chemical reactions involve the breaking and forming of bonds, creating new substances with different properties. For instance, when hydrogen reacts with oxygen, water ($H_2O$) is formed, which has entirely different properties from both hydrogen and oxygen.
2. Irreversible process: Chemical changes are typically not reversible by simple physical means. For example, when food is cooked, it cannot be turned back into its original raw state.
3. Considerable change in temperature (either rise or fall): Chemical reactions often release or absorb energy in the form of heat. For example, burning a match produces heat and light (exothermic reaction).
4. Change in color, smell, or emission of light: Chemical changes can involve the formation of substances that have different colors, smells, or even emit light. For example, when iron rusts, it changes color from shiny silver to reddish-brown.
5. Gas may be formed: Many chemical reactions produce gases. For example, when baking soda reacts with vinegar, carbon dioxide gas is released.

Examples of chemical change

1. Burning of wood: Wood reacts with oxygen to form ash, carbon dioxide, and water vapor. This process is irreversible and releases significant heat and light.
2. Rusting of iron: When iron reacts with oxygen in the air, it forms iron oxide (rust). This process changes the properties of the metal, and it cannot be reversed.
3. Baking a cake: The heat causes ingredients to react and form new substances like gases (carbon dioxide) and changes the color and texture of the batter, making it different from the original ingredients.
4. Decomposition of hydrogen peroxide: Hydrogen peroxide breaks down into water and oxygen gas when exposed to light or heat. The formation of gas and new substances makes it a chemical change.

Summary of differences

Physical Change	Chemical Change
No new substance is formed.	New substance(s) are formed.
Generally reversible.	Irreversible under normal conditions.
No significant temperature change.	Significant temperature change, rise or fall.
No formation of gas (usually).	Gas may be formed.
Mass remains the same.	Mass may change (due to formation of new substances).