When granulated sugar is first heated, it begins to melt. Melting a sugar cube is a physical change because the substance is still sugar. Burning a sugar cube is a chemical change. Fire activates a chemical reaction between sugar and oxygen.

Chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. These processes are called chemical reactions and, in general, are not reversible except by further chemical reactions.

A yellow banana with a few brown spots is perfectly normal, but extensive browning or mold on the skin (especially near the stem) may mean that it’s best not to eat. If you aren’t lucky, the banana may have already started to rot inside.

A change which cannot happen backward, that is, it cannot be reversed is called an irreversible change. When you burn a piece of paper, it turns to ash. It cannot become paper again. Your height cannot decrease.

In a physical change the appearance or form of the matter changes but the kind of matter in the substance does not. However in a chemical change, the kind of matter changes and at least one new substance with new properties is formed. The distinction between physical and chemical change is not clear cut.

There are five signs of a chemical change:

• Color Change.
• Production of an odor.
• Change of Temperature.
• Evolution of a gas (formation of bubbles)
• Precipitate (formation of a solid)

Chemical Changes are also called Chemical Reactions. The chemical reaction produces a new substance with new and different physical and chemical properties. Matter is never destroyed or created in chemical reactions. The particles of one substance are rearranged to form a new substance.

Adding or removing energy from matter causes a physical change as matter moves from one state to another. Physical changes can also be caused by motion and pressure. Melting and freezing. When heat is applied to a solid, its particles begin to vibrate faster and move farther apart.

it is irreversible as sour milk can’t be turned to normal milk again. the souring of the milk is an irreversible change …..

Souring milk results in curdling, a physical reaction. Yet, chemically it changes because of increase acidity. The proteins become denatured in the process and acid levels increase changing the chemical composition.