oxygen

3. Remember:
 Physical change occurs when materials undergo change in physical
characteristics such as size, shape, texture without changing the
composition of a substance. The application of heat, force and
pressure can cause a material to undergo physical change.
Chemical change occurs when the original properties of materials
are lost and there are new materials formed. Application of heat
can cause a material to undergo chemical change. This process is
irreversible.
 Application of heat may change the materials physically and
chemically.

4.  Materials can also change chemically when there
is presence or lack of oxygen.
Oxygen is a part of the air that surrounds us. It can
cause changes on different materials and food
inside our body. Digested food is metabolized or
used up in the body by combining with oxygen. This
process releases energy from the food so that the
body can use it for biological processes or for work.

5. People and animals use oxygen in respiration. As oxygen is inhaled,
some materials inside the body combine with it and undergo a
chemical reaction that gives off carbon dioxide, water and energy.
 Apples and other fruits turn brown once the cut surface is exposed to
air due to a reaction with oxygen in the surrounding air, a chemical
reaction called oxidation.
The chemical reaction that occurs in burning is also called oxidation
or combustion. Oxidation may be slow or fast. The rusting of metal is
slow oxidation. Burning or combustion is a form of fast oxidation.