Physical And Chemical Changes
Introduction
- In our day to day life we come across many changes around us. For instance, dissolving sugar in water, burning of fuels, burning of coal or wood, setting of curd, stretched rubber band when left returns back to its original position, cooking of food and many more.
- In all these reactions the starting material is undergoing changes to either give new product or remain in its changed position.
- In fact with the passage of time our body is growing and we are becoming taller. This is also a change. Our body is undergoing changes.
What are changes?
- Any alteration of a substance from its original shape, size and state is known as a change. But the changes occurring around us can sometimes be reversed and sometimes cannot.
- Those changes which can be reversed are termed as reversible changes like rolling of roti, melting of ice, melting of wax.
- Whereas those which cannot be reversed are termed as irreversible changes like cooking food, baking cake, baking roti, tearing paper and many more.
The changes can again be classified into two type’s
namely physical and chemical changes as discussed below.
Physical change
- Physical changes refer to the changes in the physical properties of the substances such as shape, size, color and state. These changes are reversible in nature. In a physical change no new substance is formed.
- Tearing a piece of paper changes its shape and size.
- Similarly kneading of flour to dough changes its state.
- Melting of ice is also a physical change where the matter is changing its state from solid to liquid.
- Rolling of chapati/roti
- Crushed can
Chemical
change
- A change leading to the production of a new substance is known as a chemical change. These changes are irreversible. Some of the chemical changes occurring in our day to day life include rusting of iron, silver jewellery getting tarnished or copper articles getting covered by green layer.
- In case of rusting of iron, the iron reacts with the oxygen present in air and moisture and develops rust (hydrated iron (III) oxide).
Corrosion
causes damage to car bodies, bridges, iron railings, ships and to all objects
made of metals, especially those of iron.
- In case of rusting of copper, the metallic copper reacts with oxygen, carbon-dioxide and atmospheric moisture and develops a green colored coating of copper hydroxide and copper carbonate.
- In case of tarnishing of silver articles, the metallic silver reacts with hydrogen sulphide or sulphur present in air and gets tarnished.
- Burning of magnesium is another chemical change. When a ribbon of magnesium is brought near flame it starts burning with a brilliant white light leaving behind a powdery ash on complete burning. Magnesium burns in air to form magnesium oxide.
- On dissolving the ash in water it forms a new substance.
- Rotten fruit
Changes
occurring in a chemical change
Chemical
changes occurring in our day to day life involves following changes.
- Change in color
During
rusting the iron articles undergoes change in color. We have also observed that
silver gets tarnished when kept for a long time. The metallic silver reacts
with hydrogen sulphide or sulphur present in air and gets tarnished.
- Change in state
On
heating liquid changes its state and converts to vapour due to reaction within
the particles caused by heat.
- Change in odour
Due
to chemical reaction food gets spoiled and smells really bad.
- Change in temperature
Lime
when mixed with water undergoes certain change that rises its temperature and
makes it evolve heat and give a clear solution. Such reactions are called exothermic
reactions.
- Release of gas
During
Aerobic cellular respiration in human body energy molecules combines with
inhaled oxygen and releases energy needed by the cells. It also releases Carbon
Dioxide gas and water. Here is the overall equation for aerobic cellular
respiration:
Rusting
of iron
We
all must have observed newly bought iron, a silver or copper article appears
very shiny but with passage of time they get dull. This is due to the layer of
metal oxide that develops on their surface. Rusting of iron, silver jewellery getting
tarnished or copper articles getting covered by green layer. Metals react with
atmospheric oxygen and produces metal oxides that are basic in nature because
they react with water to form bases.
- In case of rusting of iron, the iron reacts with the oxygen present in air and moisture and develops rust (hydrated iron (III) oxide).
Prevention
of rusting of iron
Following
methods can be used to prevent corrosion:
- Galvanization: Method to protect steel and iron articles from rusting by coating them with a thin layer of zinc.
- Alloying: Homogeneous mixture of two or more metals, or a metal and a non-metal to change the properties of the pure state metals and protect them from rust.
For
instance, Iron is used widely for different purposes but it is never used in
pure state. It is due to the fact that pure iron is very soft and stretches in
hot state. But mixing with small amount of carbon (about 0.05%) makes it strong
and tough. Mixing nickel and chromium with iron gives stainless steel, which is
hard and does not rust.
Another
vital property of alloy is that electrical conductivity and melting point of an
alloy is less than that of pure metals. For instance, brass is an alloy of
copper and zinc (Cu and Zn), and bronze is an alloy of copper and tin (Cu and
Sn), possessing poor conductivity towards electricity. On the other hand copper
in pure state is used for making electrical circuits.
Solder
is an alloy of lead and tin (Pb and Sn) possessing low melting point and is
used for welding electrical wires together
Crystallization: A physical change
- The process of deriving large crystals of pure substances from their solutions is known as crystallization. It is a physical change. Crystals of salt are obtained by the evaporation of sea water.
- Similarly crystals of copper sulphate can be obtained by adding copper sulphate powder to the boiling solution of water and dilute sulphuric acid. This mixture when filtered and allowed to cool produces crystals of copper sulphate.
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