Physical and Chemical Changes

IMPORTANT TERMS

Compound                   A substance that is formed when two or more elements combine chemically.

Mixture                        A mixture consists of two or more substances simply mixed together, but not chemically combined.

Rusting                        Depositing of a reddish brown layer on iron objects is known as rusting.

Crystallization               The process by which an impure compound is converted into its crystals.

Galvanization               The process of applying zinc coating to iron objects to prevent them from rusting.

Alloying                        The process of combining a metal with one or more other metals or non-metals.

Exothermic Reaction    Chemical reaction during which heat energy is released.

Endothermic Reaction  Chemical reaction in which heat is absorbed by the substances.

 

PHYSICAL CHANGE

 

A change, in which only physical properties of a substance change and no new substance is formed is known as physical change. Physical properties include shape, size and colour of a substance. These changes lead to a change in the shape, size and state of a substance but its chemical composition remains the same.

 

Characteristics of Physical Change

• Physical changes are generally reversible.
• Physical changes are temporary in nature.
• New substance is not formed during physical change.
• Physical changes do not change the chemical properties of the substance. The composition of the matter remains same.

 

Examples of Physical Change

Dissolving of sugar in water is an example of physical and reversible change. When sugar is dissolved in water, no new substance is formed and the properties of sugar do not change. When the sugar solution is boiled, the water evaporates and a white residue is obtained that has all the properties of sugar.

Melting of ice is a physical and reversible change, as in this case, only the state of water changes from solid to liquid. The properties of water remain unaltered. Similarly, when water is heated for some time, it starts boiling and the liquid starts to evaporate and changes into steam. This is a physical change and can be reversed as when the steam will again condense into water.

 

Cutting of wood, melting of wax, stretching of rubber band, melting, freezing, evaporation, precipitation and condensation are some other examples of physical changes.

 

Crystallization

 

Crystallization is an example of physical change. The process by which an impure compound is converted into its crystals is known as crystallization.

Common salt obtained from sea water contains many impurities such as sand and other substances like magnesium chloride. These impurities are removed from the common salt by the process of crystallization. Large crystals of pure substances can be formed from their concentrated solutions.

 

CHEMICAL CHANGE

 

A change, during which new substances with different properties are formed, is called chemical change.

 

Characteristics of Chemical Change

 

• Chemical changes are usually irreversible in nature and cannot be undone.
• During a chemical change, two or more substances (reactants) react to form a new substance.
• New substances that are formed from reactants are different in chemical properties from the reactants.
• Heat is generally absorbed or released during chemical change.
• A gas may be formed during chemical change.
• In a chemical change, the colour of the product sometimes gets changed. E.g. Copper sulphate is blue and acquires a sea-green colour when iron fillings are dropped into it.

Examples of Chemical Change

 

Rusting of iron

Rusting of iron is an example of chemical and irreversible change. When iron is exposed to air, it forms a substance known as iron oxide or rust. Presence of oxygen and moisture in air is necessary for rusting of iron objects.

Iron + oxygen + water ----------> iron oxide

Iron objects become weak by the process of rusting. Applying coat of paint, oil or grease on iron objects can prevent them from rusting. Galvanisation is also done to prevent iron objects from rusting. It is the process of applying a protective zinc coating to iron objects to prevent them from rusting.

Browning of vegetables and fruits

There are certain compounds that are present in vegetables and fruits. These compounds react with oxygen and cause browning of vegetables and fruits. Browning of fruits and vegetables such as apple, brinjal etc. is caused because of chemical reaction between certain compounds and atmospheric oxygen.

Burning of a Substance

Burning of any substance is a chemical change. When a substance such as paper or wood is burnt, the carbon reacts with oxygen in the air and produces ash and smoke and heat and light energy. This is a permanent and irreversible change.

Burning of magnesium ribbon is a chemical and irreversible change. When magnesium ribbon is burnt, magnesium oxide is formed. The word equation is:

Magnesium + Oxygen -------------> Magnesium oxide

Magnesium oxide is the product in this chemical reaction.

Formation of curd, burning of fuel and cooking of food are some other examples of chemical changes.

 

INTERESTING FACTS

• Iron is an element that has been known in its pure form for at least 5,000 years.
• Many people think that all crystals are the precious jewels. One of the examples of crystal is salt, which is created from the crystallization process.
• Nearly all reactions involve energy. Some involve light or electricity. Most involve heat.
• Human body uses chemical energy to perform daily functions.
• Carbon comes in various forms which include diamond, graphite and impure forms such as coal.
• Hot molten iron is a liquid and when left to cool will from solid steel and can take many forms.