7th Class Science Physical and Chemical Changes

Physical and Chemical Changes

Heat

Heat is a form of energy and energy is the capacity to do work. This clearly indicates that work can be done with the help of heat.

Temperature

The decree of hotness and coldness is called the temperature of the body. The two common scales that are used to measure the temperature of the body are Celsius scale and Fahrenheit scale. On the Celsius scale, the melting point of ice is and boiling point of water is On the Fahrenheit scale, the melting point of ice is  and boiling point of water is

Thermometer

• A thermometer is a device for measuring the temperature of an object. There are two common types of thermometers: Laboratory thermometer and Clinical thermometer. Both of these thermometers are based on Celsius scale of temperature.
• The range of a laboratory thermometer is generally from  to.
• The range of Clinical thermometer is from  to 

Effects of Heat

Heat brings tremendous changes in objects by acting on it. Heat causes the following changes:

Contraction and Expansion of Substances: All substances, whether solid, liquid or gas expand on heating and contract on cooling. On heating gases expand the most, the liquids expand less than gases and solids expand the least. You must have observed the electric wire on the street poles which becomes loose in summer and tight in winter. This happens just because of the effect of heat.

Change in State: Solid changes into liquid on heating and liquid changes into gas on further heating. For example, ice changes into water and water changes into vapour. On the other hand, when vapour cools, it changes into liquid and when liquid cools, it changes into solid.

Flow of Heat

Heat can flow through solids, liquids, gases and even through vacuum. Heat can flow through three ways namely: conduction, convection and radiation.

Conduction: The flow of heat from hotter part of a body to colder part of the body. When two bodies are in contact is called conduction. The transfer of heat from one body" to another is stopped when both the bodies acquire same temperature.

Convection: Liquids and gases are poor conductors of heat. Heat travels in liquids and gases by the process called convection. In the process of convection, heat travels in liquid and gas by the displacement of liquid or gas.

Radiation: In addition to travelling through solid, liquid or gas, heat can also travel through space or vacuum. Heat from the sun reaches to the earth by travelling through vacuum. All objects radiate heat. The amount of heat radiated depends on the temperature of the object. The heat radiated by a substance is called radiant heat or radiant energy.

Physical Change

Change in the physical properties (such as state, shape, size and colour) of a substance is called physical change. For example, breaking of glass, melting of ice, expanding of balloon, etc.

Characteristics of Physical Change

• No new substances are formed.
• Changes are temporary and can be reversed.
• Very little amount of energy is involved.

Chemical Changes

The changes m which two or more than two substances react with each other to form a new substance with different properties are called chemical changes. For example. rusting of iron, burning of wax, turning milk sour, etc.

Characteristics of Chemical Changes

• New substances are formed.
• Changes are permanent and usually irreversible.
• Great amount of energy is involved.

Rusting of Iron

Rusting of iron is a chemical change. When iron is exposed to moist air, it reacts with oxygen and water present in air and formed iron oxide which is red-brown flaky substance

Called rust.

Crystallization:

The process in which large crystals of pure substances are obtained from their solution is known as crystallization.

Crystallization is a physical change. Crystal of sugar, salt and alum can be obtained from their solution.