Changes Around Us

Key Points of the Chapter:

· Introduction

· Physical Changes

· Changes of state

· Chemical changes

· Conditions needed for a chemical change

· Indicators for a chemical change

· Exothermic and Endothermic chemical change

· Periodic and Non-periodic change

Introduction

“Changes take place around us all the time”. A change refers to an alteration in physical properties or alteration in the composition of matters, For example -The water upon heating becomes just hotter and hotter and at some time become water vapour. It remains water only its volume changes and this is called “physical change”. Whereas in case of burning of paper we cannot get back the paper after burning as there is a change in chemical nature this is called “chemical change”.

Physical and chemical changes:

Ø Matter is made up of tiny particle, atoms and molecules .we know that matter is made up of:

MATTER

Gases

Solid

Liquid

Effect of Heat on Solid, Liquid and Gases:

Upon heating, particles arrangement within the state of matter gets disturbed .The disturbance is seen either by “EXPANSION” or “CONTRATION”. When a glass of water is heated, its volume increases and if a gas of water is cooled its volume decreases.

Physical Changes:

Physical changes are the changes in which only physical properties of a substance undergo a change. There is no new substance formed in a physical change. Physical properties include Lustre, Malleability, Ductility, Mass, Volume etc.

Characteristics of Physical Changes:

· No new substances are formed.

· This change is usually Temporary and Reversible

· In this, the chemical properties of a substance do not change

· In this, the physical properties such as colour, size ,shape of a substance may undergo a change

Changes of State:

Changes of a state of a substance are one of the major physical changes we encounter in daily lives. The following are some of the changes of state:

From Solid – To Liquid Is Melting

From Liquid –To Gas Is Vaporization

From Gas – To Solid Is Freezing

From Solid – To Gas Is Sublimation

Melting, Vaporization And Sublimation Occur When Heated And Hence It Is Called As “Endothermic Process”.

In Freezing And Condensation, Heat Is Removed, Resulting In Decreases The Speeed Of The Molecule And Hence It Is Called As “Exothermic Process”.

Melting:

Melting is the changing of a solid into its liquid state and it happens by heating, whereas Freezing is the changing of a liquid into its solid state.

Vaporization:

There are two types of vaporization boiling and evaporation. The first one is by heating and second type of vaporization is natural.

Evaporation:

It is the changing of a liquid into its gas state and it happens by heating. This is the technique used to extract salt from sea water in salt pans.

Freezing:

A change in state take place during the freezing of water to form ice, but no new substance is formed so, the freezing of water is a physical change.

Condensation:

Condensation Is The Conversion Of Gas Into Its Liquid State. The Liquid Obtained After Condensation Can Be Converted Back Into Gas On Heating So, Condensation Is Also A Physical Process.

Sublimation:

The process in which solid is converted directly into gas is called sublimation.

Crystallization:

The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallization. It is a method of separation as well as method of purification.

Chemical Changes:

Changes that occur with the formation of new substance with different chemical composition are termed as chemical changes.

FOR EXAMPLE- metals, medicines.

Rusting Of Iron:

In this we learn why rusting of iron is termed as chemical change. Rusting is one change the affects articles and slowly destroys them. The process of forming rust is represented as follow:

IRON +OXYGEN+WATER =RUST

2Fe +2O2 from air +2H2O=2Fe2O3.H2O

For rusting both oxygen and water is essential, even when air is humid then rusting is fast.

To prevent rusting we apply paint or grease on iron articles.

Burning:

A chemical changes, especially through the rapid combination of a substance with oxygen, producing heat

Curdling Of Milk:

It is a process in which liquid gradually turns into solid. It is an “IRREVERSIBLE CHANGE “and “CHEMICAL CHANGE” as well.

Fermentation:

It is a process in which microorganisms such as yeast and certain bacteria break down sugar solution into alcohol and carbon dioxide. It is an

“IRREVERSIBLE PROCESS” and chemical change as well.

Chemical Reaction of Baking Soda With Lemon:

When these two substances are mixed together, then a chemical change takes place to form three new substances

Sodium Hydrogen Carbonate + Citric Acid -> Sodium Citrate + Carbon Dioxide + Water

Conditions Needed For A Chemical Change:

We can say that many chemicals changes occur only when the substances are made to physically contact with each other. We conclude that contact of the substances, applying pressure is some of the different conditions needed for chemical changes to occur.

Indicators of a Chemical Change:

There can be evolution of bubbles, evolution of heat, change of odour, change in colour or formation of a precipitate that serve as indicators for us to understand that a chemical change had taken place.

Exothermic and Endothermic Chemical Changes:

Changes in which heat is released are known as exothermic changes.

Changes in which heat is absorbed are known as endothermic changes

Periodic and Non-Periodic Change:

Changes that repeat themselves after a definite period of time are called periodic changes.

For Example- Rotation of earth.

Changes that do not repeat themselves after a definite period of time are called non-periodic changes.

For Example-Movement of legs while dancing.

FIGURE SHOW A PERIODIC CHANGE

FIGURE SHOW EXAMPLE OF NON PERIODIC CHANGE.