6th Chp 6 Notes

                          Class 6th Science Notes             

             Chapter 6 Changes Around Us            

Introduction

→ Changes can occur everywhere at any time.

→ Change is a continuous process.

→ Some changes are reversed and some cannot be reversed.

→ Change may occur by heating a substance or by mixing it with some other substance.

Common changes taking place in nature

→ Change in position, sun rise and set.

→ Change in colour, sky at day and at night time.

→ Change in shape, shape of moon.

→ Change in size, a germinate seed.

→ Change in temperature, hot summer and cold winter.

→ Change in state or form, water on cooling and heating.

→ Rusting of iron.

Types of changes

• Reversible changes

• Irreversible changes

• Chemical changes

• Physical changes

• Expansion And Contraction

Reversible changes

→ It is an effect on substance in which substance get returned to its original state.

→ The change only alters the physical state of the substance that means there is no new substance is formed.

→ Examples: melting of ice, water turning into vapour, inflation of balloon etc.

Irreversible change

→ In this process, the changes cannot be reversed are called irreversible changes.

→ In irreversible changes new material being produced, which may or may not be useful.

→ Examples: Ripening of fruits is an irreversible change because it is not possible to get back the raw fruits from ripened or mature ones.

→ Blooming of flowers is an irreversible change because flowers cannot change back into buds.

Physical changes

→ In this changes, there is no new substance is formed.

→ It is reversible

→ Examples: melting of wax, Melting of ice, crushing can etc.

Chemical 

→ In this change there is a new substance is formed after the change.

→ It cannot be reversed by simple physical means.

→ Examples: rusting of iron, burning of candle etc.

Expansion and Contraction

→ Some materials expand on heating and some material contract on cooling.

→ The amount of expansion differs in solids, liquids, Physical change and gases.

→ Gases material expands the most while solids material expands the least.

→ Solids material contract the least while gases material contract the most.

Causes of change

Force

→ It can bring changes in certain thing. It can change motion, shape and size of objects by push or pull.

→ Examples

(i) When a potter applies force on a lump of soil, it gives shape to the lump.

(ii) When a goldsmith hammers a piece of gold, he makes intricate designs on it.

Temperature

→ This brings changes by causing increase or decrease in temperature.

→ Examples

(i) When ice is kept on room temperature, it melts and turns into water.

(ii) When a candle begins burning, its wax start melts because of higher temperature. 

 

6th Chp 5 Notes

             Class 6th Science Notes        

     Chapter 5 Separation of Substances    

What is Separation?

• The removal of substances from a mixture of two or more substances is known as separation.

• Some examples of separation are:

→ Separating stones from rice

→ Churning milk to obtained butter

Why do we need to separate substances?

→ To obtained two different components

→ To remove impurities or harmful components

→ To group substances of different sizes

Separation of substances

• Pure substance: The substance which contain only one type of particles.Examples: Water

• Impure substances: The substance which contain more than one kind of particle.

Examples: A plate of rice is mixed with some stones.

Separation of components of mixtures

• Mixture of solid with solid

(i) Hand picking

→ The component of a solid-solid mixture can be separated by hand picking.

→ It is used for separating large size impurities like stones and husk.

→ For example: separating grass from mint leaves, separating pebbles from dal.

(ii) Threshing

→ The process in which the stalks are beaten to free the grain seeds.

→ Threshing is done with the help of bullocks. For large quantity of grains machines are used for threshing.

→ This method is used by the farmers to separate the grains from the stalks after harvesting.

(iii) Winnowing

→ It is process which is used to separate heavier and lighter components of a mixture by wind or by blowing air.

→ This method is used by farmers to separate lighter husk particles from heavier seeds of grain.

(iv) Sieving

→ It is a process in which a components of a mixture with different size can be separated by sieve.

→ The smaller particles remove out through the pore of sieve and the larger particle is left behind it.

→ This method is used to separate wheat bran (the bigger particles) from flour.

(v) Sublimation

→ The process in which the solid changes directly into gaseous state on heating is called sublimation.

Mixtures of solids with liquids

(i) Evaporation

→ It is a process which is used when we want to get back a substance that has been dissolved. In this process liquid changes into gaseous form on heating.

→ Example: Getting salt out of salty water.

(ii) Crystallization

→ It is a process which separates a pure solid in the form of crystals from a saturated solution.

Separation of solids that do not dissolve in liquids

(i) Sedimentation: The process of separation of 2 components in such a way that the heavier substance settles down.

(ii) Decantation: separation of 2 components which do not mix together.

Example: Water and oil.

(iii) Filtration: The process in which separation of solid from liquid by the filter paper or strainer.Example: separating mud from water by using filter paper.

• Condensation: It is a process by which a substance changes from the gas phase to liquid phase.

Can water dissolve any amount of a substance?

→ Water dissolves different amounts of soluble substances in it.

→ A saturated solution is one which cannot dissolve more of that substance.

→ To dissolves more of a substance in a solution we have to heating it.