k8schoollessons.com

Free Worksheets, Lessons, Quizzes

FREE PDF Worksheets!!! All our online worksheets & quizzes are now downloadable & printable. Download all of them as FREE PDF worksheets or print them for FREE!

Reversible changes


Reversible changes are changes that can be undone or reversed. Melting, freezing, boiling, evaporating, condensing, dissolving and also, changing the shape of a substance are examples of reversible changes.

Reversible changes

 






 

What are reversible changes?

Reversible changes are changes that can be undone or reversed.

 

Give another name for reversible changes.

Physical changes

 

Why we call reversible changes, physical changes?

Reversible changes usually change the physical appearance of a substance. Therefore, reversible changes are also called physical changes.

 

Characteristics of a reversible change

  • A reversible change changes how a substance looks or feels (changing the physical appearance), and it is easy to turn it back again.
  • A reversible change may change the state of a substance such as, solid, liquid and gas.
  • However, a reversible change does not change the amount of matter in a substance.
  • Also, a reversible change doesn’t produce new substances.

 

E.g. – Water can change into ice. Ice can change into water. Here, only the state of the substance liquid water changes, but not the substance water.

 

Processes of reversible changes

Reversible changes fall under different processes. They are;

  • Melting
  • Freezing
  • Boiling
  • Evaporation
  • (Boiling and evaporation are the two types of vaporization)

  • Condensation
  • Sublimation
  • Dissolving
  • Changing the shape of a substance
  • Making mixtures
  • Magnetism
  • Crystallization

 

Now let’s see the examples of reversible changes or physical changes involved in these different processes.

 

Examples of reversible changes

 

Following are some of the reversible changes examples.

 

Melting

 

Example (1) – Melting chocolate

When chocolate is heated until it melts, the melted chocolate can be changed back into solid chocolate by cooling or freezing.

 
Examples Melting Chocolate
 
Examples Melting Chocolate
 
Examples Melting Chocolate
 

Example (2) – Melting candle wax

When candle wax is heated, the solid wax melts and becomes a liquid. If you cool the molten wax, it becomes a solid again.

 
Examples Melting wax
 

Example (3) – Melting ice cubes

When you keep some cubes of ice outside the freezer for a few minutes, you can see how they melt and turn into liquid water.
 
Examples Melting ice cubes
 

If you pour this water in a tray of ice cubes and keep in the freezer for some time, you can make ice cubes back.

 

Example (4) – Melting butter

Heat some butter and see how it melts. Keep the melted butter back in the fridge and get solid butter back.

 
Examples Melting butter
 
Examples Freezing butter
 

Example (5) – Melting metal

People cast metal into shapes by melting them into a liquid. Melted metal is poured into molds. As the metal cools, it solidifies. This is how people make metal objects with different shapes. Alluminium, iron, lead, copper, gold and silver are popular types of metal that are melted. Also, brass, bronze and steel are some examples of metal alloys created by melting metal.

 
Examples Melting metal
 

Example (6) – Melting plastic

Plastic can also be melted and pour into different molds to get different shapes. After cooling down, you can get solid plastic back.

 

Freezing

Example (1) – Making ice lollies

When orange juice is frozen to make ice lollies, the ice lollies can be changed back into liquid orange juice by heating.

 
Examples Freezing Ice lollies
 
Examples Making ice lollies
 

Example (2) – Making ice cubes

When we put some water in the freezer of a refrigerator it will turn into ice. If we then warm ice it melts and changes back into water.

 
Reversible Changes Examples Freezing Ice
 
Reversible Changes Examples Making ice cubes
 

Example (3) – Making jelly

You can freeze a mixture of liquid jelly and turn it into solid jelly. If you keep solid jelly outside the fridge for sometime you can get liquid jelly back.

 
Reversible Changes Examples Making jelly
 

Example (4) – Freezing lava

Lava is molten rock. Rock can be melted due to the extreme heat at the centre of the earth and turns into a liquid called ‘magma’. Magma gushes out of the earth’s weakest surfaces as a volcanic eruption. The runny magma that flows from the volcano is called ‘lava’. Lava that runs out of volcanoes cools down quickly.

 
Reversible Changes Examples Freezing Lava
 

Boiling, evaporating and condensing

You can boil some water and turn it into a gas, which is water vapour. We also call this ‘steam’. This process is called ‘evaporation’.

 

If you could capture all the steam that is made when a kettle boils, you could turn it back to water by letting it cool. This process is called ‘condensation’

 
Reversible Changes Examples Evaporating
 

Sublimation

Substances like iodine and dry ice can directly change to the gaseous state when they are heated. This process is called ‘sublimation’.

 

Example (1) – Heating iodine crystals

When solid iodine crystals are heated they directly form a purple gas without passing through a liquid state.

 
Iodine crystals-sublimation
 

Example (2) – Heating dry ice

Dry ice is carbon dioxide in its solid form. It turns directly into a gas when heated, instead of melting into a liquid.

 
Dry ice sublimation
 

Dissolving

Example (1) – Salt solution

When salt is mixed with water it disappears because it dissolves in the water to make salty water. We call this salt solution. The salt in the solution can be recovered by boiling off the water.

 
Dissolving
 
Reversible Changes Examples Heating salt solution
 

Example (2) – Sugar solution

You can make a sugar solution by dissolving sugar crystals in water. When you heat the solution, the water in it will evaporate leaving behind the sugar.

 
Reversible Changes Examples Heating sugar solution
 

Changing the shape of a substance

We know that physical changes are reversible. But not all of them. Some physical changes occurred by changing the shape of a substance are not reversible. Let’s see which are reversible and which are not.

 

Example (1) – Blowing up a balloon

When you blow up a balloon the shape of the balloon is changed. If you let the air inside the balloon go it will come back to its usual shape. This physical change is reversible.

 
Reversible changes examples - Blowing up a balloon
 

Example (2) – Crumpling a piece of paper

When you crumple a piece of paper it will become creased and wrinkled. However, neither the amount of matter in the paper nor the material the paper is made is changed. This change is reversible.

 
Reversible changes examples - Crumpling paper
 

Example (3) – Cutting, tearing and breaking substances

When you cut a piece of wood in half, you change its shape. But you do not alter the way the wood is made.

 

Similarly, when you cut, tear or break something you just alter its shape by dividing the substance into some parts. The amount of matter in the substance and the material which the substance is made are not changed.

 

These types of changes are physical changes which are not reversible.

 
Reversible changes examples - Cutting, tearing and breaking substances
 

Example (4) – Molding clay

When you mold some clay into a pot, the shape of clay is changed, but no new matter or material is formed. You can turn the pot back into the same chunk of clay before you bake it in the kiln. This physical change is reversible.

 
Reversible changes examples - Molding clay
 

Example (5) – Sharpening a pencil

When you sharpen a pencil, you change the physical appearance or the shape of the pencil. Also, some amount of matter is separated from the pencil, but no matter or material of the pencil is chemically changed. This physical change is not reversible.

 
Reversible changes examples - Sharpening a pencil
 

Example (6) – Stretching a rubber band

When you stretch a rubber band, you alter its shape, but it is still made of rubber. When you let it go it will come back to its usual shape. This physical change is reversible.

 
Reversible changes examples - Stretching a rubber band
 

Example (7) – Squashing a sponge

Similarly, when you squash a sponge, you alter its shape, but it is still made of sponge. When you let it go it will come back to its usual shape. This physical change is also reversible.

 
Reversible changes examples - Squashing a sponge
 

Making and separating mixtures

Example (1) – Sand and gravel mixture

A sand and gravel mixture consists of solid particles of different sizes. Sieving is the best method to separate mixtures like this.

 
Reversible changes examples - Sand and gravel mixture
 

Example (2) – Water and sand mixture

Filtering is used to separate mixtures like water and insoluble substances such as sand.

 
Reversible changes examples - Water and sand mixture
 

Example (3) – Sand and salt mixture

To separate a mixture of sand and salt, first the mixture should be mixed with water and then filtered, lastly evaporated.

 
Reversible changes examples - Sand and salt mixture
 

What is happening in these steps? First the salt dissolves in the water, then sand filters off and lastly, water evaporates to leave the salt.

 
Reversible changes examples - Sand and salt mixture
 

Magnetism & Crystallization

Magnetism and crystallization are some advanced processes of physical changes.

 

The process of magnetism is reversible, and no new materials are formed as it does not affect the chemical composition.

 

The type, shape and the size of crystals can be changed by heating, rolling or hammering. However, no new materials are formed.

 

Also, read the lesson ‘Irreversible Changes‘ and try Reversible Changes and Irreversible Changes quizzes and worksheets.