7 C - SCIENCE CH PHYSICAL AND CHEMICAL CHANGE ( TOPICS TAUGHT AND HOMEWORK)

2021-22 SCIENCE CH  PHYSICAL AND CHEMICAL CHANGE

(TOPICS TAUGHT AND HOMEWORK)

DATE: 17/8/2021

GRADE: 7 C

CHAPTER: PHYSICAL AND CHEMICAL CHANGES

Diagnostic test-  Discussion

TOPICS TAUGHT: REVISED THE TOPICS OF TYPES OF CHANGES 

Examples Explained.

HOMEWORK: FIND MORE EXAMPLES OF PHYSICAL CHANGES AND COMPLETE THE TUNE IN GIVEN IN T.B

DATE: 18/8/2021

GRADE: 7C

CHAPTER: PHYSICAL AND CHEMICAL CHANGES

TOPICS TAUGHT

Some of the properties of a chemical change or chemical reaction are:

Change in energy

Change in color

Evolution of gas

Formation of precipitate

 

Change in energy: Almost all chemical reactions involved energy change. Some chemical reactions produce heat while some take away heat.

A change or reaction in which heat is released is called an exothermic reaction.

For example; In a reaction between carbon and oxygen, carbon dioxide is formed with the generation of heat.

Carbon + Oxygen ……. Carbon dioxide + Heat

 

A change or reaction in which heat is absorbed from the surrounding is called an endothermic reaction.

For example in a reaction between carbon and sulphur, carbon disulphide is formed with the absorption of heat.

Carbon + Sulphur + Heat …….. Carbon disulphide

 

Change in colour: The colour of the product may differ from that of the reactants.

For example; copper sulphate is blue in colour, but after reacting with iron, the product formed ( iron sulphate) is of green colour.

 

Evolution of gas: Some chemical changes also produce gases. Generally the gases produced can be carbon dioxide, hydrogen, ammonia etc. The presence of carbon dioxide can be confirmed as it turns lime water milky.

HOMEWORK: Write down difference between Physical changes and Chemical changes

DATE: 19/8/2021

GRADE: 7 C

CHAPTER:

Activity:

Reaction between vinegar and baking soda:

Take vinegar (acetic acid) in a glass beaker and add a pinch of baking soda (sodium hydrogen carbonate) to it. We will observe gas bubbles coming out of the beaker. This is due to the release of carbon dioxide. This can be confirmed by making the gas produced pass through freshly prepared lime water. The lime water turns milky when carbon dioxide is passed through it due to the formation of calcium carbonate.

Carbon dioxide (CO2) + Lime water [Ca(OH)2 ] -> Calcium Carbonate (CaCO3) + Water (H2 O)

Formation of precipitate: In some chemical changes, the product formed separates from the solution and form a different mass of layer. This insoluble solid is called a precipitate.

For example; in a reaction between copper sulphate and hydrogen sulphide, copper sulphide and sulfuric acid are formed. The copper sulphide formed is given out as a precipitate.

HOMEWORK:  Read Chapter Physical changes and Chemical changes.

DATE: 19/8/2021

GRADE: 7 C

CHAPTER: Physical changes and Chemical Changes

Rusting of iron

A reddish-brown layer of iron oxide forms on iron articles which are exposed to air and moisture for a period of time. This reddish-brown layer is known as rust. The process of

formation of rust is known as rusting.

The process of rusting is represented by the following chemical equation:

Iron + Water + Air (oxygen) Iron oxide (rust)

The following methods can be used to prevent rusting:

i. Galvanisation

The process of coating iron articles with zinc is known as galvanisation. This prevents the

direct exposure of iron to moisture and air and thus prevents rusting.

ii. Application of oil, grease or paint

Rusting can also be prevented by applying oil, grease or paint on the iron surface.

iii. Alloying

Painting and galvanisation are sometimes not enough to prevent rusting. For example, if the

iron articles such as a ship are exposed to moisture or kept under water for a long period of

time. In such cases, alloys are used. Iron is mixed with other metals or non-metals to form

alloys. Stainless steel is an alloy of iron made by mixing iron with carbon and metals like

chromium, nickel, and manganese.

Displacement reactions:

All metals do not have the same reactivity. The more reactive metals can replace the less

reactive metals in chemical reactions. The reaction in which a more reactive element displaces

a less reactive element from its solution is known as a displacement reaction. Reactivity

series is a table which shows the arrangement of metals on the basis of their reactivity. The

most reactive metal is placed at the top of the series

Homework- Complete the Exercise from chapter Physical changes and Chemical Changes.

DATE: 19/8/2021

GRADE: 7 C.

Chapter- Physical changes and Chemical Changes

Revision and Worksheet taken .

Homework- Kindly prepare Chapter for AT.