Class 7th Science Physical and Chemical Changes Chapter notes

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PHYSICAL AND CHEMICAL CHANGES

When we dissolve sugar in water to make a cold drink. Making a sugar solution is a change. 

Setting curd from milk is a change.

Milk becomes sour. Souring of milk is a change. 

Stretched rubber band also represents a change.

Changes are of two Types, physical and chemical.

PHYSICAL CHANGES

Properties such as shape, size, colour and state of a substance are called its physical properties. 

A change in which a substance undergoes a change in its physical properties is called a physical change. 

A physical change is generally reversible. 

In a change no new substance is formed. 

CHEMICAL CHANGE 

A change with which you are quite familiar is the rusting of iron. 

If you leave a piece of iron in the open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting. 

Iron gates of parks or farmlands, iron benches kept in lawns and gardens, almost every article of iron, kept in the open gets rusted. At home you must have seen shovels and spades getting rusted when exposed to the atmosphere for some time. 

In the kitchen, a wet iron pan (tawa) often gets rusted if left in that state for some time. 

Rust is not iron. It is different from iron on which it gets deposited.

A small piece of a thin strip or ribbon of magnesium. Clean its tip with sandpaper. Bring the tip near a candle flame. It burns with a brilliant white light. When it is completely burnt it leaves behind a powdery ash. 

The change can be represented by the following equation:
Magnesium (Mg) + Oxygen (O2 ) → Magnesium oxide (MgO)

Collect the ash and mix it with a small amount of water. It forms a new substance. This change can be written in the form of the following equation:
Magnesium oxide (MgO) + Water (H2O) → Magnesium hydroxide [Mg(OH)2 ] 

Magnesium hydroxide is a base. Magnesium oxide is a new substance formed on burning of magnesium. 

Magnesium hydroxide is another new substance formed by mixing magnesium oxide with water.

Dissolve about a teaspoonful of copper sulphate (blue vitriol or neela thotha) in about half a cup of water in a glass tumbler or a beaker. Add a few drops of dilute sulphuric acid to the solution. You should get a blue coloured solution. Save a small sample of the solution in a test tube or a small glass bottle. Drop a nail or a used shaving blade into the remaining solution. After half an hour or so.  

Take out the nail or the blade. The changes that you notice are due to a reaction between copper sulphate and iron. 

The change of colour of the solution from blue to green is due to the formation of iron sulphate, a new substance. 

The brown deposit on the iron nail is copper, another new substance. We can write the reaction as: 

Copper sulphate solution (blue) + Iron → Iron sulphate solution (green) + Copper (brown deposit) 

Take about a teaspoonful of vinegar in a test tube. Add a pinch of baking soda to it. You would hear a hissing sound and see bubbles of a gas coming out. Pass this gas through freshly prepared lime water.

The change in the test tube is as follows: 

Vinegar (Acetic acid) + Baking soda (Sodium hydrogencarbonate) → Carbon dioxide + other substances 

The reaction between carbon dioxide and lime water is as follows: 

Carbon dioxide (CO2 ) + Lime water [Ca(OH)2 ] → Calcium Carbonate (CaCO3 ) + Water (H2O) 

When carbon dioxide is passed through lime water, calcium carbonate is formed, which makes lime water milky. 

The turning of lime water into milky is a standard test of carbon dioxide.

The reaction of copper sulphate with iron produced iron sulphate and copper. Both of these are new substances.  

A change in which one or more new substances are formed is called a chemical change. 

A chemical change is also called a chemical reaction. 

Chemical changes are very important in our lives. 

All new substances are formed as a result of chemical changes. For example, digestion of food in our body, ripening of fruits, fermentation of grapes, etc., happen due to series of chemical changes. 

A medicine is the end product of a chain of chemical reactions.

Plastics and detergents, are produced by chemical reactions. 

One or more new substances are produced in a chemical change. 

New products, the following may accompany a chemical change: 

  • Heat, light or any other radiation (ultraviolet, for example) may be given off or absorbed. 
  • Sound may be produced. 
  • A change in smell may take place or a new smell may be given off. 
  • A colour change may take place . 
  • A gas may be formed.
Burning of coal, wood or leaves is also a chemical change. Burning of any substance is a chemical change. Burning is always accompanied by production of heat.

Explosion of a firework is a chemical change. An explosion produces heat, light, sound and unpleasant gases that pollute the atmosphere. 

When food gets spoiled, it produces a foul smell. we call this change a chemical change. 

A slice of an apple acquires a brown colour if it is not consumed immediately. If you have not seen this change in colour, cut a fresh slice of apple and keep it away for some time. Repeat the same activity with a slice of potato or brinjal. 

The change of colour of a slice of potato or brinjal is due to the formation of new substances.

RUSTING OF IRON 


Rusting is one change that affects iron articles and slowly destroys them. 

Iron is used in making bridges, ships, cars, truck bodies and many other articles, the monetary loss due to rusting is huge. 

The process of rusting can be represented by the following equation: 

Iron (Fe) + Oxygen (O2 , from the air) + water (H2O) → rust (iron oxide Fe2O3 ) 

For rusting, the presence of both oxygen and water (or water vapour) is essential. 

The content of moisture in air is high, which means if it is more humid, rusting becomes faster. 

Prevent iron articles from coming in contact with oxygen, or water, or both. One simple way is to apply a coat of paint or grease. 

Coats should be applied regularly to prevent rusting. Another way is to deposit a layer of a metal like chromium or zinc on iron.

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