Physical And Chemical Changes

* Note:

Chemical change is a change where a substance is changed and a new substance is formed.

Physical change: A physical properties change in which a substance undergoes a change in its physical properties is called a physical change. change. In such a change, no new substance is formed.

(a), (c), (f)→Chemical Changes.

(a) Photosynthesis is a chemical process by which green plants convert light energy (Sunlight) into chemical energy (Carbohydrates).

(c) Burning of coal is a chemical change because new substances carbon dioxide and water are formed.

(f) Digestion of food is a chemical change as food is broken down into its constituent’s chemicals by the help of enzymes.

(b), (d), (e)→Physical Changes.

(b) Dissolving sugar in water is a physical change because when sugar molecules are mixed in water, they dissolved in water molecules and no new substance is formed.

(d) Melting of wax is also physical change because there is no new substance is formed on melting of wax.

(e) Beating aluminum to make aluminum foil is a physical change because no new substance is formed and only the physical properties like shape, size, etc. change.

(i) Rusting: If a piece of iron is open for some time, it acquires a film of brownish substance. This substance is called rust and the process is called rusting.

Rusting is harmful because it destroys the iron objects. Iron is the most widely used metal and so rusting is such a serious problem.

Prevention of rusting: Rusting can be prevented by preventing iron articles from coming in contact with oxygen, or water, or both.

• One simple way is to apply a coat of paint or grease. In fact, these coats should be applied regularly to prevent rusting .

• Another way is galvanization, i.e., to deposit a layer of metal like chromium or zinc on iron.

(ii) Spoilage of food: Food item when kept carelessly, get spoiled. This is a chemical change and obviously harmful for us.

Food is spoiled by microorganisms.

Prevention of food spoilage: Microorganisms do not survive at high or low temperature. So, food items stored in refrigerator do not spoil. Also we should keep them covered so that microorganisms do not get any chance to enter and spoil them.

(a) False.

Cutting of log of wood into pieces is a physical change because no new substance is formed and only the physical properties like shape, size, etc. change.

(b) False.

Formation of manures from leaves is a chemical change because manure had different composition of leaves.

(c) True.

Iron pipes coated with zinc do not get rusted easily because zinc prevent iron articles from coming in contact with oxygen, or water, or both.

(d) False.

When iron piece is left in the open for some time, it acquires a film of brownish substance called rust. Hence, iron and rust are different substances.

(e) True.

Condensation of steam is a physical change because water changes its state from vapour to solid.

The nails in bottles B rust a little, nails in A are the most rusted and that in C remain unchanged.

For rusting both water and oxygen are necessary. Both the factors are present in the bottle A since oxygen is dissolved in water.

In bottle B, water is boiled and hence dissolved air is removed. Due to lack of oxygen, iron nails do not rust much.

In bottle C, the layer of oil present prevents dissolving of atmospheric air in the water and hence no rusting occurs.

A cupful of water taken in beaker and a few drops of dilute sulphuric acid are added into it. The water is heated. When it starts boiling alum powder is added slowly while stirring continuously. Alum powder is added continuously till no more powder can be dissolved. The solution is filtered and allowed to cool down. Crystals of alum slowly form at the bottom of the beaker.

Crystals of Alum

(a) When carbon dioxide is passed through lime water, it turns milky due to the formation of Calcium carbonate.

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

(b) The chemical name of baking soda is sodium hydrogen carbonate.

It is popularly called cooking soda. The chemical formula of baking soda is NaHCO₃).

(c) Two methods by which rusting of iron can be prevented are painting and galvanization.

Painting and galvanization are two methods to prevent iron articles from coming in contact with oxygen, or water, or both. Thus, prevent rusting of iron.

(d) Change in which only physical properties of a substance change are called physical changes.

A physical change is the one in which a substance undergoes a change in its physical properties. In such a change, no new substance is formed.

(e) Changes in which new substances are formed are called chemical change.

Chemical change is a change where a substance is changed and a new substance is formed.

The different fuels used for cooking are wood, charcoal, cow-dung cake, kerosene, biogas, LPG, etc.

Among all these, biogas and LPG are least polluting. Both of these burn completely and do not give smoke. Also they do not leave any residue (ash, unburnt part, etc.)

When baking soda is mixed with lemon juice, a chemical change occurs. In this reaction, new substances like carbon dioxide is formed. Since a new substance, carbon dioxide is formed so it is a chemical process.

1) When a candle burns, both physical and chemical changes take place.

Physical change: Melting of wax is a physical change since it again turns into solid wax on cooling.

Chemical change: The wax near to flame burns and gives new substances like carbon dioxide.

2) Lighting torch bulb using the dry cell is another example where both physical and chemical changes take place.

Physical change: The lighting of a bulb is a physical change since no new substance is formed.

Chemical change: The current is obtained from the chemical substances present inside the dry cell. Inside the cell, the chemicals react with each other and produce light substances and hence the cell ultimately becomes useless. So, it is a chemical change.

Setting of curd is a chemical change because once curd is formed, milk cannot be reobtained from it. The new substance i.e., curd is different from the milk in taste, smell and other chemical properties.

Burning of wood is a chemical change because in burning new substance like ash, smoke, etc. are formed.

But cutting it into small pieces is physical change because no new substances are formed. Only the physical properties like shape, size, etc. change.

Crystals of copper sulphate are prepared by the method of crystallization. It is an example of physical change. The process is as follows:

(1)A cupful of water is taken in a beaker and a few drops of dilute sulphuric acid are added in it.

(2)The water is then heated and when it starts boiling, copper sulphate powder is added slowly with stirring.

(3)Copper sulphate powder is added continuously till no more powder can be dissolved.

(4)Then solution is filtered and allowed to cool down.

(5)Do not disturb the solution, when it is cooling.

(6)Crystals of copper sulphate slowly form at the bottom of the beaker.

Crystals of Copper Sulphate are formed as describe in the method given above.

Rusting of iron articles is due to the exposure of iron to the atmosphere for some time. Painting of iron gate prevents iron from rusting because the layer of paint prevents the iron gate to come in direct contact of atmosphere, water and both. Thus, rusting of iron can be prevented.

Both air and moisture are necessary conditions for rusting to take place. In coastal areas, the amount of moisture present in air is higher than that in desert areas. So, the process of rusting is faster in coastal areas than in deserts.

Process A is a physical change because no new substance is formed. The LPG in cylinder is in liquid form because of high pressure. When it comes out from the cylinder, it turns into a gaseous form. Thus, it is a physical change.

Process B is a chemical change because burning of gas is a chemical change.

Production of biogas from animal waste by the action of bacteria is chemical change because new substance biogas is formed. Thus, process A is a chemical change. Biogas works as fuel and produces heat. Hence, process B is also a chemical change.