Living Science 2020 2021 for Class 6 Science Chapter 6 - Changes Around Us

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In a chemical reaction, new substances having different properties are formed are regarded as chemical changes.

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No, all physical changes are not reversible.
For eg. tearing of paper, sharpening of pencil, etc.

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A physical change is one which involves changes in the physical properties of a substance such as a state, shape, size, and colour, etc. No new substances are formed in a physical change. Also, the original substance can generally be recovered again in a physical change i.e. most physical changes are reversible, as the melting of ice only includes the change in physical properties like state, shape, etc. moreover no new chemical products is formed. Therefore, it is a physical process.

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(a)  The molecules of the substances do not change.

Physical changes are reversible, the molecules of the substance remain exactly the same before and after the physical change.

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(b) a chemical change
Growth occurs due to the intake of food. The substances present in food are used to nourish the cells and allow them to grow/divide. Hence, growth is a chemical change in which the molecules present in the food undergo changes to form molecules that enhance growth.
  

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(c) either a physical change or a chemical change, depending on the conditions.
When two substances are mixed together, it can result either in a physical or chemical change. This depends on the substances that are mixed and also on the conditions.

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(a) always irreversible.
Chemical reactions always result in forming new substances with different molecules. It is not easy to get back the original substances. Hence, the chemical changes are permanent and irreversible.

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(d) burning of wax.
In the process of burning of wax, it is neither the solid wax nor the liquid wax that is burnt. Wax vapours are burnt giving off heat and light. Once burned, it is not possible to get back the wax. Burning of wax is a chemical process and hence, is irreversible.

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(c) melting of wax.
The melted wax gets solidified after cooling. The melting of wax, therefore, is a reversible change.

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(b) melting of ice to form water

The melting of ice to give water is a physical and reversible change. The water can be frozen to again form ice.

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(c) breaking of a glass tumbler

The glass tumbler or its broken pieces are made of glass and there are no new molecules created, so its is a physical change. However, you cannot get back the glass from the broken pieces so, the change is irreversible.

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True.
A substance continues to remain either in its physical state or chemical state until and unless it is acted by an external factor like heat, motion, wind etc. So, every change has a cause.

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True.
Sometimes a change in a substance may result in the formation of new substance and sometimes may not. Depending on this, every change can be classified as either physical or chemical.

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Sugar dissolving in water is a physical change. The sugar can be obtained back by evaporation of water and the water by condensation of water vapour.

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The change of state of a substance does not involve the formation of new molecules. A change in temperature or pressure can reverse the change in state. Therefore, a change in state is a physical change.

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Most chemical changes cannot be easily reversed. Such changes are called irreversible changes.

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No, changing of milk is not a reversible change. When the milk changes into curd, the molecules of the milk undergo change resulting in the formation of curd.

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When a paper is burnt, the molecules of paper undergo change forming new substances like smoke and water vapour. The change here is irreversible. So, a burnt piece of paper cannot be brought back to its original form.

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Substances which expands on heating, regain their original shape on cooling without forming any new substances. This change is a physical change.

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       (a) Physical changes:
         Example: Change in the size of the pencil or eraser with continuous use.
       (b) Chemical changes:
          Example: Burning of wood.
       (c) Reversible changes:
          Example: Salt dissolving in water.
       (d) Irreversible changes:
          Example: Curdling of milk.

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A potter shapes pots out of clay. This is a physical change which can be easily reversed. During this change, only the shape of the substance changes, without any change in the texture of the clay. 
The potter then bakes the pots in an oven that makes the clay to becomes hard and brittle. Thus, there is a change in the internal structure of clay which makes soft clay hard and brittle. This is a chemical change which is irreversible.

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Cooking food is an irreversible, chemical change. During cooking, the molecules that are present in food changes to form new substances. Also, cooked food cannot be reverted to the raw state.

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No, there are some physical changes which are irreversible. For example, tearing of a paper. When a paper is torn, the size and the shape of the paper changes, but the molecules of the paper remains the same. Since, no new molecules are formed, tearing paper is a physical change. At the same time, we cannot get back the sheet of paper from the pieces. Thus, the physical change is irreversible.

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Growth results in increase in size due to the intake of food. The food consumed by a living organism, is absorbed by the body and is used to nourish the cells for growth. Therefore, this is a chemical change, which includes the formation of new substances during the process of growth.

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Consider the example, when salt is mixed with water. The salt dissolves in water forming a solution. The salt can be obtained back by the evaporation of water and water by condensation of the water vapour. Such changes which can be easily reversed are called reversible changes. 

Consider the example of rusting of iron. When an iron is kept in humid air for some days, a brown substance (rust) deposits on it, which has very different properties from that of iron. There is no simple way to get back the iron from the rust. Such changes which cannot be reversed are called irreversible changes. 

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A chemical reaction is the process by which reactants react with each other to yield products. In chemical reactions, the reactant molecules are completely different from the molecules of the products formed.
Example: Take some washing soda and mix it with water in a bottle. After making sure that the washing soda is fully dissolved in the water, add a few drops of lemon juice to the contents of the bottle. Hold a lit match stick near the mouth of the bottle. You will see that the match is extinguished showing that the gas produced is carbon dioxide.

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When substances are mixed together, it can result either in a physical change or a chemical change.
Example: If iron and sulphur are mixed, no chemical change occurs. But when the mixture is heated, it glows after some time and a black substance is formed which is different from iron and sulphur. So, heating a mixture of iron and sulphur results in a chemical change.
Similarly, when sugar and water are mixed together, sugar dissolves completely in water. No new substances are formed during the change. The sugar and water can be obtained back from the solution. This is a physical change which can be reversed.

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When a candle burns, wax in the candle melts and is then vaporised as it is drawn up the wick. Melting and vaporisation are physical changes. The wax vapours then burn at the wick to leave behind soot and water vapour, while emitting heat and light. The burning of wax vapours is a chemical change. Thus, wax undergoes both physical and chemical changes when a candle burns.

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The tearing of paper is an irreversible change as we cannot get back the original sheet of paper from the pieces. However, there are no new substances created by tearing paper. The pieces of paper obtained have the same molecules as the original sheet of paper. Thus, tearing of a paper sheet is a not a chemical change, but it is a physical change.

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When a potato is cooked, heating the potato cause changes in the molecules of the potato. Therefore, upon cooking, the potato does not remain the same at a molecular level. Thus, cooking is a chemical change, not a physical change.

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When sugar dissolves in water to form a sugar solution, the sugar disappears. However, the molecules of sugar remain intact and are distributed uniformly in the water. Also, the water molecules are not changed when sugar is dissolved in water to form a sugar solution.
We can easily get back the sugar by evaporation of water from the sugar solution. The evaporated water can then be condensed to recover the water also. Hence, it is a physical change and not a chemical change.

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When we boil liquid water, its molecules get separated from each other on vaporization but dissociation of water molecule doesn't take place.
So, H₂O will remain in the vapour phase as well.

Hence, the correct answer is option "a".