Reversible and Irreversible Changes PDF

## Reversible and Irreversible Changes We experience many changes in our everyday lives, such as the occurrence of day and night, change of seasons, growth and development in plants and animals, formation of a rainbow in the sky after rains, and so on. Based on whether a change can be reversed or n...

## Reversible and Irreversible Changes We experience many changes in our everyday lives, such as the occurrence of day and night, change of seasons, growth and development in plants and animals, formation of a rainbow in the sky after rains, and so on. Based on whether a change can be reversed or not, changes around us can be broadly grouped into two types - reversible or irreversible. ### Reversible Changes A change that can be reversed is called a reversible change. It is a temporary change which may occur due to the change in the shape, size and appearance of the substance undergoing the change. In a reversible change, the properties of a substance do not change. The original substance can be obtained back in its original form. Some examples of reversible changes are given below: - **Melting of ice:** Water obtained after melting of ice can be converted back to ice by cooling. Similarly, melting of an ice cream is a reversible change - **Folding of paper:** Folded paper can be unfolded again to obtain the original paper. - **Rolling out a roti from a ball of dough:** A rolled out roti can be made into a ball of dough again. Making toys from clay is also a reversible change - **Stretching a rubber band:** When the rubber band is released, it regains its original shape. **Science In Action** The process of recycling water in nature through the water cycle involves reversible changes. ### Irreversible Changes A change that cannot be reversed is called an irreversible change. It is a permanent change in which the properties of the substance undergoing a change also change. The original substance cannot be obtained back in its original form. Some examples of irreversible changes are given below: - **Burning of paper:** After the paper is burnt, a new substance called ash is obtained. This new substance differs from the original paper in its appearance and properties. Ash cannot be converted back into the original paper form. - **Cooking of food:** Cooked food cannot be converted back into the original ingredients. - **Formation of curd from milk:** The curd obtained has different properties from milk, such as smell, taste and texture. Milk cannot be obtained back from the curd. - **Ripening of fruits:** Unripened fruits cannot be obtained back from the ripened fruits. - **Growth in plants and animals:** This is an irreversible change. **Remember** Rolling out of roti from dough is a reversible change as it only changes the shape of the dough. However, baking of the roti on a hot iron tawa is irreversible as it cannot be reversed to get the dough back. ## Physical and Chemical Changes Based on whether a change leads to the formation of a new substance or not, changes can be further grouped as - physical or chemical. ### Physical Changes A change in which no new substance is formed is called a physical change. For example, melting of wax is a physical change as it only changes the state of wax from solid to liquid and no new product is formed. Tearing of paper, breaking of glass, heating of milk, melting of ice, freezing of water and cutting of fabric are some other examples of physical changes A physical change can be reversible or irreversible. For example, folding of paper is a physical change as no new substance is formed. It is also a reversible change as the original paper can be obtained by unfolding it. Tearing of paper is also a physical change as no new substance is formed. But, the original paper cannot be obtained back from the torn pieces of paper. So, tearing of paper is an irreversible change. ### Chemical Changes Change in which a new substance with different properties is formed is called a chemical change. Burning of paper is a chemical change. During this change, paper changes into ash which has different properties from that of the original paper. Heat and light are also produced during this change. Burning of kerosene,petrol, LPG gas, coal and wood, rusting of iron, cooking of food and respiration are some other examples of chemical changes. Chemical changes are always irreversible, and hence, these changes are permanent. For example, ash obtained after a piece of paper is burnt cannot be converted back into the original paper. Hence, burning of paper is an irreversible chemical change. **Let's Investigate** Is popping of popcorns a physical or chemical change? Can this change be reversed? ## Expansion and Contraction of substances Some substances expand on heating and contract on cooling. An increase in the size of an object on heating is called expansion and decrease in the size of an object on cooling is contraction. The extent to which substances expand or contract differs for solids, liquids and gases. This is because of the different arrangements of molecules in each state. The amount of expansion and contraction is maximum in gases, less in liquids, and the least in solids. Expansion and contraction can be utilized in several day-to-day activities. ## Practical applications of expansion and contraction 1. **Thermometers:** work on the principle of expansion. As temperature around the thermometer bulb rises, the mercury present in the bulb expands and rises in the tube, indicating the temperature. 2. **Metal rims:** can be fitted on a wooden wheel by using the principle of expansion and contraction. On heating, the metal rim expands and easily slips onto the wooden wheel. On cooling, the metal rim contracts and fits tightly on the wooden wheel. 3. **Tightly closed metal lids of food jars:** can be easily opened by dipping the lid part in hot water. Hot water heats up the metal lid. Heated metal lid expands and opens easily. 4. **Metals:** such as iron, gold and silver expand and become soft on heating. These softened metals are then moulded and beaten into desired shapes. 5. **Hot-air balloons:** When air heats up and expands, its density decreases. The warmer, less dense air rises above the colder, denser air below it. Hot-air balloons are able to rise due to the thermal expansion of air. The air in the balloon is heated, which causes this air to expand. The density of the air in the hot-air balloon is lower than the density of the cooler air outside, which causes the balloon to rise. Hot-air balloons are used to collect data from remote areas, that is, they are used in scientific research or in weather forecasting. ## Drawbacks of expansion and contraction Apart from benefits, contraction and expansion may cause some serious issues too: 1. **Bridges:** With seasonal changes in temperature, bridges made of concrete can be stressed, and eventually crack. Therefore, special joints are needed in concrete to be stressed, and eventually crack. Therefore, special joints are needed to prevent the bridges from bending out of shape. 2. **Railway Tracks:** Railway tracks expand in summer. Therefore, they are laid with gaps. The gap is provided to allow the expansion. If no gap is left, the expansion in summer will cause the rails to bend sideways. Bent railway tracks may lead to serious accidents. 3. **Electrical Wires:** The electrical wires sag in summer. 4. **Concrete Roads:** Concrete road develops cracks due to the continuous contraction and expansion in different seasons. **My Dictionary:** **Sag:** Hang down loosely. **Let's Investigate** Most substances expand upon heating. Find the name of a substance that expands on cooling. **Real World** When two glass tumblers get stuck in each other, they can be separated by putting them in hot water. Hot water expands the outer tumbler slightly and the two tumblers can be separated easily.