\n6.<\/td>\n | Souring of milk<\/td>\n | No<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Question 4. \nA drawing sheet changes when you draw a picture on it. Can you reverse this change? \nAnswer: \nBy erasing the drawing, we can undo the change. For example, if we draw with the help of pencil, we can erase the drawing by using an eraser. Then, it is a reversible change. If we draw using a pen, we cannot erase the drawing. Then this change cannot be reversed.<\/p>\n <\/p>\n Question 5. \nGive examples to explain the difference between changes that can or cannot be reversed. \nAnswer: \nExamples of Reversible Changes:<\/p>\n \n- Melting of ice into water: By freezing the water we can obtain ice again.<\/li>\n
- Folding a paper: By unfolding it, we can undo the change.<\/li>\n
- Flot milk to cold milk: By boiling milk, we can make it warm again.<\/li>\n<\/ol>\n
Examples of Irreversible Changes:<\/p>\n \n- Bursting a balloon.<\/li>\n
- Bud becoming a flower.<\/li>\n
- Cutting a wood into wood pieces.<\/li>\n<\/ol>\n
Question 6. \nA thick coating of a paste of Plaster of Paris (POP) is applied over the bandage on a fractured bone. It becomes hard on drying to keep the fractured bone immobilised. Can the change in POP be reversed? \nAnswer: \nWhen water is added to POP, it undergoes a chemical change. Its composition changes and is converted into another substance. Since it is a chemical change, it cannot be reversed.<\/p>\n Question 7. \nA bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement, could be reversed? \nAnswer: \nDue to water, cement hardens and its composition changes. Chemical changes, in general, are irreversible changes. Hence, it is a chemical change which cannot be reversed.<\/p>\n NCERT Extended Learning Activities And Projects<\/span><\/p>\nQuestion 1. \nTake a lemon, a paintbrush and a piece of paper. Cut the lemon and squeeze out its juice in a cup. Dip the brush in the lemon juice and write a message on the paper. Let the paper dry and you find that the letters of your message become invisible. Now, press the paper with hot iron or warm it by holding it above the flame of a candle (Take care that it does not catch fire). As the paper gets warm, invisible letters change into dark brown colour. Identify the changes that can be reversed in this process. \nAnswer: \nIrreversible changes: Cutting and squeezing of lemon, writing message on paper. \nReversible changes: Drying and wetting of the paper.<\/p>\n Question 2. \nObserve preparation of dishes at your home. Identify two changes that can be reversed. \nAnswer: \nDo it yourself.<\/p>\n Question 3. \nMaintain a record for one year of the seasonal changes in vegetables, clothing, nature and events around you. Identify the changes that can or cannot be reversed. \nAnswer: \nDo it yourself.<\/p>\n Activity 1<\/span><\/p>\nObjective: To show that a balloon changes its size and shape on blowing air into it and the change is reversible. \nMaterials Required: A balloon. \nProcedure:<\/p>\n \n- Take a balloon and blow air in it. Take care that it does not burst.<\/li>\n
- Now allow the air to escape from the balloon.<\/li>\n<\/ul>\n
\nObservations: On blowing, the size and shape of the balloon have changed. It regains its shape when air is released. \nConclusion: It is possible to get the balloon back to its original shape and size. Therefore, it is a reversible change.<\/p>\n
<\/p>\n Activity 2<\/span><\/p>\nObjective: To make a toy aeroplane by folding paper and show that the change is reversible. \nMaterials Required: A piece of paper. \nProcedure:<\/p>\n \n- Take a piece of paper and fold it to make an aeroplane.<\/li>\n
- Now unfold the paper again.<\/li>\n<\/ul>\n
Observation: The sheet of paper has changed into a toy aeroplane. On unfolding, the paper regains original form. \nConclusion: It is a reversible change.<\/p>\n Activity 3<\/span><\/p>\nObjective: To show that rolling of roti out of dough is a reversible change. \nMaterials Required: Dough (kneaded flour) and rolling pin. \nProcedure:<\/p>\n |