Page 1 :
STD . 7th, , Sub:Science, 13. Changes Physical and Chemical, , SUMMATIVE EVALUATION, QUESTIONS AND ANSWERS, , Q. 1. Select a proper word and fill in the blanks :, (Irreversible, non periodic, slow, natural, physical, periodic, galvanization), (1) Change like spoiling of the milk is a natural change., (2) Ripening of the fruit is a slow change., (3) If the properties of the original substances remain the same, then this type of, change, is called physical change., (4) The change that can occur in a forward and reverse directions again and again is, periodic change., (5) The changes in which the time interval is not fixed are non periodic change, (6) In the process of galvanization iron articles are given a thin coat of zinc to, prevent corrosion., Q. 2. Write whether following statements are true or false :, (1) Evaporation is a physical change., Ans. True, (2) Baking bread is a harmful change., Ans. False. Baking bread is a useful change, (3) Rusting of iron is a physical change., Ans. False. Rusting of iron is a chemical change, (4) The objects become durable due to corrosion., Ans. False. The objects become weak due to corrosion, (5) A number of different criteria can be applied to the same change., Ans. True., Q. 3. Distinguish between the following:
Page 2 :
(1) Physical change and chemical change., Physical change, 1. The properties of the original, substances remain the same in the, physical change., 2. Their original composition remains, unchanged., 3. e.g.. Evaporation, boiling, dissolution, etc., , Chemical change, 1. The properties of the original substance, alter in the chemical change, 2. Their original composition changes, 3. e.g., Photosynthesis, corrosion., , (2) Periodic change and non-periodic change., Periodic change, 1. Changes occurring again and again after, a definite interval of time are called, periodic changes, 2. There is constancy in periodic changes., e.g. high and Low tide of the sea, movement of hand of clock, , non-periodic change, 1. Changes not occurring again and again, after a definite interval of time are called, non-periodic changes, 2. There is no constancy in non-periodic, changes, e.g., Breaking of storm., , (3) Natural change and man-made change., Natural change, 1. The change occurring automatically by, natural cause is called natural change., 2. Most of the natural events are the, example of natural changes., e.g. Ripening of fruits, , man-made change, 1. The changes induced by man are called, man – made changes, 2. Man – made changes are due to man ‘s, wish to satisfy his needs., e.g. cutting down the trees, , Q.4 Under which different types can be following changes be classified?, Questions:, (1) Conversion of milk into yoghurt, , (1) Chemical, irreversible, slow, , (2) Bursting pf a cracker, , (2) Chemical, irreversible, fast., , (3) Occurrence of an earth quake, , (3) Physical, Chemical, irreversible, natural., , (4) Revolution of the earth around the, , (4) Periodic, slow, natural., , (5) Stretching of a spring, , (5) Physical, reversible, , 5. Give reasons:, (1) While purchasing canned food, its expiry date should be checked.
Page 3 :
Ans. Every canned item has a limited shelf life. This is the time limit during which, the food item is safe to eat. This is mentioned on the can. But later, after the expiry date is, over. the food may not remain in a good condition due to irreversible chemical changes in, it. Consuming such items can harm our health. Therefore, while purchasing canned food,, its spiry date should be checked., (2) An iron article should be given a coat of paint., Ans. An iron article rusts naturally which is called corrosion. Corrosion takes place, due oxygen, moisture, vapours of chemicals in the air. Corrosion makes articles weak and, unusable. Therefore, to keep the iron articles in better condition, it should be given a coat, of paint, (3) A wooden article should be polished., Ans. A wood is an organic substance that is affected by termites or fungus. This, natural change can result into irreversible damage. If polish is applied to such wooden, articles, the rate of natural changes slows down. Thus to prevent this harmful change, a, wooden article should be polished., (4) Copper and brass utensils should be tinned., Ans. The metals like copper and brass gather a greenish layer due to slow chemical, changes. This is corrosion which happens due to oxygen, moisture and vapours of, chemical in the air. For the prevention of such corrosion, copper and brass utensils are, tinned. Moreover, the tinned utensils are safer for cooking., (5) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet, handkerchief to dry., Ans. A dry handkerchief absorbs the water, once it is dipped into the water. This is, a cy in non-pe fast change. But drying such moist handkerchief is a slow process which, occurs due to evaporation. Evaporation is the physical change which depends upon, atmospheric temperature and humidity. It may therefore take long for drying a wet, handkerchief., 6. Which points will you consider while classify the following changes into two groups – a, fruit falling from a tree , rusting of iron , raining , lighting an electric bulb , cutting, vegetables?, Ans. The points to consider while classify the changes (1) Whether the changes are, natural or man made? (2) The man- made changes whether it is a physical change or, chemical changes, Q.6. What will you take into account while identifying the following?, (1) A physical change in a substance., Ans. It will be seen that whether the substance has changed its properties and, composition. If both of these have remained unchanged then it is a physical change.
Page 4 :
(2) A chemical change in a substance., Ans. It will be seen that whether, the substance is changed in its properties., Whether the composition of the substance is changed, whether the change is irreversible?, If all these changes are taking place then it will be a chemical change., Q. 7. Read the paragraph and note down the various types of changes it mentions, It was nearing six o'clock in the evening. The sun was setting. A breeze was blowing, Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet sod, and shaping them into various toys. Then he felt hungry and went into the house. Mother, made a dough from wheat flour and fried purees. While eating hot purees, his attention w, drawn outside the window. It had started raining. There was lightning, too. Sahil was, enjoying his dinner in the dim light., Ans. (1) It was nearing six o'clock in the evening: (Time-reversible, periodic), (2) The sun was setting: (Sunset Periodic, reversible), (3) A breeze was blowing: (Blowing of wind-Physical, non-periodic), (4) Leaves on the tree were shaking: (Movements due to wind Physical, periodic, (5) Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them in various, toys. (Making toys of soil-Physical), (6) Then he felt hungry and went into the house. (Hunger-Periodic), (7) Mother made a dough from wheat flour and fried purees. (Frying-Physical irreversible), (8) While eating hot purees, his attention was drawn outside the window. (Sering, randomly-Non-periodic), (9) It had started raining. (Raining-Physical, Periodic if raining in specific season), (10) There was lightning. (Lightning-Physical, Chemical, Fast), Q.8. Use your brain power!, (1) What kind of change is the falling of a tree in a storm?, Ans. The falling of a tree in a storm is a natural change.., (2) What kind of change is the conversion of milk into yoghurt?, Ans. Change is the conversion of milk into yoghurt is a natural chemical change., (3) Which type of change is the change of seasons from summer to rains to winter?, Ans. After summer, comes the rainy season. After rainy season follows the winter., This cycle of seasons goes on continuously on the earth. This occurs in specific time., Therefore change of seasons is a periodic change.
Page 5 :
(4) Which hands of a clock show periodic changes from 6.00 am to 6.00 pm How many, times?, Ans. Hands of clock show following movements during time period of 6 am to 6, pm. Hour hand: 1 periodic movement. Minute hand: 12 periodic movements. Second, hand: 12 x 60= 720 periodic movements., (5) Which of the following are physical changes and which are chemical ACTIVIT changes, making a table from wood, burning wood, breaking of a glass object ripening of a tomato,, rusting of iron?, (1) Making a table from wood:, , Physical change, , 2) Burning wood:, , Chemical change, , (3) Breaking of a glass object:, , Physical change tomato: Chemical change, , (4) Ripening of a tomato :, , chemical change, , (5) Rusting of iron, , Chemical change.
