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UNIVERSAL EDUCATION CISCE 2024-25 CLASS VII CHEMISTRY Ch. 2: Changes Around Us Unit Review 1. Fill in the blanks: i. Most changes can be clas...

UNIVERSAL EDUCATION CISCE 2024-25 CLASS VII CHEMISTRY Ch. 2: Changes Around Us Unit Review 1. Fill in the blanks: i. Most changes can be classified as more than one type of change. ii. A snowstorm is an undesirable change. iii. In a chemical change, the substance formed has a different chemical composition. iv. Shape of a substance is a physical property. v. All changes of state are physical changes. 2. Write three differences between physical and chemical changes. Ans. Sr. Physical change Chemical change No. 1. It is temporary and reversible in 1.It is a permanent and most cases irreversible change 2. No new substance is formed 2.New substances are formed 3. The chemical composition of the 3. The chemical composition of substance formed remains the the substance formed changes same 3. Meera carried out the following experiment. She took a piece of potassium metal and added it to a test tube of water. She noticed that bubbles started to form. The temperature of the water rose too. Is the change a physical change or a chemical change? Explain your answer. Ans. Reaction of potassium metal with water is a chemical change because new substances, Potassium hydroxide is formed and hydrogen gas is evolved. 4. Classify the following changes: Ans. a. Chemical change f. Chemical change b. Physical change g. Chemical change c. Chemical change h. Chemical change d. Periodic change i. Physical change e. Physical change j. Physical change 5. Write what you will observe when i. milk is not boiled or refrigerated for a day ii. An iron nail is kept half immersed in a test tube of water for many days iii. A match stick is burnt Ans. i. When milk is not boiled or refrigerated for a day, we observe souring of milk. ii. When an iron nail is kept half immersed in a test tube of water for many days, a reddish brown coating on the nail called rust is formed. iii. When a match stick is burnt, there is evolution of energy in the form of heat and light. 6. The set-up below is used to split water into hydrogen and oxygen. Electricity is needed for this to happen. Give three reasons to prove that this is a chemical change. Ans. Electrolysis of water is a chemical change as i. New substances hydrogen and oxygen is formed. ii. The chemical composition and properties of hydrogen and oxygen is different from original substance water. iii. Change is permanent and irreversible. 7. Kartik carried out the following experiment. He took copper sulphate solution in a beaker. He added iron nails to the beaker and left it for few hours. i. Note the observations that he may have made. ii. Is the above change physical or a chemical change. Ans. i. There is a colour change from blue to green due to the formation of Iron sulphate. ii. The change is a chemical change as a new substance Iron sulphate is formed. The change is permanent and irreversible. 8. A burning candle is an example of both physical and chemical change. Do you agree with this statement? Explain your answer. Ans. When a candle burns the wax melts and turns into a liquid state, which on cooling solidifies again. There is a change in state; hence it is a physical change. Simultaneously, when a candle burns two new substances are formed: water vapour and carbon dioxide, hence it is chemical change. Thus melting of wax is a physical change and burning of a candle is a chemical change. 9. Write whether the changes given below are physical or chemical. i. tearing of paper ii. evaporation of water iii. cooking an omelette iv. a sapling growing into a tree Ans. i. tearing of paper - physical ii. evaporation of water - physical iii. cooking an omelette - chemical v. a sapling growing into a tree - chemical 10. Classify the changes involved in the following processes as physical and chemical change.  Sr. Examples Physical change Chemical change No. i. Bread becomes toast Chemical change ii. On rubbing, sanitiser evaporates Chemical change when rubbed on your hand Iii. Butter melts on warm toast Physical change iv. Burning of coal Chemical change v. Crumpling a sheet of aluminium Physical change foil vi. Baking a cake Chemical change vii. Shredding paper Physical change 11. Multiple choice questions: (a) Which of the following indicates a chemical change? A) Change in color B) Change in state C) Change in size D) Change in shape Answer: A) Change in color (b) Which of the following is not a characteristic of a chemical change? A) Formation of a new substance B) Irreversibility C) Change in energy D) Change in state only Answer: D) Change in state only (c) Which of these changes is irreversible? A) Freezing of water B) Evaporation of water C) Burning of paper D) Melting of ice Answer: C) Burning of paper (d) What is a common sign that a chemical change has taken place? A) Change in temperature B) Change in shape C) Change in size D) Change in texture Answer: A) Change in temperature (e) Which of the following is a chemical change? A) Boiling of water B) Dissolving sugar in water C) Cutting of wood D) Burning of wood Answer: D) Burning of wood