BASIC SCIENCE FOR CLASS 7 AMARNATH MISHRA,PHD GEETA MEHTA ,PHD DEEPTI LEHRI,PHD RAJESH MOHAN, PHD

           Chemistry

             Ch-3 

              Change and reaction

A. Answer the following in not more than 20 words. 

Question 

 1. Define a physical change 

 Short Answer 

 A physical change is a type of change in matter where the substance's physical properties are altered, but its chemical composition remains the same.

 2. Define a chemical change 

 Short Answer 

 A chemical change is a process in which one or more substances are transformed into new substances with different chemical properties.

3. 3. Classify the following into physical and chemical charges.

(a) The cooking of food- chemical change 

(b) The evaporation of a liquid - physical change 

(c) The digestion of food - chemical change 

(d) Burning - chemical change 

(e) The condensation of water vapour - physical change .

Question 

 4. What are the following called?

(a) A reaction in which two or more reactants add up to form a product . 

 Short Answer.

Addition reaction

(b) A reaction in which one substance breaks down into two or more simpler substances. 

 Shorts Answer 

 Decomposition reaction.

(c) A reaction in which one element displaces another from a compound and takes its place in the compound. 

 Short Answer 

 This type of reaction is called a displacement reaction. 

(d) A reaction in which an acid reacts with a base to form a salt and water 

 Short Answer 

 Acid-base neutralization reaction.

B.Answer the following in not more than 40 words. 

Question 

 1 . How would you decide whether a change is physical or chemical ? Mention two characteristics of each kind of change. 

 Short Answer 

 The change is physical if the substance's composition remains the same, and it is chemical if the substance's composition is altered.

 2. Discuss why the dissolving of salt in water is a physical change. 

 Short Answer 

 The dissolving of salt in water is a physical change because no new substance is formed and the process can be reversed by evaporation.

 

 3. Discuss an example of physical and chemical changes happening together. 

 Short Answer 

 An example of physical and chemical changes happening together is the process of rusting. 

C.Answer the following in not more than 100 words. 

Question 

 1. Giving one example of each kind, show that a change in energy takes place when a physical or a chemical change occurs. 

 Short Answer 

 In physical changes, energy is often only transferred between systems, while in chemical changes, energy is transformed.

 2. Describe an activity to find the conditions required for rusting 

 Short Answer 

 Conducting a rusting experiment using different conditions and observing their effects on the process.