Q1. i. In what two ways combustion can be useful to us? Explain by giving examples. ii. Explain why decomposition of sugar on heating is an irreversible change. Q2. In order to answer this question you need to look around your immediate surroundings to find examples of rusting of iron or any other metal. i. What makes iron rust? ii. Why is the rusting of iron a problem? Give any 3 examples from daily life where rusting has been a disadvantage. iii. Suggest any two ways that can help prevent rusting of iron. Q3. i. Give two differences between a physical change and a chemical change. ii. Explain whether condensation of water vapour is a physical or a chemical change. iii. Write word equations for the following reactions: a. Breaking down/ electrolysis of water b. synthesis of water c. decomposition of sugar d. photosynthesis in plants

Q1. i. In what two ways combustion can be useful to us? Explain by giving examples.      

Ans1 i.Combustion is the burning of fuels in oxygen to release energy. It is useful to us in the following ways:

Combustion helps us to cook food.

Combustion helps us to power vehicles by burning fossil fuels to produce energy.

Combustion helps in keeping our home warm by burning coal and wood.

ii. Explain why decomposition of sugar on heating is an irreversible change.  

Ans1. ii.  Decomposition of sugar is an irreversible change because new substances are formed and a chemical reaction has taken place. It is therefore not possible to change the sugar back to its original form.                          

Q2. In order to answer this question you need to look around your immediate surroundings to find examples of rusting of iron or any other metal.

What makes iron rust?  

Ans 2 i. Oxygen and water vapour in the air react with iron to form iron oxide. Iron oxide’s common name is rust. This causes the iron to rust.                

Why is the rusting of iron a problem? Give any 3 examples from daily life where rusting has been a disadvantage.  

Ans 2 ii. Rusting of iron is a problem because it happens on the surface of iron objects and makes them coarse and flaky. It also makes the iron objects fragile. The rust can also eat through the material and causes it to weaken.  

Rusting of car bodies causes the body of the car to weaken and become more fragile and prone to breaking.

Oxygen and water vapour can react with iron bodies of water tanks and pipes and cause them to rust. This causes the metal to become weak.

Rusting can occur on iron towers and thus causes the quality of the metal to diminish.

Suggest any two ways that can help prevent rusting of iron.  

The two ways that can help prevent rusting of iron are:  

Keeping iron clean and dry can keep it from rusting because oxygen in water causes the iron to rust.

Coating the surface of a metal object with paint or varnish can also cause it to become immune to rusting.                                        

Q3. i. Give two differences between a physical change and a chemical change.  

      Ans 3. i. The three differences between physical change and chemical change are:

During a physical change no new substance is formed whereas during a chemical change a new substance is formed.

During a physical change no energy is absorbed or given out whereas during chemical change energy is given out or absorbed.

During a physical change there is no change in the type of molecules. However, in chemical change there is a change in the type of molecules.                

ii. Explain whether condensation of water vapour is a physical or a chemical change.  

Ans 3 ii. Condensation of water is a physical change as no new substance is formed during condensation. Only the state of water is changed. Also, as no energy is given out or absorbed,condensation of water is an example of a physical change.    

iii. Write word equations for the following reactions:                      

Breaking down/ electrolysis of water

Ans a.        Electric current

Water        Hydrogen + Oxygen

synthesis of water                

                                          Heat

                       Hydrogen + Oxygen          Water

decomposition of sugar          

                                              Heat

                                                Sugar                                 Carbon + Water

photosynthesis in plants          

                                                Light

Carbon dioxide + water                                   Glucose + Oxygen + Energy

Explanation: