Q1: We usually shorten the length of the pant by folding it, when walking through a waterlogged area. Is the change reversible or not?
Answer: Yes, The change is reversible and it can be done by unfolding the pants.
Q2:You accidentally drop your Barbie doll and break it. Is this reversible or not?
Answer: No, The change (breaking of Barbie doll) cannot be reversed.
Q3:Among the examples listed down, state whether they are reversible or not.
| S.no | Changes | Reversible or irreversible |
| 1 | Dissolving salt in water | |
| 2 | Melting of ice cubes | |
| 3 | Release of steam from the cooker | |
| 4 | Ripening of a fruit | |
| 5 | Freezing orange juice by cooling | |
| 6 | Melting of wax |
Answer:
| S.no | Changes | Reversible or irreversible |
| 1 | Dissolving salt in water | Reversible |
| 2 | Melting of ice cubes | Reversible |
| 3 | Release of steam from the cooker | Irreversible |
| 4 | Ripening of a fruit | Irreversible |
| 5 | Freezing orange juice by cooling | Reversible |
| 6 | Melting of wax | Reversible |
Q4:Is the change where you draw a picture on a drawing sheet, reversible?
Answer: If we draw a picture using a pencil sketch, then the change is reversible.
But, if the picture is drawn using an oil color, sketch, watercolors or pen, then the change is irreversible.
Q5:State the examples for reversible and irreversible changes.
Answer:
i) If we draw a picture using a pencil sketch, then the change is reversible. However, if the picture is drawn using an oil color, sketch, watercolors or pen, then the change is irreversible.
ii) If we inflate a tire, the shape and size of the tire changes. This change can be reversed, but if the tire bursts while inflating, then the change is irreversible.
iii) Melting of ice is a reversible change whereas cooking of food is an irreversible change.
Q6:For a fractured bone, Plaster of Paris bandage is preferred. Why? State whether the change is reversible or irreversible.
Answer: Plaster of Paris bandage is preferred because it becomes hard on drying to keep the bone immobilized which help in recovering at a faster rate. Since there is a change of state, a new product is formed which cannot be brought back to its original form. So the change is irreversible.
Q7:If you leave a piece of iron outside for a long time it turns brown leading to the formation of rust. Is the change reversible or irreversible?
Answer: The formation of rust is completely a new substance due to the reaction of oxygen with the iron, which is termed to be a chemical change. Therefore, it not possible to avoid the rust formation and the change is irreversible.
Q8:State a few examples for slow changes.
Answer:
i) Germination of seed
ii) Rusting of iron
iii) Curdling of milk
Q9:State a few examples for fast changes.
Answer
i) Burning of candle
ii) Burning of paper
iii) Inflating tire
Q10:State any two examples of reversible changes.
Answer:
i) Melting of ice
ii) Melting of candle wax
Q11:State any two examples of irreversible changes.
Answer:
i) The ingredients like flour, egg and sugar is converted to cake
ii) Cooking of food
iii) Burning of fuel
Q12:Deforestation is an irreversible change. Why?
Answer: Once the trees are cut, the same tree cannot be planted. So deforestation is considered as an irreversible change.
Q13:The explosion of a cracker is a chemical change. Explain.
Answer:The explosive reactants of a cracker are transformed into gaseous products along with heat and light. Thus, it cannot be brought back to the original form. Hence, it is a chemical change.
Q14:Blacksmith heats the metal rim to fix it on the cartwheel. Explain.
Answer: The metal rim will be smaller than its usual size. On heating, the metal rim expands and fits onto the wheel. Similarly, on cooling, the metal rim contracts and fits tightly to the wheel.
Q15: Define slow changes and fast changes. State a few examples.
Answer:
Slow changes:
The changes, which take place slowly over a period is called slow changes.
Ex-Germination of seed, formation of day and night and Rusting of iron
Fast changes:
The changes which take place quickly is called fast changes.
Ex: Burning of paper, Melting of a candle and blowing of balloons.
Q16: Classify the following as slow or fast changes:
i) Growth of a plant
ii) Formation of clouds
iii) Formation of sugar solution
iv) Inflation of tire
v) Melting of chocolate
Answer
i) Slow change
ii) Slow change
iii) Fast change
iv) Fast change
v) Fast change
Q17: What are the changes taking place in our body?
Answer:
a) The height of the person increases
b) Nails grow
c) The weight of the person increase
d) Our hair grows.
Q18: Formation of clouds is usually a physical change. Justify the statement.
Answer: The formation of clouds is a physical change because there is a phase transformation cycle of natural water from liquid to gas and then, gas to liquid. Hence, the property of water never changes.
Q19 : Explain the physical changes and chemical changes with an example.
Answer:
Physical change:
i) The changes where the physical property of the substance varies without any formation of new products.
ii) These changes are reversible and the products are identical to the reactants.
Ex: Melting of ice, during this change the water changes from its solid form to liquid form, melting of wax.
Chemical change:
i) The changes where the chemical property of the substance varies with the formation of new products.
ii) These changes are irreversible and the products are not identical to the reactants.
Ex: Formation of curd from milk, rusting of iron.
Q20:What will be the change when sugar is heated?
Answer: When sugar is continuously heated, a black powdery substance is formed which is called charcoal. Since a new product is formed, it is termed to be a chemical change.
Q21.Give some examples of changes, which take place on their own.
Answer:
The changes that take place on their own are
i) Shedding of leaves
ii) Ripening of fruits
iii) Growth of the body
iv) Blooming of flowers
Q22:How do we convert milk into curd and what type of change is it?
Answer: The milk is taken in a vessel and a small quantity of curd is added into it. The milk is stirred and it is kept aside for few hours without any disturbance. Finally, the milk is converted into curd. However, curd formed cannot be converted into milk again indicating that it is an irreversible change.