Short-answer type questions (II):
1. What do you observe when magnesium is burnt? What is the new substance produced called? Is it acidic or basis?
2. Describe a method to prepare a crystal of copper sulphate.
3. (a) How would you show that curdling of milk is a chemical change!
(b) Explain why burning of wood and cutting it into small pieces are considered as two different types of change?
4. (a) Explain why rusting of iron objects is faster in coastal areas than in deserts.
(b) How can you explain that a reversible change is a physical change?
5. Curdling is a desirable change, formation of ice from water is also a desirable change. But one is a chemical change
and the other is a physical change. Decide which one is a physical and which one is a chemical change? Why are both
changes desirable?
6. (a) Suggest three ways of preventing rusting of iron objects.
(b) Raw rice boiled with water turns to cooked rice, why is it called a chemical change?
(c) Bursting of a fire cracker is a chemical change. Justify.
Inno-answer type questions:
Answers
Answer: When magnesium is burnt it burns with a sparkling light. It then turns to ashes . The new substance is known as magnesium oxide . It is basic , when we add water to make it dilute it becomes magnesium hydroxide .
We can prepare a crystal of copper through the process of crystallisation .
Describe how crystals of copper sulphate are prepared.
Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.
Heat the water.
When it starts boiling, add copper sulphate powder slowly while stirring continuously.
Continue adding copper sulphate powder till no more powder can be dissolved.
Filter the solution.
Allow it to cool. Do not disturb the solution when it is cooling.
Crystals of copper sulphate will be seen at the bottom of the beaker.
In the setting of curd, milk changes into new substance, curd. This is an irreversible process that means milk cannot be made from curd. Formation of new substance and irreversible process shows that setting of curd is a chemical change.
In burning of wood, new substances are formed and hence it is a chemical change. While cutting the wood into small pieces no new substance is formed, thus it is a physical change.
Hence, burning of wood and cutting it into small pieces are considered as two different types of changes.
Iron gets rusted because of reaction with oxygen present in moist air. In coastal areas, air contains more moisture because of sea or ocean, while in deserts air is dry and hot. Thus because of getting more moist air iron objects getting rusted faster in coastal areas than in deserts.
Because in reversible change only the physical properties and composition of the substantially are change and no new substance is formed in reversible change
curdling is a chemical change because it is made by milk n it will change within hours as curd ..
Explanation:
Answer 1 :
When Magnesium burns, it produces a dazzling white flame. The new substance produce is Magnesium Hydroxide [ Mg(OH)₂ ] which is basic in nature.
Answer 2 :
• Take a cup full of water in a 250 mL beaker and add a few drops of dilute sulphuric acid.
• Heat the water. When it starts boiling add copper sulphate powder to the water and stir it continously.
• Continue adding copper sulphate powder till no more powder can be dissolved in water.
• Filter the solution and leave it for some time.
• Later we could see cystals of copper sulphate.
Answer 3 :
(a) While curdling of milk, the liquid of milk is converted into solid form i.e. curd and the chemical properties of curd is different from the chemical properties of milk. Hence, it is a chemical change.
(b) When we burn the wood, it change into ashes and we cannot convert it back into wood, hence, it is a chemical change while cutting of wood doesn't change the chemical properties of wood, hence, it is a physical change.
Answer 4 :
(a) In coastal areas, air contains high percentage of moisture because of sea or ocean, while in deserts air is dry and hot. As the percentage of moisture in air is more in coastal area compared to desert, rusting becomes faster.
(b) This can be understood by taking an example :
When ice is converted into water on melting, we can convert the water back into ice by freezing it. Hence, a physical change is a reversible change.
Answer 5 :
Curdling is a chemical change while formation of water from ice is a physical change.
These changes are desirable because we let these to be happen.
Answer 6 :
(a) Painting, oiling and greasing
(b) Cooking rice is a chemical change because during this process one substance reacts with another to undergo a change in chemical composition. Chemical changes brings change in the chemical properties of rice and we get a new substance i.e. cooked rice.
(c) Because after bursting of crackers, one cannot make it ( the burst crakers ) again as the chemical properties are changed.