Solutions-Class 7-Science-Chapter-13-Changes-Physical & Chemical-Maharashtra Board

(a) Conversion of milk into yoghurt- Chemical, irreversible

(b) Bursting of a cracker- Chemical, irreversible, fast

(c) Occurrence of an earthquake- Natural, Physical, irreversible, non-periodic.

(d) Revolution of the earth around the sun- Periodic, Natural

 (e) Stretching of a spring- Physical, reversible

• Every canned food item has a limited shelf life.
• An expiry date indicates that for how long a canned food can be safe for consumption.
• Canned foods after expiry date over are not considered worth for consumption.
• As they could affect your health due to irreversible chemical change in it.
• They might get contaminated with microorganisms like bacteria or may have no nutritional value left after its expiration.

Therefore while purchasing canned food their expiry date should be checked because

• Iron articles often undergo rusting resulting in the formation of a reddish brown layer on its surface is called corrosion.
• Corrosion irreversible chemical change, caused by the exposure of iron to the air, moisture and chemical vapours in the surroundings.
• Corrosion makes article weak and unusable.
• To prevent rusting, iron article is coated with a paint.
• Paint coating prevents the interaction of iron with the surrounding air and moisture, thus protects it from rusting.

A wooden article should be polished because, A wood is organic substance and can be affected by fungus or termite, can results into irreversible chemical damage.

If polish applied, it protects the wooden article from termite and all other external conditions and increase the life of wooden article.

• Copper and brass materials also undergo corrosion resulting in the formation of a greenish layer on this surface.
• When the surface of copper/ brass comes in contact with moisture, vapours of chemical presents in the air, slow irreversible chemical change occurred, this can results in corrosion.
• To prevent this, copper and brass utensils are coated with tin.
• The tinned utensils are safer for cocking and storing food.

A dry handkerchief gets wet at once because it quickly absorbs the water in which it is dipped. However, it takes longer for a wet handkerchief to dry because the water needs to evaporate from the handkerchief. Evaporation is a slow process which is depend upon atmospheric temperature and humidity.

Chemical changes are changes that involve reaction of substances with one another. Such reactions result in alterations in the properties and chemical compositions of the substances involved. These changes lead to the formation of new substances. If

• One or more new substances are formed during a change.
• Changes are irreversible.
• The chemical composition of a substance does not remains the same.

Then it is a chemical change. A chemical change is always accompanied by a change in energy.

• It was nearing six o'clock in the evening (Time-Irreversible, Periodic).
• The sun was setting (Periodic change).
• A breeze was blowing (Natural, non-periodic change).
• Leaves on the tree were shaking (Physical change).
• Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys (Physical change).
• Then he felt hungry (Chemical, periodic change) and went into the house.
• Mother made a dough from wheat flour and fried purees (Physical Irreversible change).
• While eating hot purees, his attention was drawn outside the window. It had started raining (Natural change).
• There was lightning (Natural, Physical, Chemical change).