Maharashtra Board Class 7 Science Changes – Physical and Chemical
Distinguishing Between Types of Changes:
Physical change and chemical change:
Physical Change: In a physical change, the substance’s composition remains the same, and no new substance is formed. It involves changes in the physical state or appearance. Example: Melting ice into water.
Chemical Change: In a chemical change, one substance is transformed into another with different properties, and a new substance is formed. Example: Burning wood to produce ash and smoke.
Periodic change and non-periodic change:
Periodic Change: Periodic changes are those that occur at regular intervals of time, and their recurrence is predictable. Example: Day and night.
Non-Periodic Change: Non-periodic changes are those that do not have a fixed time interval for recurrence, and their timing is not predictable. Example: Earthquakes.
Natural change and man-made change:
Natural Change: Natural changes occur without human intervention and are a part of the natural processes. Example: Ripening of a fruit.
Man-Made Change: Man-made changes are brought about by human actions and interventions. Example: Baking bread.
Classifying the Changes:
Conversion of milk into yogurt:
This is a chemical change because the original substance (milk) is transformed into a new substance (yogurt) with different properties.
Bursting of a cracker:
This is a fast/quick change and a non-periodic change because it happens suddenly, and the timing is not predictable.
Occurrence of an earthquake:
This is a non-periodic change because earthquakes do not occur at regular intervals, and their timing is not predictable.
Revolution of the earth around the sun:
This is a periodic change because it occurs at regular intervals and is predictable. It’s also a natural change.
Stretching of a spring:
This is a physical change because the material of the spring retains its composition, and no new substance is formed.
Giving Reasons:
While purchasing canned food, its expiry date should be checked:
Canned food has a limited shelf life due to the potential for chemical changes and microbial growth over time. Checking the expiry date ensures that the food is safe to consume.
An iron article should be given a coat of paint:
A coat of paint acts as a barrier, preventing exposure to oxygen and moisture, which can cause iron to rust (a chemical change).
A wooden article should be polished:
Polishing helps protect the wood by providing a protective layer, enhancing its appearance, and preventing it from getting damaged or deteriorating.
Copper and brass utensils should be tinned:
Tinning forms a protective layer on the copper or brass, preventing them from corrosion when they come in contact with acidic or alkaline foods.
A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry:
This is due to the difference in water absorption rates. A dry handkerchief quickly absorbs water upon contact, but evaporation, the process of drying, takes longer as it requires heat and air circulation to remove the moisture from the fabric.
