Question

Classify the following as physical change or chemical change. I) cooking of food II) boiling of water III) cutting of trees IV) dissolving salt in water V) digestion of food VI) melting of ice

Answer 1

Answer and Explanation:

$\mathbb{\bold {\underline {Physical \ Changes}}}$

$\implies$are changes that does not make any difference in the chemical constitution of a substance

$\implies$these changes are generally reversible

$\mathbb{\bold {\underline {Chemical \ Changes}}}$

$\implies$are changes that makes differences in the chemical constitution of a substance

$\implies$these changes are generally irreversible

From the given examples,

1. Cooking of food => Chemical change
2. Boiling of water => Physical change
3. Cutting of trees => Chemical change
4. Dissolving salt in water => Physical change
5. Digestion of food => Chemical change
6. Melting of ice => Physical change

Cooking of food changes the chemical constitution of the ingredients due to breakdown and formation of many new substances

Boiling of water does not change the chemical constitution as it is just a phase change [  H₂O(l)   -------->    H₂O(g)  ]

Cutting of trees is a chemical change since all chemical reactions stop and we cannot get back the original tree

Dissolving of salt does not changes the chemical constitution and we can regain the salt back by cooling it

Digestion of food changes the chemical constitution as enzymes act upon the food to break it down into simpler forms

Melting of ice does not change the chemical constitution as it is just a phase change [  H₂O(s)   -------->    H₂O(l)  ]

Answer 2

$\large{\underline{\rm{\red{Solution:-}}}}$

I) Cooking of food: Chemical change

II) Boiling of water: Physical change

III) Cutting of trees: Physical change

IV) Dissolving salt in water: Physical change

V) Digestion of food: Chemical change

VI) Melting of ice: Physical change

$\large{\underline{\rm{\red{Explanation:-}}}}$

I) Cooking of food is a chemical change since it involves changes in physical composition of food. When we cook food it can change colour or have a difference in smell which indicates that is a chemical change.

II) Boiling of water is a physical change because the water vapor still has the same molecular structure as liquid water. If the water boils it just rises the temperature and there is no difference in the internal structure.

III) Cutting of the tree is a physical change but a irreversible change as there is only a change of appearance and the composition is the same.

IV) Dissolving of salt in water is a physical change as only the state of the salt is getting changed. If you evaporate the salt water, you can get back the salt in the solid form.

V) Digestion of food is a chemical change as it involves changes in the composition of food. Enzymes help to break down large molecules into smaller ones, so that the food can be absorbed easily.

VI) Melting of ice is a physical change as only the temperature or pressure gets changed. Freezing of melted ice can get back the ice, therefore there is no permanent change occurring here.

$\large{\underline{\rm{\red{Physical \: Changes:-}}}}$

A change of a physical properties of a substance is called a physical change.

Physical changes are usually reversible as no new substance is formed.

Physical properties include size, shape, colour and state of a substance.

Example: Melting of ice, dissolving of salt in water, boiling etc.

$\large{\underline{\rm{\red{ Chemical \: Changes:-}}}}$

A change in which one or more new substances are formed is called as a chemical change. Chemical changes are also known as chemical reactions.

A chemical change occurs when two substances react chemically to form a new substance with different chemical properties.

Chemical changes are permanent and cannot be reversed.

Example: Digestion of food, rusting of iron, burning wood, souring milk etc.