Question:medium

Give reason for the following observations.
(a) Naphthalene balls disappear with time without leaving any solid.
(b) We can get the smell of perfume sitting several metres away.

Updated On: Jan 19, 2026
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Solution and Explanation

Given Observations and Reasons: 

(a) Naphthalene balls disappear with time without leaving any solid:

Reason: This occurs due to the process of sublimation. Naphthalene, which is a solid, directly changes into its gaseous form when exposed to air, without passing through the liquid state. This is known as sublimation. The naphthalene molecules escape into the air, and eventually, the solid balls disappear completely.

(b) We can get the smell of perfume sitting several meters away:

Reason: This happens because of the process of diffusion. The molecules of perfume spread through the air from the place where they are sprayed. The perfume molecules move from an area of higher concentration to an area of lower concentration, allowing us to smell the fragrance even from a distance. This diffusion of molecules is facilitated by air particles, which carry the scent over long distances.

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Questions Asked in CBSE Class IX exam

(Street Plan) : A city has two main roads which cross each other at the centre of the city. These two roads are along the North-South direction and East-West direction.

All the other streets of the city run parallel to these roads and are 200 m apart. There are 5 streets in each direction. Using 1cm = 200 m, draw a model of the city on your notebook. Represent the roads/streets by single lines. There are many cross- streets in your model. A particular cross-street is made by two streets, one running in the North - South direction and another in the East - West direction. Each cross street is referred to in the following manner : If the 2nd street running in the North - South direction and 5th in the East - West direction meet at some crossing, then we will call this cross-street (2, 5). Using this convention, find: 

(i) how many cross - streets can be referred to as (4, 3). 

(ii) how many cross - streets can be referred to as (3, 4).

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