NCERT Solutions For Class 9 Science Chapter 1
Matter in Our Surroundings

Steam contains more heat, in the form of latent heat, as compared to boiling water. So when steam comes in contact with skin it will give more heat than boiling water, so steam causes more severe burns.

Burns caused by steam are more severe than those caused by the boiling water; though both are at the same temperature (100 °C).This is because, steam has 540 kcal/kg (latent heat) more energy than boiling water. When steam condenses on skin, it gives out 540 kcal/kg more energy than the boiling water.

Ice is solid form of water which at its freezing point (273K) absorbs heat to get converted into liquid form at this constant temperature which is called the latent heat of fusion. When completely converted into water (liquid), it will absorb heat till its temperature becomes equal to that of the surrounding or the body to which it has contact.

But water at 273K will only absorb heat till its temperature becomes equal to that of the surrounding or the body to which it has contact.

Therefore, ice at 273 K more effective in cooling than water at the same temperature.

(a) Water has no fixed shape but has a fixed volume that is,it occupies the shape of the container in which it is kept. Boiling point of water is 100°C and its freezing point is 0°C but the room temperature is between 0 and 100 0°C, therefore, it is a liquid at room temperature.
(b) All the atoms in iron are very tightly packed at the room temperature. Also its melting and boiling point is very high as it is a metal. So it is a solid at room temperature.

(a) At 25°C water exists as a liquid, with some water vapour above the water surface
(b) At 0°C water can exist as a solid as well as a liquid. Small amount of water vapour will be present in the air above the water/ice mixture.
(c) At 100°C water can exist in both liquid and gaseous states.

Oxygen is a gas; therefore, the force of attraction is negligible between its particles. Water is a liquid, so the force of attraction between its particles is more than liquid and less than solid. Since sugar is a solid, thus force of attraction between particles is greatest. Hence, the correct increasing order of forces of attraction between the particles is as follows

Oxygen < Water < Sugar

(a) Naphthalene is a sublimating substance. Therefore, naphthalene balls sublimate that is turns into vapour without changing into liquid. Therefore, they disappear with time without leaving any solid.
(b) We can get the smell of perfume sitting several meters away because of diffusion. The molecules of perfume spread out through the air and reach to great distance.

A. Fusion or melting
B. Vaporisation
C. Condensation
D. Solidification
E. Sublimation
F. Deposition