Chapter 1: Matter in Our Surroundings

Answer: Anything that occupies space and has mass is called matter.

The water vapour present in air, comes in contact with the cold outer surface of the container thereby condensing it to form water droplets.

Solids have fixed shape due to strong intermolecular force of attraction between them. The liquids and gases have molecules with less intermolecular force of attraction and hence they can flow and take shape of the container.

Liquids and gases have intermolecular space, on applying pressure externally on them the molecules can come closer thereby minimizing the space between them. But in case of solids there is no intermolecular space to do so.

The balloon has air filled in it. The balloon when kept in sun gets heated and the air inside it also gets heated. The molecules of air get energy, and vibrate faster thereby exerting large force on the walls of the balloon. Due to this expansion of gases the balloon bursts.

On a hot, humid day, due to the heat our body starts sweating for the cooling mechanism i.e., by evaporation and gets cooling effect. But the air cannot hold any more water on a humid day and therefore the sweat or perspiration is seen.

At higher altitudes, the atmospheric pressure is low and the water boils very fast and evaporates at faster rate therefore the pressure is required to increase the cooking process and this is done by using pressure cooker which increases the pressure inside the container and cooks food faster.

The states of matter that can flow due to less intermolecular force of attraction, are liquids and gases and are called as fluids.

1 kg sand is more denser than 1 kg cotton because density = mass/volume. The volume required by cotton is more than the sand and density and volume are inversely proportional.

Pressure and temperature determine the state of a substance.

• Connect Pages: Any matter i.e., solid, liquid or gas when experiences an increase in temperature then they change their state. Take ice cubes in a beaker or heat them slowly, the temperature increases and the ice melts to form liquid. Heat this liquid further it will become steam.
• On lowering down the temperature of any matter, show change in their state. Take the steam that is coming out of a boiling water and allow it to cool down, it condenses to form water and on further cooling of this water we get ice.
• On applying pressure and reducing temperature we can liquefy gases or change them into solid.
• Example: Take carbon-dioxide gas, reduce its temperature and apply lot of pressure on it so that it changes into solid carbon dioxide, called diy ice, which is used as refrigerant for cooling.
• If the pressure on it is decreased, it directly changes into gas. In LPG cylinders, the petroleum gas is cooled and with lot of pressure changes it into liquid state. While using this LPG, we release the pressure exerted on it and hence it comes out in the form of gas.

The rate of evaporation depends on the following factors:

• Surface area: If the surface area is increased the rate of evaporation also increases. (a) To dry the clothes we spread them to dry faster. (b) Tea in saucer cools faster than in a cup.

• Temperature: If the temperature is increased the rate of evaporation also increases. Due to increase in temperature the particles gain more kinetic energy and change their phase from liquid to gaseous. Water will evaporate faster in sun than in shade.

• Humidity: It is the amount of water vapour present in air. The air can hold definite amount of water vapour, at a given temperature. If the amount of water vapour is high in the air then the rate of evaporation decreases. On hot and humid day, desert coolers are not effective as the air cannot hold any more moisture to get the cooling effect.

• Wind speed: With the increase in wind speed, the rate of evaporation increases. The particles of water vapour move away with the wind, decreasing the amount of water vapour in the surrounding.