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Chapter 9 — The Amazing World of Solutes, Solvents, and So147ions Crust Let us dig deeper! Did you know that our planet, Earth, is Upper mantle composed of several layers, such as crust, upper mantle, lower mantle, outer core, and Lower mantle inner core, each with its particular range of density? The outermost layer, called the crust, is the lightest and the density of the different Outer core (liquid) layers increases as we move towards the centre(Fig.9.20).Asonemovesdeeperintothe Inner core (solid) Earth, both the pressure and the temperature Fig. 9.20: Layers of Earth rise significantly, making the materials heavier and more compact. Ever heard of ... In ancient times, before large ships were invented, people used bamboo and wooden logs to travel across rivers and seas (Fig. 9.21). Bamboo was used because it is light, hollow, and floats easily on water. People tied bamboo poles together to make rafts and small boats for fishing, trading, and crossing water bodies. Wooden Fig. 9.21: Bamboo raft floatlogs, especially from strong trees were on water either hollowed out to make boats or used as rafts. These simple boats, made from locally available materials, were important for moving around and connecting different places. Even today, similar traditional boats made of bamboo or wood are used in some regions—not just for transport, but also as tourist attractions. 9.5.2 Effect of temperature on density Generally, the density of a substance decreases with heating and increases with cooling. This can be explained on the basis of what you have learnt in chapter ‘Particulate Nature of Matter’. As temperature increases, the particles of a substance whether, solid, liquid, or gas, tend to move away and spread. This results in an increase in volume but there is no change in mass. Since the Density = Mass/Volume, upon heating, the volume increases and the density decreases. This explains why hot air moves up as it is less dense than the cool air around it. The hot air balloonworksonthesameprinciple(Fig.  9.22). Fig. 9.22: Rising of hot air balloons Reprint 2026-27