Bubbles often form at the bottom of the containing vessel due to the presence of nucleation sites, convection currents carrying bubbles upwards, and the decrease in pressure at the bottom as the liquid gets heated.
The theory section of the experiment suggests that bubbles should form near the surface when a liquid starts to boil. However, in reality, bubbles often form at the bottom of the containing vessel due to several factors:
1. Nucleation sites: Bubbles tend to form at locations where there are nucleation sites, which are tiny irregularities or impurities on the surface of the container. These nucleation sites provide a surface for the formation of gas bubbles. The bottom of the vessel often has more nucleation sites, such as scratches, dust particles, or imperfections, compared to the smooth surface near the top.
2. Convection currents: As the liquid is heated, convection currents are set up within the liquid. Hotter regions near the heat source rise while cooler regions sink. This creates circulation within the liquid. As the liquid near the bottom of the vessel gets heated, it rises towards the surface. Along with it, any bubbles that have formed at the bottom are carried upwards, causing them to be released near the surface.
3. Increased pressure at the bottom: The pressure at the bottom of the vessel is higher compared to the surface due to the weight of the liquid above it. This higher pressure can prevent the formation and release of bubbles at the bottom. However, as the liquid gets heated and its density decreases, the pressure at the bottom decreases, allowing bubbles to form and rise.