Water often has ions dissolved in it: post 16.39 explains why and gives more information on ions. Two common ions found in drinking water are calcium ions (Ca2+) and magnesium ions (Mg2+). When these ions interact with soap, they form a soft, insoluble deposit called scum. Water that forms scum, in this way, is called… Continue reading 19.1 Hard water

Before you read this, I suggest that you read posts 16.34, 16.35 and 18.29. In post 16.34, we noted that heat flows spontaneously from a hot object to a cold object. This process is shown in the picture above, where the temperature T1 is greater than the temperature T2, and the arrow shows the direction… Continue reading 18.30 Heat pumps

Before you read this, I suggest you read post 18.27. The picture above is almost the same as the second picture in post 18.27; a membrane (green) separates water molecules (blue) from a solution of big molecules (red) in water. The big molecules are too big to pass through the pores of the membrane. The… Continue reading 18.29 Reverse osmosis

Before you read this, I suggest you read posts 16.37, 18.25 and 18.27. In post 18.27 we saw that molecules in a solution are free to move around like molecules in a gas. There are two main differences. The first is that, in a gas, movement of the molecules is constrained only by the walls… Continue reading 18.28 Applying the ideal gas equation to solutions

Before you read this, I suggest you read post 18.26. Molecules in solution can diffuse through the pores in a membrane, if the pores are big enough. The membrane is coloured green in the picture above. On the left-hand side of the picture , bigger molecules (red) are dissolved in water molecules (blue); on the… Continue reading 18.27 Diffusion through membranes, osmosis and dialysis

In post 16.37, we saw that the molecules in a liquid or a gas are free to move; in a gas they are free to move anywhere, in a liquid they are free to move anywhere below the surface. Now let’s think about what happens when we add a coloured liquid to a transparent liquid,… Continue reading 18.26 Diffusion

Before you read this, I suggest you read post 16.37. A gas will fill all the space available in a sealed container; it has no surfaces except where it meets the walls of its container (post 16.37). Let’s think about a gas in the cylinder (coloured yellow in the picture above) that is sealed by… Continue reading 18.25 An Ideal Gas

It might be useful to read post 17.39 before you read this. Let’s suppose we use a spring (post 16.49) to push an object (mass m – post 16.13), through a viscous liquid (post 17.17). Three things happen at once: (1) the force compresses the spring (post 16.49), (2) the force overcomes the drag force… Continue reading 18.24 Analogies between electrical and mechanical systems

Before you read this, I suggest you read posts 17.44, 18.20 and 18.22. The picture above shows a resistor (of resistance R, post 17.44), a capacitor (of capacitance C, post 18.20) and an inductor (of inductance L, post 18.22) connected together; I have used standard symbols to represent these components. There is a potential difference… Continue reading 18.23 Frequency response and resonance in electrical systems

Before you read this, I suggest you read posts 17.44, 18.20 and 18.21. The picture above shows an electric circuit containing a resistor of resistance R (post 17.44) and an inductor of inductance L (post 18.21). I have used standard symbols for a resistor and an inductor. There is a time-dependant potential difference, V, between… Continue reading 18.22 Inductance and impedance