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Adding salt decreases the freezing point of ice. We know pure ice melts at 0°C and if we add salt to it it starts melting at a lower temperature, let's say at -6°C. That means ice start melting at a faster rate than pure ice. That is why it is used in melting snow at roads. But do you wonder how is it helpful in freezing ice cream? The answer lies in the huge difference in latent heat of melting and specific heat. The latent heat of fusion of ice is approx. 334 KJ/Kg while the specific heat of ice is 2.09 KJ/Kg°C. Let's consider an example, if we have 1 kg of an ice at -10°C. It will absorb 20.9 KJ heat from surrounding and start melting at 0°C. But if we add salt 100 gm of salt to 1 Kg of ice it will approximately decrease the freezing point to -6°C. So it will absorb 8.36 KJ of heat and reaches to -6°C and then start absorbing 334 KJ heat to melt. This means the temperature will be -6°C for quite long time and it will help infreezing ice cream.

In series  The charged particles move in only one path so the current i.e. charge/time will be same in all the appliances connected The ammeter which is used to measure current is connected in series The resistance of ammeter is zero ideally to avoid any potential drop The total current should pass through the fuse which is used to protect the appliances, as the current exceeds the limit of fuse, it gets heated and melts. The potential drops as the current passes through the appliances. In parallel The charged particles have different paths to pass, so the total current divides in all the paths All the appliances are connected to the same potential terminals, so the potential difference is same across all the appliances. Voltmeter which is used to measure potential difference is connected in parallel The resistance of voltmeter is infinite ideally to avoid any current through it.