 In this video, let us look at the rules which help us determine which digits in a measurement are significant and which digits are not significant. So first, let us look at all the non-zero digits. The rule is that any non-zero digit in a measurement is significant. For example, if we have a measurement like 232 grams, then there are three significant digits in this measurement. All three of the digits 2, 3 and 2 are significant. Similarly, a measurement like 5.78 milliliters. How many significant digits does this have? Again, 3. All three of these are significant. So any non-zero digit in any measurement is always significant. So that takes care of 1 through 9. That takes care of all these numbers. These numbers are always going to be significant. What is left? What is left is zeros. Let us talk about zeros now. Now zeros can be before non-zero digits. They can be between non-zero digits or they can be after non-zero digits. First let us talk about when the zeros are before non-zero digits. Whenever we have zeros before non-zero digits, we can safely say they are not significant. So for example, in a number like 0.0032 kgs, this is a measurement. But in this, if someone asks us how many significant digits there are, then the answer is only 2. This 3 and this 2 are significant because these are non-zero digits. But all of these zeros are not going to be significant. So any zeros that are before a non-zero digit are not significant. Okay, what about zeros between non-zero digits? These are always going to be significant. For example, a measurement like 40.009 litres. If someone asks us how many significant digits there are in this measurement, then the answer is 5. 400009. Wait, I think I said an extra zero there. But all of these zeros and the 4 and the 9, all of these are going to be significant. So there are 5 significant digits in this number. Okay, finally we are left with the zeros that come after the non-zero digits. In this case, there are two situations possible. The first situation is that there is a decimal in the number. If there is a decimal in the number, then those zeros are significant. For instance, if we have 521.00 centimeters, if this is a measurement, then all five of these digits, 521 because these are non-zero and both of these zeros are going to be significant. There are 5 significant digits here. These zeros are coming in a number where there is a decimal. So both of them are going to be significant. Similarly, 491.30 grams. How many significant figures here? Again, 5, all of these are significant. Finally, the only case we are left with are when zeros are after non-zero digits but there is no decimal in the number. For example, 52,000 grams. How many significant figures here? Only two, 5 and 2, the other zeros are not going to be significant because there is no decimal in this number. So 5 and 2 are significant, the other three are not. There are only two significant digits here. What about 350 grams? How many significant digits here? Only two. The last zero is not significant because there is no decimal in this number. All right. So we have gone through all of these rules, absorbed all of these rules and then let us look at some examples and try to find out the number of significant digits. So the first number we have is 0.00034 grams. How many significant digits here? Pause the video and give it a shot. So all of these zeros are they going to be significant? No. All of these are not going to be significant because these are zeros that come before non-zero digits. These two are going to be significant because they are non-zero digits. So only two significant figures in this number. What about 0.0209 centimeter cube? How many significant digits in this measurement? Again pause and give it a shot. There are only three significant digits here. These two are not significant because these come before non-zero digits. This zero is significant because this is between non-zero digits. So there are three significant figures here. What about 258,000 seconds? How many significant digits here? Pause and try. There are three significant figures here as well. One, two and three. These are all not significant because these are zeros that come at the end of a number without a decimal. Finally, this is the last one I'm going to give you. 230.00 milliliters. How many significant figures here? The answer is all of them are significant. One, two, three, four, five, five significant figures. Because there is a decimal here, so all zeros that come after the non-zero digits are going to be significant. So all of them are significant and we have five significant figures.