Question

The melting point of mercury is -36°F and its boiling point is 672°F. What is the difference between the boiling point and the melting point?

Answer 1

704

Explanation:

Whatever the problem context or topic, the math relationships are the same. In this case a number line is a very useful device for visualizing what we do with combinations of negative and positive numbers. When you place a value on a number line, you can see that adding or subtracting one value from another is just moving “up” or “down” the line in the direction of the sign.

So, for this problem we have a number on one side of zero (negative) and another on the other side (positive). You can try to remember “rules” about adding positives and negatives, but you can also look at the number line and see that “-36” is 36 ‘units’ away from zero in one direction, and 672 is in the other direction. Thus, the TOTAL ‘distance’ or value between them is the SUM of the distances from zero, or ‘absolute’ values.

So the difference (subtraction, algebraically) is 672 - (-32) = 672 + 32 (that’s the algebra rule) = 704 – what you see if you look at a number line, or do the algebra correctly.