Measuring heat effect of reaction

I did an experiment to determine enthalpy change when Zinc is added to Copper(II) sulphate solution.

I recorded values and plotted a temperature-time graph to obtain values of the temperature change to calculate heat transfer .

The trend line that I obtained is when temperature of the mixture started to drop.

I never knew why I needed to extend that trend line and draw a vertical line at the point when Zinc is added and thus never understood why the difference between the interception point and the point when Zinc is added is $\Delta T$

To summarise, I don’t understand why I need to extend the trend line and the interception point gives the highest ‘T’ value...

• Thanks alot! One more question from me, I did another exercise where I just recorded the initial and highest temperature it reached to calculate enthalpy change. As compared to the style I went for above (when I recorded the temperature in intervals), why would I get a more accurate $\Delta H$ value for the style I went for above rather than the exercise when I record the initial and highest temperature ? – user175089 Jan 12 '18 at 16:29