How can I improve this experiment so that it is more accurate/reliable?

We measured out 20 cm^3 of sodium thiosulfate solution in to a 100ml conical flask and then we added 30 cm^3 of water to it. We placed the flask on an "x" marked on paper. Then we added 5 cm^3 of hydrochloric acid to the flask. I recorded the temperature immediately after adding hydrochloric acid. My friend started the stopwatch and recorded time taken for "x" to disappear. We did this experiment for five different temperatures.