Science Buddies: "Ask an Expert"

Hello, my name is Huthayfah and I have an important question regarding my science fair project,

I need my third trial to be done by 12/24/2019. The purpose of my project was to find out which based fertilizer (Urea 46-0-0, Diammonium Phosphate 0-46-0, or Potassium Chloride 0-0-60) affects algae the least, and is the safest for our ecosystems (lakes, oceans, and rivers). The procedure involved pouring 10ml of algae populated water into 4 different clear containers, which each had a lid with a tiny hole in them. Next, I added ⅛ tsp of fertilizer 1 to container A. 1/8 tsp of Fertilizer 2 to container B, and 1/8 tsp of fertilizer 3 to container C. Container D was the control. Next, I weighed them (twice to make sure), and placed them on a window sill. Finally, I took the data on the weight of each container after 3 days and compared it to the weight at the beginning of the experiment. The only problem was for my first trial the wight decreased from all of them, I thought that maybe the wholes were too big, so on my second trial, I did the same thing except with tiny holes. The wight still decreased. Is this because of evaporation, what should I do for my next trial? Please answer quick. My science teacher doesn't know what to do either.

Hi and welcome to Scibuddies. I will try to help you figure out your experimental results.

My first question would be what algae did you use and where did you get it? What did you grow the algae in? Most city water contains chlorine that will kill algae or at least keep it from growing.

My next question is how accurate is your scale? Your experiment was to measure the increase in weight due to the growth of the algae but you only allowed them 3 days to grow which may not be enough time. If your scale is only accurate to 0.1 g then you probably won't be able to read such a small change accurately.

Since the weight of all the containers with the algae decreased, I think you have to assume that it was due to evaporation. You could test this by using a container with a tight fitting lid and comparing its weight after three days to a container with a loose lid from which water can evaporate. If the containers were on a sunny window sill that got quite warm then the water would evaporate. That would be the only way that the weight could decrease.

A better way to measure the effect of fertilizer on algae is to measure their production of oxygen. The method requires some time and apparatus to collect and measure the gas, however. Here is the procedure from a Scibuddies project on yeast:
https://www.sciencebuddies.org/science- ... #procedure

Yeast produces carbon dioxide, not oxygen, but the method for collecting and measuring gas is the same.

I hope this helps. If you have more questions, please post again.

Sybee