Did your teacher give you this assignment, or did you just think, “I bet I can stump the folks at Ochef?” There are many differences and similarities between butter and margarine, but there are also many different variations primarily among margarines, and that makes your question a lot more complicated. Spreads is another category entirely, and makes this even more complicated. Why couldn’t you just produce a volcano and make it erupt with baking soda and vinegar like everyone else at the science fair?
First of all, butter, of course, is made from cream, produced by cows, and by law in this country, has a butterfat content of at least 80%. There are some premium butters on the market with 81% up to 85% butterfat. Water, milk solids, and – often – salt make up the rest. The amount of salt in salted butter ranges from 1.5% to 3%.
Margarine is made from a vegetable oil, although when it was developed in 1869 by Hippolyte Mèges-Mouriés in France, it was based on beef fat (suet) flavored with milk. Vegetable oils are liquid at room temperature, but a process called hydrogenation was developed in the early 1900s that makes them solid at room temperature. Most margarine today is made with corn oil or soybean oil.
Like butter, margarine is 80% fat and 20% water and solids, of which about 3% is salt. It is often flavored with skim milk or a synthetically produced chemical compound that mimics the flavor of butter. It is sometimes fortified with vitamins A and D to match the nutritional make-up of butter, and includes salt, artificial color, and preservatives.
Most solid fats do not melt suddenly at a precise point, but do so gradually over a range of 10 to 20 degrees. There are different compounds with different characteristics in most fats, and these melt at different temperatures. So instead of turning instantly from a solid to a liquid, certain compounds melt at a lower temperature, weakening the overall structure (think of butter getting soft at room temperature). Eventually, all of the compounds melt and you are left with a liquid.
The melting point of butter is between 90°F and 95°F (32°C and 35°C). The melting point of margarine appears to be a hair warmer, at 94°F to 98°F (34°C to 37°C). But margarines can be formulated to have melting points ranging from 91°F to 109°F (33°C to 43°C). Many of the higher-melting-point margarines are manufactured for the baking industry. The presence of salt lowers the melting point of both butter and margarine.
Spreads do not have a specific amount of fat in them (some are as much as 50% water), and their melting points are all over the map. Because they are so inconsistent, they are not reliable for cooking.
Nothing in the vegetable-oil/margarine kingdom can truly match the flavor of butter – although some come quite close. Butter is not well suited to frying, because the milk solids burn at a low temperature. The milk solids can be removed by clarifying the butter, though, which makes it a tasty and indulgent medium for frying. Margarine, again, with 20% mystery ingredients, is also not a great choice for frying. But in cooking tasks, most recipes let you use butter or margarine interchangeably, and with recipes that do specify butter exclusively, people who prefer margarine generally use it anyway.