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A colour change is one indication that a chemical change has occurred. However, mixing a chemical with a substance or food usually does not make food change colour — for example table salt is a “chemical” called sodium chloride, but mixing this chemical usually does not make food change colour. A “chemical” is a pure substance. Usually chemicals that make food change colour are coloured substances and therefore add colour rather than really change colour — food dyes, coloured spices, etc. A small number of “chemicals” added to food do cause chemical change, for example, the process of making corned beef, which genuinely causes the meat to change colour.
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A chemical looks a certain colour because of the way light is absorbed and reflected by the chemical.
For example, chlorophyll is a molecule in plant cells. Chlorophyll captures light energy, helping the plant make it’s own food (by photosynthesis). Chlorophyll is also what makes leaves appear green, because it reflects green wavelengths of light and absorbs the blue and red wavelengths.
But if the structure of a chemical may change if it is mixed with something else and a chemical reaction occurs, like Kieran said. If the new chemical produced by the reaction reflects and absorbs different wavelengths of light to the original chemical, it will look a different colour.
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Molecules absorb energy. That’s one of the most basic things they do. What energy is absorbed depends on a bunch of things, but one is the molecule’s size. Long molecules, with things called “double bonds” in them, often absorb light that’s in the visible region of the electromagnetic spectrum. The molecule that’s responsible for the colour of a tomato, lycopene, is like this. We know enough about molecules to be able to design new molecules that will absorb certain wavelengths of light and give certain colours.
Cooking and food, though, are pretty complicated. There are lots of chemicals in there. The chemistry going on is really complicated, so you get a lot of different molecules, all with different structures. Very often the mixture of all those colours will give you a kind of average of “brown”. If you mix a bunch of paints together, you’ll eventually get that kind of colour! So if you cook things by grilling or frying, often you’ll get brown colours. Much of the chemistry involved is described by something called the Maillard reaction. Molecules are made with lots of different structures, giving a kind of brown average!
So the answer is that the colour is determined by all the molecules. In some cases, as Bridget said, one molecule may dominate because there’s a lot of it. Chlorophyll is a great example. In other cases, there’s a complicated mess, giving an average. Food colouring is different – those are pure, synthetic molecules we add in. Did you know before we could make food colouring cheaply, people used to use beetle blood as a red food colouring. My grandma said it was the best thing for cakes…
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A colour change is one sign that a chemical change has taken place but a colour change doesn’t always happen when you mix a chemical with a substance or food. Using water, (which is a chemical), to wash a lettuce leaf doesn’t cause the lettuce leaf to react. It will get wet but that’s about it. Everything around us is made of chemicals and not everything reacts.
Commonly when it comes to food, most of the chemical reactions take place in cooking where lots of complicated chemical reactions take place. One particular reaction, the Maillard reaction is one that is studied by chemists, chefs and contestants on reality cooking shows. It’s the reaction that is responsible for things going brown and adding taste to food. It happens in breads, biscuits, and meats to name a few examples.
As for the red food colouring from bugs that Mat mentioned, this is still made today. The red food colouring is called cochineal and can be found on the shelves of grocery stores. Though it isn’t made from the blood of the insect but from a type of acid it produces mixed with aluminium and calcium salts to produce a red colour. http://en.wikipedia.org/wiki/Cochineal
If you have the time, you can make a pH indicator from red cabbage leaves. http://museumvictoria.com.au/scienceworks/discoverycentre/iyc-video-1-/
When testing the pH of substances, a chemical reaction takes place changing colour of the indicator used to indicate whether a solution is an acid or a base.
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