How to Tell a Redox Reaction? What You Need to Know

Do you know how to tell a redox reaction? Well, redox reactions are important to understand in biology and chemistry, since many of the reactions that occur in living organisms and cells involve redox processes, such as photosynthesis and respiration. However, it can be difficult to tell whether a reaction you are studying is redox or not, especially if you're just beginning your study of chemistry or biology. Luckily, there are five ways to tell whether a reaction you're looking at is redox or not! If you want to learn more about these methods, keep reading!

1. The Color

That's one of the easiest ways to tell. A redox reaction produces a change in color if you start with colorless reactants and end up with colored products. For example, if you dissolve iodine in water, it gives you a deep-brown solution (iodine is an unstable compound that decomposes when it comes into contact with water). If we mix potassium dichromate and sulfuric acid, we'll get an orange solution (the sulfuric acid reduces chromium from +6 oxidation state to +3).

But watch out, not all color changes indicate redox reactions. Some chemicals change color as they react, but don't undergo redox reaction (see next point). Similarly, certain metals will turn black as they react, but no change in oxidation state occurs.

2. How to Tell a Redox Reaction? The Temperature

If your reaction is exothermic (meaning it produces heat), that is often a good indication of oxidation/reduction. When two molecules exchange electrons, there is generally an increase in energy that gets released as heat. There are exceptions, but for many common redox reactions, you can tell if there's oxidation/reduction by looking at whether your reaction is exothermic.

Likewise, if you have an endothermic reaction (meaning it absorbs heat), that can also be an indication of redox chemistry. An important thing to note here is that just because something is exothermic or endothermic doesn't mean it's necessarily a redox reaction. For example, combustion is both exothermic and a redox reaction; however, dissolving salt in water has no change in temperature, but it's still definitely a redox reaction.

Although temperature may give you some clues as to what kind of chemistry you're dealing with, don't rely on temperature too much. A lot of non-redox reactions can produce heat! Also, keep in mind that some types of reduction (like breaking up a metal oxide into metal + oxygen) actually release more energy than most oxidation do.

3. Addition/Subtraction of Reagents

To check if a reaction is redox, check to see if there's an addition or subtraction of reagents. If you put in more oxidizing agent than reducing agent, then it would not be redox and vice versa. In addition, much oxidation/reduction reactions involve metals (like Cu⁺²), so adding or removing ions can also clue you into whether a reaction is redox. A great example of an oxidation/reduction reaction involving ions is AgNO₃ + NaCl → AgCl + NaNO₃, and it's hard not to notice that NaCl disappeared from solution!

4. How to Tell a Redox Reaction? The Smell

One of my favorite rules for redox reactions is If it smells like garlic, it's probably a reducing agent. That's not always true, but it is often and if something smells strongly enough like garlic that you want to stay away from it, then you know that you probably don't want it in your reaction. What's more, an odd smell can suggest instability (which usually isn't good).

A bad odor generally indicates some kind of byproduct left over from what should have been a clean reaction. To be clear: most odors are temporary and harmless, so I wouldn't stress too much about them unless they make you worried that there could be long-term damage or health hazards involved with whatever product created that stench.

5. Addition/Subtraction of Oxygen

Do you know how to tell a redox reaction? Well, One of your two chemicals is losing/gaining oxygen. If you add or subtract oxygen from an element, it changes its oxidation state and becomes more stable. This could be something as simple as combining hydrogen with oxygen (H₂ + O₂ = H₂O) to make water or complex as adding iron to carbon dioxide (Fe + CO₂ = FeCO₃). Regardless of how simple or complex your reaction is, if you see a reaction that involves reduction or addition of oxygen, you can bet one of those chemicals is changing its oxidation state.