It’s a common misconception that specific heat can be negative. It is possible for the values to be positive or even zero, but never negative. The misunderstanding comes from not understanding what specific heat is in the first place. Specific heat refers to how much energy it takes to raise one gram of material by one degree Celsius.
For example, if one gram of water takes four joules to raise its temperature by one degree Celsius, it has a specific heat value of four. If the same amount of iron needs six joules for the same change in temperature (one degree), then it would have a specific heat value of three. The average values are zero and two respectively – but never negative.
The misconception probably comes from not understanding what “specific” means: that only pertains to certain materials rather than every possible material on earth. Specific heats can be different because some substances require more energy or less energy to increase their temperatures by one degree Celsius. Some people might think when they hear about specific heating that this is referring to all forms of matter.