I did an experiment to compare whether salt water (5% concentration of salt) or fresh water of the same volume took longer to heat up to a certain temperature. We found that salt water took longer to heat up than fresh water.
Is this due to density? specific heat capacity? or should I have gotten different results.
The thermal conductivity of saline is less than water. See this page for graphs of thermal conductivity against salt content.
Note that a secondary effect is that adding salt to water actually lowers the specific heat, and this will increase the rate of temperature change. See the question Why does salty water heat up quicker than pure water? and it's answers. In particular follow the link I provide to the paper by Zwicky.
However you're comparing the same volume you have more mass to heat up because the density of sea water is greater than the density of pure water. If you take sea water (about 3.5% salt - I chose this because data is easily Googlable) the specific heat is 3.993kJ per kg per degree, compared to water at 4.184kJ/kg/K. However the density of seawater is 1037kg/m$^3$ so the specific heat per cubic metre is almost exactly the same as pure water.
