It is safe to eat warm butter that has been out for up to 2 hours or less or if the butter was warmed up by heating it to melt it.
However if the butter has been left out longer than 4 hours and is warm from just being left out then it's not considered safe.
However pasteurized butter does not have to be refrigerated to keep it safe.
It is OK to eat melted butter as melted butter is just butter that has been changed into a liquid by melting it.
As long as the melted butter is not contaminated or spoiled it's safe to eat the melted butter although it should be consumed in moderation as part of a healthy balanced diet.
Melted butter is often used in baking and cooking and on bread and as a topping for pancakes, popcorn vegetables etc.
Melted butter does go rancid and even non melted butter does eventually go rancid especially when left out at room temperature or a long time.
Exposure to heat, oxygen and light can accelerate the oxidation process of the melted butter and non melted butter and cause it to develop an off flavor and turn rancid.
Oxidation of the butter is when the fats in the butter break down when they're exposed to air and cause it to go rancid.
Melted butter and other butter often goes rancid within a few days if not refrigerated.
The white stuff in melted butter is simply milk solids that are made of fat and milk proteins.
Butter is made of water, fat and protein that form an emulsion and when the butter melts the emulsion then separates into layers.
And the white foam which floats to the top is made of milk proteins and the white stuff or solids that settle to the bottom of the melted butter is just milk solids.
Butter melting including on bread is not a chemical change and is instead a physical change.
The butter melting including on bread is a physical change and not a chemical change because the butter is simply changing from the state of being a solid to the state of being a liquid.
And the chemical composition of the melted butter still remains the same.
This means that the butter that is melted can be reversed by cooling the butter back down to a solid state.
Toasting bread causes a chemical change known as the Maillard reaction and spreading and melting butter on warm toast is a physical change that is related to state transition.
When butter is heated up, the emulsion then separates and the fat also separates from the water which forms 2 layers.
The separated fat layer is also commonly known as ghee which is clarified butter.
Ghee and butter have different properties though and you cannot extract butter from ghee so that is a chemical change.
Melting anything including butter is a physical change as it changes the state of matter from a solid to a liquid.
And during the melting process the chemical composition of the substance doesn't change.
When an ice cube for example melts, it's still water and the chemical composition of the water is H2O.