Hemoglobin: the molecule which transports oxygen through the blood stream.  Click here for more information about and images of hemoglobin.

Hess's law: the enthalpy change for any chemical reaction can be calculated from tables of standard heats of formation using the relationship:

To use this formula, sum all the heats of formation of all the products, then subtract the sum of all the heats of formation of all of the reactants.  The standard heat of formation is the enthalpy change when 1 mole of a compound is made from the elements in their standard states.

Heterogeneous: two distinct phases of matter that do not mix, such as immiscible liquids like oil and water, or a solid and gas

Heterogeneous reaction: a reaction which takes place between two heterogeneous phases of matter.  For example in burning wood, the wood is a solid and the oxygen is a gas.   Increasing the surface area in contact may cause a significant increase in the reaction rate.

Hydrazine: hydrazine, N₂H₄, and its derivatives such as methylhydrazine CH₃N₂H₃, or dimethylhydrazine (CH₃)₂N₂H₂, are oily liquids at room temperature with a slightly fishy odor.  They are commonly used propellents, either pure, or in various mixtures, together with N₂O₄ as an oxidizer, since the mixture is hypergolic.

Hydrocarbons: molecules containing only carbon and hydrogen.  Crude oil is largely made of hydrocarbons, though it will contain small quantities of other molecules, and impurities (especially sulfur containing compounds).  Typical crude oil will have 200 or more different organic compounds, mostly hydrocarbons. Different crudes contain different combinations and concentrations of these various compounds.  The simplest of all hydrocarbons is methane, CH₄.  Gasoline is a mixture of many kinds of hydrocarbons, but is often represented in chemistry by octane, C₈H₁₈.  Kerosene which was the fuel in the Saturn V booster rocket, is made of hydrocarbons from C₁₂H₂₆ to about C₁₆H₃₄.  Heavy crude oils will contain many large molecules, and asphaltenes which may have formulas as complex as C₂₀₀H₂₄₆N₂S₇O₄.   More infomation about heavy oil is available here.

Hypergolic: fuels which catch on fire spontaneously, without requiring an external source of ignition.  Hydrazine and nitrogen tetroxide are commonly used hypergolic fuels.

Hypothesis: a tentative (as opposed to a theory which is well tested) explanation for observed events. An hypothesis is not a prediction itself, but it must allow you to make predictions which can be tested by experiment. When the results of those experiments are as predicted, it lends support to the hypothesis as a good explanation, and its eventual acceptance as a theory. If the results are not as predicted, the hypothesis must be modified, or replaced with a better explanation. No statement is an hypothesis unless it suggests a cause for an effect.