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How can we determine if zinc hydride is a ionic compound? If it is a ionic compound then why doesn't it dissociate when put in water?

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Zinc(II) hydride is not at all an ionic compound rather the elements are arranged in a one-dimensional network bonded covalently. From wikipedia article of zinc(II) hydride:

New evidence suggests that in zinc(II) hydride, elements form a one-dimensional network (polymer), being connected by covalent bonds.(...)Solid zinc(II) hydride is the irreversible autopolymerisation product of the molecular form, and the molecular form cannot be isolated in concentration. 

Molecular zinc hydride in the gas phase was found to be linear with a Zn-H bond length of 153.5 pm. Quantum chemical calculations predict the molecular form to exist in a doubly hydrogen-bridged, dimeric groundstate, with little or no formational energy barrier.The dimer can be described as di-μ-hydrido-bis(hydridozinc), according to IUPAC additive nomenclature

As it is not an ionic compound, it does not dissociate in aqueous solution, rather it gets hydrolyzed slowly. If you want to decompose into component element, just heat it at 90℃.

Zinc(II) hydride slowly decomposes to metallic zinc and hydrogen at room temperature, with decomposition becoming rapid if it is heated above 90°C.

$$\ce{ZnH2 → H2 + Zn(0)}$$

It is readily oxidised and is sensitive to both air and moisture; being hydrolysed slowly by water .

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