> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.snomed.org/education/snomed-ct-education-editorial-guide/readme/authoring/domain-specific-modeling/substance/substance-naming-and-modeling-conventions/guidelines-for-specific-substance-types/hydrates-and-anhydrous-substances.md).

# Hydrates and anhydrous substances

<mark style="color:$danger;">**This Editorial Guide is used for Education Purposes Only. It is used in the Authoring Courses and Certifications. It is based on the January 2026 Editorial Guide.**</mark>

## Modeling

<table data-header-hidden><thead><tr><th width="199.83203125"></th><th></th></tr></thead><tbody><tr><td><strong>Parent concept</strong></td><td>Most distal appropriate descendant of 105590001 |Substance (substance)|</td></tr><tr><td><strong>Semantic tag</strong></td><td>(substance)</td></tr><tr><td><strong>Definition status</strong></td><td>Primitive</td></tr><tr><td><strong>Attribute</strong></td><td><p>738774007 |Is modification of (attribute)| relationship to the substance with unspecified hydration</p><p>726542003 |Has disposition (attribute)|, as appropriate</p></td></tr></tbody></table>

## Hydrates

Hydates are modeled with an 738774007 | Is modification of (attribute)| relationship to the substance with unspecified hydration.

For example,

* Ferrous oxalate dihydrate (substance) is modification of → Ferrous oxalate
* Caffeine hydrate (substance) is modification of → Caffeine
* Zinc sulfate heptahydrate (substance) is modification of → Zinc sulfate

### Naming

### FSN

Pattern:

* X hydrate (substance)
* X dihydrate (substance)
* X monohydrate (substance)

For example,

* Copper sulfate pentahydrate (substance)

***

### Preferred Term

Pattern:

* X hydrate
* X dihydrate
* X monohydrate

For example,

* Copper sulfate pentahydrate

### Exemplar

The following illustrates the **inferred** view.

Caffeine hydrate (substance) IS MODIFICATION OF Caffeine (substance)

<figure><img src="/files/j7w2XOzanB9AhqD4sCH1" alt=""><figcaption></figcaption></figure>

## Anhydrous compounds

Anhydrous compounds have an 738774007 | Is modification of (attribute)| relationship to the unspecified substance, i.e., where the level hydration is not specified.

For example,

* Sodium carbonate anhydrous (substance) is modification of → Sodium carbonate
* Theophylline anhydrous (substance) is modification of → Theophylline

### Naming Guidelines

### FSN

Pattern:

* X anhydrous (substance)

For example,

* Theophylline anhydrous (substance)

***

### Preferred Term

Pattern:

* X anhydrous

For example,

* Theophylline anhydrous

***

### Synonyms

Anhydrous X

### Exemplar

The following illustrates the **inferred** view.

|Theophylline anhydrous (substance)| IS MODIFICATION OF |Theophylline (substance)|

<figure><img src="/files/uyNvUKQYF5EmQnfQNZ1o" alt=""><figcaption></figcaption></figure>

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