Adding text labels and arrows
To label your diagram, there are 2 options:
| Label Diagram Tool (recommended) | Label Item |
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Using the Label Diagram Tool
To add text and arrows to your diagram
First, Select the "Label diagram" tool. 
- Click to draw a label
- Click + Drag to draw a label with an arrow
In the property panel on the right, you can edit the text as well as add adjust various properties:
Notes
- To move the label, select the "Select & Move items" tool and drag the label or the arrow.
- To change where the arrow is pointing, click and drag the yellow diamond shapes.
- To hide the arrow head, toggle "Show arrow head"
- To hide the arrow completely, toggle "Show arrow"
- See this article on how to write chemical formulas, subscripts, units etc.
Alternative: Use the "Label" item
An alternative is to add the "Label" from "Accessories > Labelling" section. Note this is intended to represent a physical paper label, to be used on glassware, bottles etc. but it can also be used for labelling the diagram itself. 