Watch glass

<p>One of the generic uses of a watch glass is as a lid for beakers. In this case, a watch glass is placed above the container, which makes it easier to control and alter vapor saturation conditions.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> Moreover, a watch glass is often used to house solids being weighed on the scale. Before weighing desired amount of solid, a watch glass is placed on the scale, followed by taring or zeroing the scale so that only the weight of the sample substance is obtained.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> A watch glass can also be used for observing precipitation and crystallization patterns. When further drying of a substance is required, a watch glass is often used in cases where a particular type of solid needs to be separated from its comparatively volatile solvent. The solid is spread on a watch glass and, often, a folded piece of <a href="/facts/Filter_paper/FVbDyXQ3">filter paper</a> is placed above to keep airborne particles from contaminating the product. To maximize the drying rate, a watch glass can be placed inside a <a href="/facts/Fume_hood/dIsosPMN">fume hood</a> to provide good air circulation for an adequate amount of time. Another technique used in chemistry laboratories to increase the drying rate is passing a gentle stream of dry air or nitrogen gas over the watch glass from an inverted <a href="/facts/Funnel/nA6YywU2">funnel</a> clamped above it.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a>
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<h2 id="types">Types</h2>

<p>There are two types of the watch glass, glass and plastic, used in the <a href="/facts/Laboratory/kqR3Vtx9">laboratory</a>. Watch glasses are available in various sizes and widths, and are usually thicker than any other glass or plastic labware.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a>
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<ul><li>Glass watch glasses – These can be reused after they are <a href="/facts/Sterilization_(microbiology)/ajVXFl7a">sterilized</a> in an <a href="/facts/Autoclave/bg1J3cXJ">autoclave</a> or a laboratory oven. The glasses provide high resistance to <a href="/facts/Thermal_shock/2vDgN5HQ">thermal shock</a>, chemical resistance, and mechanical durability.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a></li>
<li>Plastic watch glasses – These are disposable watch-glass used in laboratory work to avoid cross-contamination while preparing the samples. They are very good at implementing in low temperatures and have an operating span of −57 to 135 °C (−71 to 275 °F); they can also resist <a href="/facts/Ultraviolet/kncBUqn5">UV light</a> degradation. These plastic watch glasses are less expensive and light in weight.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a></li></ul>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Laboratory_glassware/M4CMuHRh">Laboratory glassware</a></li></ul>

<h2 id="external-links">External links</h2>
<ul><li> Media related to Watch glasses (laboratory) at Wikimedia Commons</li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>Graham, Lawrence. "E-pistle 2007/030 – Whisky Glasses; a Study". Malt Maniacs. Canada. Retrieved June 28, 2015. <a href="http://www.maltmaniacs.net/" target="_blank">http://www.maltmaniacs.net/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>Helmenstine, Anne Marie. "Watch Glass – Photo". About. Retrieved June 28, 2015. <a href="http://chemistry.about.com/od/chemistrylabexperiments/ig/Chemistry-Laboratory-Glassware/Watch-Glass.htm" target="_blank">http://chemistry.about.com/od/chemistrylabexperiments/ig/Chemistry-Laboratory-Glassware/Watch-Glass.htm</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Jones, Chad. "Chemistry Lab Equipment: Watch Glass". Answers. Archived from the original on November 6, 2013. Retrieved June 28, 2015. <a href="https://web.archive.org/web/20131106180203/http://chem.answers.com/chemistry-basics/chemistry-lab-equipment-watch-glass" target="_blank">https://web.archive.org/web/20131106180203/http://chem.answers.com/chemistry-basics/chemistry-lab-equipment-watch-glass</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>Lehman, John W. (2008). The Student's Lab Companion: Laboratory Techniques for Organic Chemistry (2nd ed.). Prentice Hall. pp. 156–157. ISBN 9780131593817. <a href="9780131593817" target="_blank">9780131593817</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"Watch Glasses Information | IHS Engineering360". www.globalspec.com. Retrieved February 17, 2016. <a href="http://www.globalspec.com/learnmore/labware_scientific_instruments/labware_consumables/watch_glasses" target="_blank">http://www.globalspec.com/learnmore/labware_scientific_instruments/labware_consumables/watch_glasses</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>"Watch Glasses Selection Guide: Types, Features, Applications". GlobalSpec. <a href="http://www.globalspec.com/learnmore/labware_scientific_instruments/labware_consumables/watch_glasses" target="_blank">http://www.globalspec.com/learnmore/labware_scientific_instruments/labware_consumables/watch_glasses</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>"Watch Glasses Selection Guide: Types, Features, Applications". GlobalSpec. <a href="http://www.globalspec.com/learnmore/labware_scientific_instruments/labware_consumables/watch_glasses" target="_blank">http://www.globalspec.com/learnmore/labware_scientific_instruments/labware_consumables/watch_glasses</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
</ol>

Watch glass open-in-new

Watch glass