The thing in the lab that breaks most easily is also the sharpest once it has broken. This note covers how glass apparatus breaks, what keeps your hand safe before it does, and which glass can take the flame.
1. How glass apparatus breaks
Lab glassware includes the test tube, beaker, funnel, conical flask, measuring flask, pipette, burette, measuring cylinder, round-bottom flask, glass rod, glass tube and Liebig condenser.
All of it breaks easily on impact. Broken glass is sharp and can cut the skin and make it bleed.
There are six risks: knocking the tap or basin while washing, too much pressure fitting a tube in a holder, too much pressure in a clamp, heating with water on the outer wall, pushing a tube or thermometer into a stopper, and a fall from a height. Match the pairs.
match themmatch the six pairs
Which job, which risk?
Pick up a job, then drop it in the box of the risk it brings.
Right. Next one.
Not yet. Ask yourself: doing this job, how could the glass break?
All six pairs match. These are the book's six risks.
2. Fitting a tube into a stopper
Pushing a glass tube into a bored cork or rubber stopper is one of the riskier jobs. Wrap both the tube and the stopper in a towel and press the tube through the hole, so that if it snaps your hand stays uncut. If it will not go in, enlarge the hole and wet it with water or glycerine.
The towel and the wetting are two small jobs. Leave one out, press the tube, and see what happens.
on the benchpress
A glass tube into a stopper
Now put the steps in order.
in orderprotect the hand first
Put the steps in order
What comes first, so that a snapped tube cannot cut your hand?
Yes. Then?
Not yet. Towel first, then pressure; wetting comes last.
Right order. The towel guards the hand. Pressing through the hole keeps the tube straight. A wider hole, wetted with water or glycerine, lets the tube slide in easily.
3. Soft glass and pyrex
By the glass used, lab glassware is of two kinds: soft glass, and hard or pyrex glass. Soft glass items are the glass tube, beaker, watch-glass, funnel, reagent bottle and Liebig condenser.
Soft glass melts and softens easily in a Bunsen flame; it takes little heat. It is not brown or very transparent.
Pyrex items: beaker, conical flask, pipette, burette, measuring cylinder, volumetric flask, round-bottom flask and distillation flask. Pyrex stands high heat and is very strong, so strong heating, round-bottom flasks for preparations, distillation flasks and vacuum-filtration apparatus are made of it.
Notice that the beaker is in both of the book's lists, so there is a third box for it below.
sort themrecall the book's lists
Which glass is it?
Pick up a chip, then drop it in a box.
Right. Next one.
Not yet. The book's soft list: glass tube, beaker, watch-glass, funnel, reagent bottle, Liebig condenser. The pyrex list has the rest, and the beaker too.
All in place. The book lists the beaker under both kinds, so it sits in its own box.
Chemically, pyrex is made from a mixture of zinc oxide and barium borosilicates, and soft glass from a mixture of sodium and calcium silicates. So to heat a mixture or solution, take a pyrex beaker or conical flask.
4. Check yourself
The first question is in the exam's style.
check yourselfpick one answer
Question 1
Which glass is used to make a burette and a pipette?
The book says burettes and pipettes are made of pyrex or hard glass; silica glass is not mentioned.
Right. Burettes, pipettes and measuring apparatus are made of pyrex or hard glass, which withstands heat and is very strong.
Pyrex is itself a borosilicate-type glass, but the question wants the book's named glass, pyrex.
Flint glass is a soft kind; the book says burettes and pipettes are of hard pyrex glass.
For the second, work out which glass it is.
check yourselfpick one answer
Question 2
The possible causes of breaking glass apparatus are-
i. Hitting the basin while washing ii. Applying too little pressure when fixing in a clamp iii. Heating on a Bunsen burner while water is on the outer wall
Statement ii is false: the book says too much pressure in a clamp breaks glass, not too little.
Right. Hitting the basin and heating a wet wall are both listed in the book as breaking risks.
Statement ii is false, so ii with iii fails; the risk is too much pressure in the clamp.
Because ii is false, all three cannot be right.
The last one is a final reminder.
check yourselfpick one answer
Question 3
What is done to avoid cutting the hand while pushing a glass tube into a bored cork?
A jerk raises the risk of breaking the tube and cutting the hand; steady pressing is the rule.
Heating the tube is not the book's method; wrapping in a towel is.
Right. This is the book's method: with the towel, a broken tube does not cut the hand.
Even an unbroken tube is wrapped, because it may break while being pushed in.
The whole note on seven cards.
cut from the notetap to turn over
Seven cards
QuestionWhat is the risk of heating glass while water is on its outer wall?
AnswerIt is one of the causes of breaking the glass
QuestionWhat is wrapped round when pushing a glass tube into a stopper, and why?
AnswerA towel, so that broken glass does not cut the hand
QuestionWhat is done if the glass tube does not enter the stopper hole easily?
AnswerEnlarge the hole and wet it with water or glycerine
QuestionWhat happens to soft glassware heated in a Bunsen flame?
AnswerIt melts and softens easily because it withstands little heat
QuestionWhich glass are the pipette, burette and conical flask made of?
AnswerPyrex (hard) glass
QuestionChemically, what is pyrex glass made of, and what is soft glass made of?
AnswerPyrex: zinc oxide and barium borosilicates; soft: sodium and calcium silicates
QuestionFor heating a solution, a beaker or conical flask of which glass is used?
AnswerHard or pyrex glass
Source: Chemistry 1st Paper, Classes 11-12 (NCTB), Chapter 1 “Safe Use of the Laboratory”, section 1.2.