Chem 23.1: Common Laboratory Techniques

What is the fuel used in a Bunsen burner?

Natural gas, mainly methane (CH₄).

Complete combustion equation of methane?

CH₄ + 2O₂ → CO₂ + 2H₂O

What color flame indicates complete combustion?

Blue, non-luminous flame.

What does a yellow, luminous flame mean?

Incomplete combustion (not enough oxygen).

What happens if there’s too little air?

Sooty, noisy, orange-yellow flame.

What happens if there’s too much air?

The flame may blow out.

How do you light the Bunsen burner?

Check gas → connect tubing → close gas control valve → open outlet → ignite with match while opening gas valve slowly.

How do you adjust flame height?

Turn the gas control valve.

Ideal working flame description?

Blue, quiet, three distinct cones.

Hottest part of the flame?

Tip of the inner blue cone.

Should you lay a glass stopper on the bench?

No, hold it between your fingers to avoid contamination.

How to pour a liquid properly?

Down a stirring rod touching the wall of the receiving vessel.

Should you return excess reagent?

Never — it contaminates the stock solution.

What do you do with screw caps?

Place them upside down on the bench.

Why do we heat liquids?

To speed up reactions or form larger crystals.

What is “bumping”?

Sudden bubble formation from superheating.

How to prevent bumping?

Add boiling chips or stir continuously.

When using a beaker over a flame, what do you use for support?

Wire gauze on an iron ring.

What’s a “cool flame”?

Non-luminous flame with reduced fuel supply.

How full should a test tube be when heating?

Less than one-third full.

What angle should the test tube be held at?

About 45°.

Should the flame be fixed at the bottom of the test tube?

No — move it around gently and never point at anyone.

How do you cool hot liquids safely?

Under running tap water or in an ice bath.

Most used and abused equipment in the lab?

Balance.

What are the two types of balances?

Top-loading (rough) and analytical (precise).

What is “taring”?

Setting the balance to zero after placing a container to measure only the sample mass.

What to never place directly on the balance pan?

Chemicals.

What must be done after weighing?

Clean the area and return settings to zero.

Why are solids heated?

To dry them or test thermal stability.

What’s used for low-temperature heating?

Drying oven.

What’s used for high-temperature heating?

Porcelain crucible.

Why use a drying oven before weighing?

To remove absorbed moisture.

What’s the typical drying oven temperature?

Around 110°C.

After drying, where do you cool the sample?

In a desiccator.

Purpose of a desiccator?

To cool samples without moisture absorption.

What’s inside it to absorb water vapor?

Desiccant (CaCl₂).

What happens to CaCl₂ when it absorbs moisture?

Forms CaCl₂·2H₂O.

Can the desiccant be reused?

Yes, after heating to remove water.

When is a graduated cylinder used?

When extreme accuracy is not required.

How should you read the volume of a liquid?

At eye level with the meniscus.

For colored liquids, use which meniscus?

Upper meniscus.

What is a pipet used for?

Delivering a definite volume of liquid.

What’s used to draw liquid into the pipet?

Rubber bulb aspirator.

Which finger controls the flow?

Forefinger (not the thumb!).

Should the pipet be held vertically?

Yes, with the tip touching the container wall.

Do you blow out the remaining drop?

No — it’s already included in the calibration.

Purpose of a buret?

To deliver varying volumes during titration.

Before use, rinse the buret with?

The titrant solution.

Why rinse?

To avoid dilution from residual water.

How do you remove trapped air bubbles?

Let titrant run quickly through the tip.

What do you do after using the buret?

Wash and store it clean.

What is the liquid that passes through filter paper?

Filtrate

What is the solid left behind?

Residue

What’s the purpose of moistening filter paper?

To seal it against air leaks.

How full should the funnel bowl be?

Less than two-thirds full.

Why should the funnel stem always be full?

It creates slight suction to speed up filtration.

What is decantation?

Pouring off the supernatant without disturbing the solid.

What is the clear liquid above the solid called?

Supernatant

What is the purpose of evaporation?

To recover dissolved solids from a solution.

What remains after evaporation?

Residue

How do you know when to stop heating?

When crystals start to appear.

Why cover with a watch glass after heating?

To prevent contamination during cooling.

What does centrifugation do?

Separates components based on density using rotation.

How full should tubes be?

No more than 1 cm from the top.

Why must opposite tubes have equal volume?

To balance the rotor and prevent vibration.

Can you stop a centrifuge manually?

No — let it stop on its own.

Blue flame

complete combustion

Yellow flame

incomplete combustion

Filtrate

passes through filter

Residue

stays behind

Supernatant

liquid above solid

CaCl₂

desiccant

Boiling chips

prevent bumping

Taring

zeroing the container’s mass

Meniscus

curved surface of liquid