C. Chemical Reactions · Chemical Reactions

Investigating Reactions: Lab Safety and Testing the Products

C2 A1.2 A1.4 A1.5 A1.6 A1.8 A1.10 A1.11 A1.13About 70 minutes

Learning goals

I can use WHMIS labels and safety data sheets to work safely with chemicals.
I can identify oxygen, hydrogen, carbon dioxide, ammonia and water with standard tests, and identify acids, bases and precipitates.
I can plan an investigation of the main reaction types and record qualitative and quantitative observations.
I can analyse lab data using correct significant digits, and write a clear lab report with sources of error.

Words to know

WHMIS 2015
Canada's Workplace Hazardous Materials Information System, aligned with the international Globally Harmonized System (GHS). It uses pictograms in a red diamond border, supplier labels and safety data sheets.

safety data sheet (SDS)
A document for each hazardous product describing its hazards, safe handling, first aid, storage and disposal.

qualitative observation
A description without numbers, such as "a white precipitate formed".

quantitative observation
A measurement with a number and unit, such as "the mass increased by 0.16 g".

limewater
A clear solution of calcium hydroxide; it turns milky when carbon dioxide is bubbled through it.

significant digits
The digits in a measurement that are known reliably, plus one estimated digit. Calculated answers should not appear more precise than the data.

Chemists as detectives

How do you know that the gas bubbling off zinc in acid is hydrogen, and not oxygen or carbon dioxide? You cannot tell by looking: all three are colourless. A chemist runs a test, a quick reaction with a result that only one substance gives. In this lesson you will learn the standard tests for the products of the reactions in this unit, and how to plan and report a safe investigation of the reaction types.

You may be doing these labs at school. Some of them use flames and hazardous chemicals, so at home you will use safe kitchen versions.

Safety first: WHMIS and safety data sheets

Every hazardous chemical in a school lab or workplace in Canada is covered by WHMIS 2015:

Supplier labels show the product name, pictograms (a black symbol inside a red diamond border), a signal word (Danger for more severe hazards, Warning for less severe), hazard statements and precautions.
Pictograms include, among others, the flame (flammable), flame over circle (oxidizer: makes other things burn more intensely), corrosion (burns skin or eyes, or corrodes metals), skull and crossbones (acute toxicity), exclamation mark (irritant or less serious health effects), health hazard (longer-term effects such as harming organs) and gas cylinder (gas under pressure).
A safety data sheet (SDS) for each product gives details on hazards, protective equipment, first aid, spills and disposal.

Household products sold in stores use a different set of symbols (with frames that show caution, warning or danger), but the idea is the same: read the label before you use it.

Lab rules for this unit: wear splash goggles; tie back long hair; never smell a gas directly (waft it toward your nose with your hand); never taste anything; add acid to water, never water to acid; heat test tubes pointed away from everyone; dispose of heavy-metal solutions (copper, lead, silver) in waste containers, not the sink.

The standard tests

Gases:

Oxygen: a glowing splint (lit, then blown out so the tip still glows red) relights in the gas.
Hydrogen: a burning splint held at the mouth of the test tube makes a squeaky "pop", as the hydrogen burns: 2H₂(g) + O₂(g) → 2H₂O(g).
Carbon dioxide: a burning splint goes out, and the gas turns limewater milky because a calcium carbonate precipitate forms: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l). (A splint going out alone is not enough, since nitrogen also puts out flames. The limewater test is the confirming test.)
Ammonia: turns damp red litmus paper blue, and has a sharp smell (waft carefully).

Other products:

Water: turns blue cobalt(II) chloride paper pink.
Acid or base: litmus or another indicator. Acids turn blue litmus red; bases turn red litmus blue.
Precipitate: a solid appears in a mixture of clear solutions; note its colour.
Halogen: colour in a non-polar solvent (bromine orange, iodine violet).

Three standard gas tests. Oxygen relights a glowing splint; hydrogen gives a pop with a burning splint; carbon dioxide turns limewater milky (and puts out a burning splint).

Reaction type	Typical school lab reaction	Test for a product
Synthesis	2Mg(s) + O₂(g) → 2MgO(s) (magnesium ribbon burned in a crucible; do not stare at the very bright flame)	white powder forms; mass increases
Decomposition	2H₂O₂(aq) → 2H₂O(l) + O₂(g) with a manganese(IV) oxide catalyst	glowing splint relights (oxygen)
Single displacement	Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)	burning splint pops (hydrogen)
Double displacement	CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g), or Pb(NO₃)₂ + 2KI for a precipitate	limewater turns milky (carbon dioxide); yellow precipitate
Combustion	a candle or burner flame	cobalt(II) chloride paper turns pink (water); limewater turns milky (carbon dioxide)

A typical plan for investigating the reaction types, with a test that confirms a product of each.

Recording and analysing data

Record both kinds of observations:

Qualitative: colour changes, bubbles, precipitates, light, heat, test results.
Quantitative: masses, volumes, temperatures, times, each with a unit and the right number of significant digits for the instrument. A centigram balance reads to 0.01 g, so record "2.40 g", not "2.4 g".

Rules for calculations: when adding or subtracting, round the answer to the fewest decimal places in the data. When multiplying or dividing, round to the fewest significant digits.

Observations are not inferences. "The splint popped" is an observation. "Hydrogen was produced" is an inference (a conclusion) based on it. A good lab report keeps them separate.

Data from burning a magnesium ribbon in a crucible. The product is heavier than the metal because oxygen from the air combined with it; the difference (0.40 g − 0.24 g = 0.16 g) is the mass of oxygen.

Worked example

Analysing a synthesis lab

A student records: mass of crucible + lid = 25.62 g; crucible + lid + magnesium = 25.86 g; crucible + lid + product (after heating) = 26.02 g. Find the masses of magnesium, product and oxygen, with correct significant digits. Was mass conserved?

Magnesium: 25.86 g − 25.62 g = 0.24 g (two decimal places, like the data).
Product (magnesium oxide): 26.02 g − 25.62 g = 0.40 g.
Oxygen combined: 0.40 g − 0.24 g = 0.16 g.
Mass was conserved for the whole system (magnesium + oxygen → magnesium oxide). The balance reading increased only because oxygen from the air, which was not weighed at the start, became part of the solid.

Mg 0.24 g; MgO 0.40 g; oxygen 0.16 g. In the Quantities in Chemical Reactions unit you will use numbers like these to find the formula of the oxide.

Worked example

Writing a conclusion and evaluation

A group added zinc to hydrochloric acid. Bubbles formed and the zinc got smaller. A burning splint at the mouth of the test tube made a pop. Write a conclusion and two sources of error.

Conclusion: the gas produced was hydrogen (inferred from the pop test), so zinc displaced hydrogen from hydrochloric acid in a single displacement reaction: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). This supports the prediction from the activity series (zinc is above hydrogen).
Source of error 1: if the test tube was not held upside down, much of the hydrogen (less dense than air) escaped, so the pop may be faint or missed.
Source of error 2: if the zinc had an oxide coating, the reaction may have started slowly.
Improvement: collect the gas in an inverted test tube, and clean the zinc with sandpaper first.

A good conclusion answers the question, cites the evidence, names the reaction type and links back to the hypothesis.

Hands-on activity

Two gas tests in the kitchen

You need: 3% hydrogen peroxide (from a pharmacy; never use stronger peroxide), 1 package of dry yeast, baking soda, vinegar, two narrow-necked glass bottles or tall glasses, a funnel, wooden barbecue skewers, a lighter (grown-up only), safety glasses or sunglasses, and a grown-up. Work beside the kitchen sink, with nothing flammable nearby.

Safety: hydrogen peroxide irritates eyes and skin: wear eye protection and rinse any splashes with water. The grown-up does every step that involves a flame. Do this at the sink so a skewer can be dropped into water.

Test 1: oxygen (decomposition).

Mix the yeast with 2 tablespoons of warm water. Pour 50 mL of hydrogen peroxide into bottle 1, then add the yeast mixture. Foam and gas form: 2H₂O₂(aq) → 2H₂O(l) + O₂(g), catalysed by an enzyme in yeast.
After 30 seconds, the grown-up lights a skewer, blows it out so the tip glows, and lowers the glowing tip into the neck of the bottle (not into the foam). It glows much brighter or bursts back into flame: oxygen.

Test 2: carbon dioxide (gas-forming double displacement).

Put 2 teaspoons of baking soda in bottle 2 and add 50 mL of vinegar.
After 20 seconds, the grown-up lowers a burning skewer into the neck. It goes out: the bottle is full of carbon dioxide (it does not support burning).

Write it up: for each test, record observations, the inference, and the reaction type. Explain why test 2 alone does not prove the gas is carbon dioxide (other gases, such as nitrogen, also put out flames; the confirming test is limewater).

Just for fun

In the carbon dioxide test, chemists say limewater turns "milky". Please do not pour it on your cereal: limewater is a calcium hydroxide solution, and the "milk" is a fine precipitate of calcium carbonate, which is basically very, very finely powdered chalk.

Your turn

A gas relights a glowing splint. What is it?

A gas makes a pop with a burning splint. What is it, and what reaction makes the pop?

Write the equation for the limewater test.

Which test shows that water is a product of combustion?

Classify as qualitative or quantitative: (a) the solution turned pale green; (b) the temperature rose by 4.5 °C; (c) a yellow precipitate formed.

What does the flame-over-circle pictogram warn about?

More practice: warm-up

What colour is the border of a WHMIS 2015 pictogram?

What is an SDS?

How should you smell a gas in the lab?

What turns damp red litmus paper blue?

Why must copper(II) sulfate solution go in a waste container?

More practice: on your own

A student heats copper(II) carbonate and bubbles the gas through limewater, which turns milky. Write equations for both reactions and classify each.

Calculate with correct significant digits: 12.47 g − 9.2 g.

A crucible + copper has mass 31.25 g; the empty crucible is 30.61 g. After heating in air, crucible + product = 31.41 g. Find the mass of copper, of product, and of oxygen that combined.

Explain the difference between an observation and an inference, using the hydrogen test as an example.

Why is a burning splint going out not enough to prove a gas is carbon dioxide?

Challenge

Design a test to tell apart three colourless solutions: hydrochloric acid, sodium chloride and sodium hydroxide, using only litmus paper and a few drops of silver nitrate solution.

In the magnesium crucible lab, a student's product mass was lower than expected. Give two specific causes, and say which direction each would push the result.

Explain why the hydrogen test tube is held upside down but the oxygen and carbon dioxide test tubes are held upright.

Check your understanding

Quiz

Which test identifies carbon dioxide?

A glowing splint bursts into flame in a gas. The gas is:

Which is a quantitative observation?

12.37 g − 10.1 g, written with correct significant digits, is:

Which test confirms water as a product?

Which practice is UNSAFE?

For parents and teachers

This lesson builds the investigation skills that run through the whole course (strand A).

How to help: read a real product label together (for example a cleaning product) and find the hazard symbols, the signal word and the first-aid instructions. Ask your teen to explain the difference between an observation and an inference.

Common difficulties:

Mixing up the glowing-splint (oxygen) and burning-splint (hydrogen) tests.
Significant digits in subtraction. Remind them that the answer cannot have more decimal places than the least precise measurement.
Vague sources of error ("human error"). Push for specific ones and their effect on the result.

Activity safety: a grown-up must handle the lighter and all flames, eye protection is worn, the work is done at the sink, and only 3% hydrogen peroxide is used.