Working Scientifically & Required Practicals

What are the four main stages of a scientific investigation?

Planning, collecting data, analysing data, and evaluation.

In data analysis, what term describes repeats that are similar to each other?

Precise.

What is an anomaly in a set of experimental results?

A result that is unusual or unexpected and may be ignored if it's due to measurement error.

What type of graph is best used when both the independent and dependent variables are continuous data?

A line graph, also called a scatter graph.

What is drawn on a line graph to show the trend in the data?

A line of best fit.

When is a bar chart the most appropriate way to display data?

When the independent variable has different categories.

What does it mean if results are described as 'repeatable'?

Similar results are obtained when the same investigator repeats the investigation using the same method and equipment.

How do you check for the 'reproducibility' of an experiment?

Get someone else to follow your method to see if their results are similar to yours.

What is the term for errors that are due to things you have no control over, such as a change in room temperature?

Random errors.

How can the effect of random errors be reduced?

By repeating measurements and calculating a mean.

What is the term for errors that are due to problems with the equipment used, affecting each measurement consistently?

Systematic errors.

A zero error, where a balance consistently reads 0.1 g too high, is an example of a _____ error.

systematic

What is a hypothesis in a scientific investigation?

A starting point for investigation; an idea or explanation to be tested.

What is the name for the variable that is changed by the investigator?

The independent variable.

What is the name for the variable that is measured in response to changes in the independent variable?

The dependent variable.

What are control variables in an experiment?

Variables that are kept constant to ensure a fair test.

In the experiment investigating how temperature affects reaction rate, what is the independent variable?

The temperature of the reactants.

In the experiment investigating how temperature affects reaction rate, what is the dependent variable?

The rate of the reaction.

What does the term 'accurate' mean in the context of scientific measurements?

A measurement is close to the true value.

Why would using a 10 cm³ measuring cylinder be more accurate for measuring 5 cm³ of liquid than a 100 cm³ one?

It has smaller scale divisions, allowing for a more precise and therefore more accurate measurement of that specific volume.

What is the purpose of the first practical: Making Soluble Salts?

To prepare a pure, dry sample of a soluble salt from an insoluble oxide or carbonate.

In the preparation of copper sulfate, why is the dilute sulfuric acid gently heated?

To speed up the reaction with the copper oxide.

In making copper sulfate, why is copper oxide added in excess?

To ensure that all the sulfuric acid (the limiting reactant) has reacted and is not left in the final salt solution.

How is the excess, unreacted copper oxide removed from the copper sulfate solution?

By filtration using a filter funnel and filter paper.

What is the liquid that passes through the filter paper called?

The filtrate.

During the preparation of copper sulfate crystals, how is the solution concentrated?

It is heated in an evaporating basin, often over a water bath, until about half the solution remains or crystals start to form.

After heating the copper sulfate solution, how are the large crystals formed?

The concentrated solution is left in a cool place for 24 hours for crystallisation to occur slowly.

What is the final step to get pure, dry crystals of copper sulfate after they have formed?

The crystals are scraped onto filter paper and gently patted dry.

What is the chemical equation for the reaction between copper(II) oxide and sulfuric acid?

CuO + H₂SO₄ → CuSO₄ + H₂O

What safety precaution is essential when working with sulfuric acid?

Wear eye protection as it is corrosive.

What are electrodes made from in the electrolysis of aqueous solutions practical, and why?

Carbon graphite, because it is inert and unreactive.

In electrolysis, what is the name of the negative electrode?

The cathode.

In electrolysis, what is the name of the positive electrode?

The anode.

What is observed at the cathode during the electrolysis of aqueous copper(II) chloride?

The electrode is coated with copper (a brown solid).

What is observed at the anode during the electrolysis of aqueous copper(II) chloride?

Bubbles of gas are produced (chlorine).

What is the test for chlorine gas?

It bleaches damp blue litmus paper.

What is observed at the cathode during the electrolysis of aqueous sodium chloride?

Bubbles of gas are produced (hydrogen).

Why is hydrogen gas produced instead of sodium metal at the cathode during the electrolysis of aqueous sodium chloride?

Because sodium is more reactive than hydrogen.

What is the test for hydrogen gas?

It produces a 'squeaky pop' with a lit splint.

What is the test for oxygen gas?

It relights a glowing splint.

In the temperature changes practical, what is the independent variable when investigating the effect of adding different volumes of sodium hydroxide to hydrochloric acid?

The volume of sodium hydroxide solution.

In the temperature changes practical, what is the dependent variable?

The maximum temperature reached.

What are two control variables in the temperature changes practical involving neutralisation?

The volume of hydrochloric acid and the concentrations of both the acid and the alkali.

Why is a polystyrene cup used in the temperature changes practical?

Polystyrene is a good thermal insulator, which reduces heat loss to the surroundings.

Why is a lid used on the polystyrene cup in the temperature changes practical?

To reduce heat loss to the air through evaporation.

In the temperature changes practical, why does the maximum temperature eventually start to decrease after a certain volume of sodium hydroxide is added?

The energy released by the reaction has reached a maximum and is being spread out into a greater total volume of solution.

The neutralisation reaction between an acid and an alkali is an example of an _____ reaction.

exothermic

In chromatography, why should the starting line be drawn in pencil?

Because ink would dissolve in the solvent and interfere with the chromatogram, whereas graphite (pencil) is insoluble.

What is the formula for calculating the R_f value in chromatography?

R_f = distance travelled by substance / distance travelled by solvent

In paper chromatography, why must the level of the solvent be below the starting pencil line?

So that the spots of ink dissolve in the solvent as it moves up the paper, rather than dissolving directly into the bulk solvent at the bottom.

What is the stationary phase in paper chromatography?

The chromatography paper.

What is the mobile phase in paper chromatography?

The solvent.

Why are measurements in chromatography often taken in millimetres rather than centimetres?

Measurements in millimetres are more precise, which gives more precise $R_{f}$ values.

What colour flame is produced by lithium ions (Li⁺)?

Crimson.

What colour flame is produced by sodium ions (Na⁺)?

Yellow.

What colour flame is produced by potassium ions (K⁺)?

Lilac.

What colour flame is produced by calcium ions (Ca²⁺)?

Orange-red.

What colour flame is produced by copper(II) ions (Cu²⁺)?

Green.

When testing for metal ions, what is added to the solution to form a coloured precipitate?

Sodium hydroxide solution.

What colour precipitate is formed when sodium hydroxide is added to a solution containing copper(II) ions (Cu²⁺)?

Blue.

What colour precipitate is formed when sodium hydroxide is added to a solution containing iron(II) ions (Fe²⁺)?

Green.

What colour precipitate is formed when sodium hydroxide is added to a solution containing iron(III) ions (Fe³⁺)?

Brown.

What is observed when sodium hydroxide is added to a solution containing aluminium ions (Al³⁺)?

A white precipitate forms which redissolves in excess sodium hydroxide to form a colourless solution.

What is the chemical test for carbonate ions (CO₃^2-)?

Add dilute acid (e.g., HCl); effervescence occurs, and the gas produced turns limewater cloudy.

What is the chemical test for sulfate ions (SO₄^2-)?

Add dilute hydrochloric acid followed by barium chloride solution; a white precipitate forms.

What is the chemical test for halide ions (Cl⁻, Br⁻, I⁻)?

Add dilute nitric acid followed by silver nitrate solution.

What result indicates the presence of chloride ions (Cl^-)?

A white precipitate.

What result indicates the presence of bromide ions (Br^-)?

A cream precipitate.

What result indicates the presence of iodide ions (I^-)?

A yellow precipitate.

What is the difference between pure water and potable water?

Pure water contains only water molecules ($H_{2}O$), while potable water is safe to drink but can contain dissolved solids.

What is the expected pH of pure water?

pH 7 (neutral).

How can you determine the mass of dissolved solids in a water sample?

Weigh an empty evaporating basin, add a known volume of the water sample, heat until all water evaporates, cool, and reweigh the basin.

What is the name of the process used to purify sea water by separating pure water from dissolved salts?

Distillation.

Describe the process of simple distillation for purifying a water sample.

The water is heated to boil, the resulting steam travels through a condenser where it cools and turns back into liquid water, which is then collected.

In the 'disappearing cross' experiment to measure the rate of reaction, what causes the solution to become cloudy?

The formation of a sulfur precipitate.

In the 'disappearing cross' experiment, what is measured to determine the reaction rate?

The time taken for the cross to become obscured by the precipitate.

How can you reduce random errors caused by parallax error when reading a thermometer?

Ensure your eye is level with the top of the liquid in the thermometer, or use a digital thermometer.

Measurements are more _____ if they are tightly clustered around the mean value.

precise

A smaller range in a set of repeat measurements indicates _____ precision.

greater

Greater precision in a set of measurements implies the presence of fewer _____ errors.

random

What is the equation to calculate the uncertainty of a set of measurements?

Uncertainty = Range of values / 2

A student measures the time for a reaction as 15.2s, 15.6s, and 15.4s. What is the uncertainty of these measurements?

The range is 15.6 - 15.2 = 0.4s. The uncertainty is 0.4 / 2 =  ±0.2s.

In a titration, why is a white tile placed under the conical flask?

To make the colour change of the indicator at the end point easier to see.

What piece of equipment is used to accurately measure a fixed volume of alkali (e.g., 25 cm³) into the conical flask for a titration?

A volumetric pipette and pipette filler.

What piece of equipment is used to add a variable but precisely measured volume of acid to the alkali in a titration?

A burette.

What are 'concordant results' in a titration?

Titres that are within 0.1 cm³ of each other.

What is the colour of phenolphthalein indicator in an acidic solution?

Colourless.

What is the colour of phenolphthalein indicator in an alkaline solution?

Pink.

Why is the first trial in a titration often considered a 'rough' trial?

It is done quickly to get an approximate idea of the end point, so subsequent titrations can be performed more precisely.