Exam code: 5070
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6.1 Physical & Chemical Changes
Give one difference between a physical change and a chemical change.
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Two changes are described.
change 1: ice is warmed until it melts
change 2: magnesium is burned in air
State whether each change is a physical change or a chemical change.
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When magnesium is burned in air, a chemical change takes place.
Describe two observations that show a chemical change has taken place.
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A student separates a mixture of sodium chloride and sand.
Water is added to the mixture and it is stirred.
Name the process used to separate the sand from the salt solution.
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The salt solution is heated until all of the water has evaporated.
State what is left behind.
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Every stage of this separation is a physical change.
Explain why.
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Seawater is a mixture of water and dissolved salts.
Name the process used to obtain pure water from seawater.
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State the two changes of state that take place during this process.
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Explain why obtaining pure water from seawater is a physical change and not a chemical change.
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A copper rod is hammered into a thin sheet.
Explain why hammering the copper is a physical change.
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Copper is heated strongly in air. A black solid forms on the surface of the copper.
Name the black solid.
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Explain why heating copper in air is a chemical change.
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State one property, other than colour, of the black solid that is different from the same property of copper.
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Solid iodine is warmed gently. It turns directly into a purple vapour without forming a liquid.
State whether this is a physical change or a chemical change.
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The purple vapour is cooled on a cold surface and grey crystals form.
Explain why the grey crystals are the same substance as the original solid.
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A student says:
> The purple colour proves that a chemical change has taken place.
Explain why the student is wrong.
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Solid carbon dioxide turns directly into carbon dioxide gas when it is left at room temperature.
State whether this is a physical change or a chemical change.
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Explain your answer to part (a).
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Green copper(II) carbonate is heated strongly. A gas is given off and a black solid is left behind.
Explain why this is a chemical change.
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Both of the changes described in this question produce a gas.
Explain why the production of a gas does not on its own show that a chemical change has taken place.
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Petroleum is separated into fractions by fractional distillation.
Explain why fractional distillation of petroleum is a physical change.
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State the property of the hydrocarbon molecules that fractional distillation depends on.
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Long-chain alkane molecules from petroleum are cracked to make smaller molecules.
Explain why cracking is a chemical change.
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A student says:
> Fractional distillation and cracking must both be chemical changes, because both of them need heat.
Explain why the student is wrong.
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Solid potassium nitrate is stirred into water until it has all dissolved. The temperature of the water falls.
State whether dissolving the potassium nitrate is a physical change or a chemical change.
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Explain your answer to part (a).
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Magnesium ribbon is added to dilute hydrochloric acid. Bubbles are seen and the temperature of the mixture rises.
Explain why this is a chemical change.
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Explain why a change in temperature does not on its own show whether a change is physical or chemical.
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Yeast is added to aqueous glucose and the mixture is left in a warm place. Bubbles of gas are seen and ethanol is formed.
Explain why this is a chemical change.
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Name the gas that causes the bubbles.
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The ethanol is separated from the mixture by fractional distillation.
Explain why this separation is a physical change.
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Suggest why the ethanol cannot be turned back into glucose by cooling the mixture.
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A large lump of marble is crushed into a fine white powder.
State whether crushing the marble is a physical change or a chemical change.
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Explain why crushing the marble changes its appearance but not the substance itself.
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The marble powder is added to dilute hydrochloric acid. Bubbles are seen and the powder disappears.
Explain why this is a chemical change.
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Suggest one other change that alters the appearance of a substance without changing the substance itself.
Explain your choice.
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Blue hydrated copper(II) sulfate crystals are heated in a test tube. Steam is given off and a white solid is left behind.
Give the formula of the blue crystals and the formula of the white solid.
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Explain why heating the blue crystals is a chemical change.
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Water is added to the white solid and it turns blue again.
Explain why being able to reverse the change in this way does not make it a physical change.
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A separate sample of the blue crystals is dissolved in water instead of being heated.
Explain why dissolving the blue crystals is a physical change.
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The mass of the solid decreases when the blue crystals are heated.
Explain why.
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State what is meant by a physical change.
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Silicon(IV) chloride is a simple molecular substance. It boils at 58 °C.
Explain, in terms of the forces that are overcome, why boiling silicon(IV) chloride is a physical change.
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Silicon(IV) oxide has a giant covalent structure. It melts at 1710 °C.
State what is broken when silicon(IV) oxide melts.
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Explain why melting silicon(IV) oxide is a physical change, even though the bonds named in part (c) are broken.
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Explain why silicon(IV) oxide has a much higher melting point than silicon(IV) chloride.
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In experiment 1, a hot saturated solution of potassium nitrate is cooled. White crystals form in the colourless solution.
State whether the change in experiment 1 is a physical change or a chemical change.
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Explain your answer to part (a).
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In experiment 2, aqueous barium nitrate is added to aqueous sodium sulfate. A white precipitate forms in the colourless solution.
Explain why the change in experiment 2 is a chemical change.
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In both experiments a white solid appears in a colourless solution.
Explain why this observation does not show which type of change has taken place.
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Describe how a student could show experimentally that the change in experiment 1 is a physical change.
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Blue cobalt(II) chloride paper turns pink when damp air is passed over it. When the pink paper is warmed it turns blue again.
State whether the colour change is a physical change or a chemical change.
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Explain your answer to part (a).
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The paper can be changed from blue to pink and back again many times.
Explain why this does not show that the change is physical.
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Explain why warming the pink paper turns it blue.
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Water is boiled and steam is produced.
State whether boiling water is a physical change or a chemical change.
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In a second experiment, water is electrolysed. Hydrogen is produced at one electrode and oxygen at the other.
Explain why the electrolysis of water is a chemical change.
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Both changes need a supply of energy.
Explain why needing a supply of energy does not show whether a change is physical or chemical.
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Steam can be turned back into water simply by cooling it. The hydrogen and oxygen cannot be turned back into water by cooling them.
Explain why.
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The electrolysis is carried out in a sealed container.
Explain what happens to the total mass inside the container during the electrolysis.
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