Mix it, then make it — questions

1. identify

Silver and sulfur powders can be stirred together. When the stirred powders are heated, they react to form silver sulfide, the same black solid that forms as a dark layer on old silver spoons. Each statement describes either the stirred powders or silver sulfide. Match each statement to the substance it describes.

Write the number of the matching option.

1. The stirred powders
2. Silver sulfide

• 1. Two kinds of grain, shiny grey and yellow, can be seen in it.: ______
• 2. Every sample of it is 87% silver by mass.: ______
• 3. It is black all through, unlike silver or sulfur.: ______
• 4. One sample can be 20% sulfur, and another 60% sulfur.: ______
• 5. Only a chemical reaction can get its silver back.: ______
• 6. Its silver grains can be picked out with tweezers.: ______

Nudge (only if you are stuck): For each statement, ask: could this be true of two powders that have only been stirred together?

2. deduce

A teacher tests three solids in a fume cupboard. J is an orange-red powder. Heated strongly, it changes into a silvery liquid and a colourless gas, always 25 g of liquid for every 2 g of gas. K is a grey solid. Heated alone, it melts and sets again unchanged. It reacts with other substances to make new ones, but no simpler substance has ever been got from it. L is a white powder with two kinds of crystal under a hand lens. Water dissolves one kind and leaves the other. Which row is correct?

Circle one answer.

• A
• B
• C
• D

Nudge (only if you are stuck): For each solid, ask: has anything simpler been got from it, and what did that take?

3. calculate / identify / explain

Zinc and sulfur always combine in the ratio 2 g of zinc to 1 g of sulfur. A pupil heats 13 g of zinc powder with 5.0 g of sulfur until there is no more change. (a) Calculate the mass of zinc sulfide formed. (b) Calculate the mass of the element that is left over.

(a) Mass of zinc sulfide formed: ______________ unit ________

(b) Mass of the element left over: ______________ unit ________

Identify the element that is left over, and explain why it is that element.

Nudge (only if you are stuck): Check each element: is there enough of the other one for all of it to react?

4. calculate / deduce / explain

When copper is heated in air, the shiny brown metal turns into black copper oxide, a compound of copper and oxygen. Copper oxide always contains 4 g of copper for every 1 g of oxygen. (a) Calculate the mass of copper needed to make 12 g of copper oxide. Give your answer to 2 significant figures. (b) Calculate the mass of copper oxide that 6.0 g of copper makes if all of it reacts.

(a) Mass of copper needed: ______________ unit ________

(b) Mass of copper oxide from 6.0 g of copper: ______________ unit ________

A pupil heats 6.0 g of copper, and the solid afterwards has a mass of 7.0 g. Deduce what the solid contains, and explain why.

Nudge (only if you are stuck): Start from the masses that combine: what mass of copper oxide forms from 4 g of copper?

5. identify / explain

Zinc is a grey metal powder and sulfur is a yellow powder. Stirred together, they make a grey-yellow powder. When the powder is heated, it reacts with a bright flash and forms a white solid, zinc sulfide. Which statements are true of zinc sulfide, but not of the powder before heating?

Tick every correct statement.

• It contains zinc atoms and sulfur atoms.
• Its zinc and sulfur atoms are joined to each other.
• Under a hand lens, it shows no grey or yellow grains.
• A liquid that dissolves sulfur can wash its sulfur out.
• Every sample of it is about two-thirds zinc by mass.
• One sample of it can be half zinc and half sulfur.

Statement 1 is true of both. Explain why it doesn't tell them apart.

Nudge (only if you are stuck): Test each statement twice: once against the powder before heating, once against zinc sulfide.

6. identify / deduce / explain

Kofi heats copper powder with sulfur in a fume cupboard. Copper is a pink-brown metal, and sulfur is a yellow powder. Copper and sulfur combine to form copper sulfide, a black solid. Afterwards, the tube holds a black solid. Under a hand lens, a few pink-brown grains can be seen in it, but no yellow powder. (a) Identify the two substances in the solid, and deduce whether the solid is an element, a compound or a mixture. (b) Kofi says that he used too little sulfur. Explain how the observations support him.

(a) The two substances, and whether the solid is an element, a compound or a mixture

(b) How the observations show that there was too little sulfur

Nudge (only if you are stuck): For (a), compare each colour with the colours of copper and sulfur. For (b), think about what is left of each element.

7. explain / predict

A balloon is filled with hydrogen and oxygen, two colourless gases. They can be put in the balloon in any proportions, and they stay unchanged for days. A spark makes the gases explode with a bang, and drops of a colourless liquid, water, form. Every sample of water contains 1 g of hydrogen for every 8 g of oxygen, whatever the proportions of the gases were. (a) Explain, using two pieces of evidence, why water is a compound and not a mixture of hydrogen and oxygen. (b) A pupil boils some of the water and cools the vapour that comes off. Predict what the pupil collects, and explain what this shows about water.

(a) Two pieces of evidence that water is a compound

(b) What the pupil collects, and what it shows

Nudge (only if you are stuck): Which part of the iron and sulfur experiment is this like: stirring them together, or heating them?