Chemical reactions, Year 7: more practice

The bubbles are just water turning to steam, so no new substance forms

HintWhat is the gas in there, and where did it come from?

The whyBoiling is a change of state, a physical change. Steam is still water, so there is no new substance, which would be the sign of a chemical reaction.

Yes, a new colour suggests new substances have formed

HintCan the crust be turned back into dough?

The whyThe browning of the crust is made by new substances, and it cannot be undone. Unexpected colour changes are a sign of a chemical reaction.

Oxygen

HintOnly one gas makes a glowing splint burst into flame again.

The whyOxygen supports burning, so a glowing splint relights in it. It is the standard test for oxygen.

Carbon dioxide

HintThe limewater reacts with the gas to form a fine white solid.

The whyLimewater turns cloudy because carbon dioxide reacts with it and makes tiny solid particles. This is the standard test for carbon dioxide.

The oxygen is used up

HintBurning needs a gas from the air as well as the fuel.

The whyThe wax is still there, but the oxygen in the jar has been used up, so the burning stops.

Magnesium oxide

HintThe metal combines with oxygen from the air.

The whyMagnesium + oxygen → magnesium oxide. The mass of the white powder is more than the mass of the ribbon, because oxygen has joined it.

Given out, as heat and light

HintThink how a campfire feels.

The whyBurning is a chemical reaction that releases stored energy. The fuel is the store of energy.

magnesium + oxygen → magnesium oxide

HintReactants go on the left, the product on the right.

The whyThe arrow means 'react to make'. The word oxide shows oxygen has joined the metal.

Zinc sulfide

HintThe product is on the right of the arrow.

The whyReactants are on the left of the arrow and the products are on the right. Zinc and sulfur are used up to make zinc sulfide.

Hydrogen reacts with oxygen to make water

HintRead the plus as 'joins up' and the arrow as 'turns into'.

The whyA word equation is a sentence in shorthand: it says what goes in and what comes out.