Chemical reactions, Year 7: more practice

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

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

为什么Boiling 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

提示Can the crust be turned back into dough?

为什么The 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

提示Only one gas makes a glowing splint burst into flame again.

为什么Oxygen supports burning, so a glowing splint relights in it. It is the standard test for oxygen.

Carbon dioxide

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

为什么Limewater 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

提示Burning needs a gas from the air as well as the fuel.

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

Magnesium oxide

提示The metal combines with oxygen from the air.

为什么Magnesium + 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

提示Think how a campfire feels.

为什么Burning is a chemical reaction that releases stored energy. The fuel is the store of energy.

magnesium + oxygen → magnesium oxide

提示Reactants go on the left, the product on the right.

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

Zinc sulfide

提示The product is on the right of the arrow.

为什么Reactants 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

提示Read the plus as 'joins up' and the arrow as 'turns into'.

为什么A word equation is a sentence in shorthand: it says what goes in and what comes out.