Chemical reactions

They are rearranged, not created or destroyed

提示Every atom is still present at the end, just joined up in a different order.

为什么Each atom that goes into a reaction comes out of it, simply bonded to different partners. That is exactly why the total mass never changes.

A gas

提示Bubbles have to be filled with something that was not there a moment ago.

为什么Bubbling is one of the classic clues that new substances are forming, alongside an unexpected colour change, a temperature change, or a solid appearing from two clear liquids.

Oxygen

提示Put a jar over a candle and the flame soon dies — something in the air has run out.

为什么Combustion is a reaction between fuel and this gas that releases energy. Cutting off the supply puts the flame out, which is precisely how a fire blanket works.

The right

提示The arrow points from what you started with towards what you finish with.

为什么Reactants sit on the left and the arrow is read as turns into — never an equals sign, because the substances themselves are not the same at the end.

They are exactly the same

提示No atom can escape from a sealed flask, so nothing is missing from the balance.

为什么Atoms are only rearranged, never lost, so a sealed reaction cannot change mass. In an open beaker the reading can drop, because a gas has drifted away.

oxidation

提示It happens fast in a flame and very slowly on a bicycle left out in the rain.

为什么Magnesium burning and iron rusting are the same kind of reaction running at wildly different speeds — in both, the metal joins with oxygen to make an oxide.

Thermal decomposition

提示One substance goes in, two or more come out, and the Bunsen burner does the work.

为什么Heating copper carbonate gives copper oxide and carbon dioxide. It is the mirror image of a reaction where several substances join to make a single new one.

Exothermic

提示Energy is leaving the reaction and warming everything around it, much like a hand warmer.

为什么Combustion, neutralisation and most oxidation reactions release energy this way. When a reaction takes energy in instead, the mixture genuinely turns cold.

Above

提示One of the two fizzes hard in acid; the other barely bothers.

为什么The series ranks metals by how eagerly they react, with potassium near the top and gold at the bottom. Its real value is predicting a result before you mix anything.

Copper

提示The less reactive of the two metals is pushed out of solution and settles as a brown layer.

为什么Iron is higher in the reactivity series, so it takes the place of the other metal in the compound. The blue fades as iron sulfate forms.

Hydrogen

提示It is the gas that fizzes off when a metal is dropped into acid.

为什么The pop is the gas burning explosively in a tiny quantity. A glowing splint relighting points to oxygen instead, and limewater turning milky to carbon dioxide.

The number of atoms of each element

提示Nothing can appear or vanish, so a tally taken before must agree with the tally taken after.

为什么You balance by changing only the large numbers written in front of a formula. Altering a small subscript would change the substance itself rather than the amount of it.