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 precipitate

提示The two liquids have made something that will not dissolve, so it hangs in the mixture and clouds it.

为什么When two dissolved compounds swap partners, one of the new pairings may not dissolve, so it appears as a fine solid spread through the liquid. Adding colourless silver nitrate to colourless sodium chloride solution gives a white one of silver chloride — the standard laboratory test for a chloride.

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.

water, then carbon dioxide

提示An acid on any carbonate always leaves a neutral liquid plus a gas — the pair that makes a chalk chip fizz.

为什么Every acid-and-carbonate reaction follows one pattern: acid + carbonate → salt + water + carbon dioxide. The acid decides the salt family, so hydrochloric gives chlorides, sulfuric gives sulfates and nitric gives nitrates, while the metal supplies the first half of the name. It is why marble chips fizz in acid and why acid rain slowly eats away a limestone statue.

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.