LearnLah
Forces & PressureElectricity & CircuitsChemical Reactions, Acids & BasesPhotosynthesis & NutritionTransport & RespirationReproductionInteractions & Ecosystems

Study notes

Chemical Reactions, Acids & Bases

Acids and alkalis, how indicators show acidity, the pH scale as a conceptual acidic-to-alkaline range, neutralisation, and the signs that tell you a chemical reaction has happened.

Learn it step by step

Acids and alkalis are everyday substances

Vinegar and lemon juice are common acids. Soap solution and baking soda solution are common alkalis (bases). Whether a substance is dangerous depends on how strong and concentrated it is, not simply on whether it is an acid or an alkali, strong versions of either can cause burns while weak versions of either, like vinegar or soap, are safe in everyday use.

Indicators change colour to show acidity

Litmus paper is a simple indicator: blue litmus turns red in an acidic solution and stays blue in a neutral or alkaline one. Universal indicator gives a whole range of colours across the pH scale, roughly green for neutral, red for strongly acidic, and purple for strongly alkaline, so it shows roughly HOW acidic or alkaline a solution is, not just whether it is one or the other.

The pH scale runs acidic to alkaline

The pH scale runs from 0 to 14: values below 7 are acidic, exactly 7 is neutral, and values above 7 are alkaline. A lower pH number means a solution is more strongly acidic; a pH of 3 is more strongly acidic than a pH of 6, even though both are on the acidic side. This is used conceptually here, not as a calculation.

Neutralisation: acid + alkali makes salt and water

When an acid and an alkali react completely, they neutralise each other, forming a salt and water (acid + alkali → salt + water). The resulting mixture is neither acidic nor alkaline. An antacid tablet works this way, its alkaline content neutralises some of the excess acid causing stomach discomfort.

Metals and carbonates react with acids differently

A reactive metal added to an acid produces a salt and hydrogen gas (acid + metal → salt + hydrogen), seen as bubbling at the metal's surface. A carbonate added to an acid instead produces a salt, water, and carbon dioxide gas (acid + carbonate → salt + water + carbon dioxide), and that gas is what turns limewater cloudy.

Chemical reactions show recognisable signs

Gas bubbles forming, a colour change, a new cloudy appearance (a precipitate), and a temperature change (getting hotter or colder) are all signs that a chemical reaction has taken place. A reaction does not need every one of these signs at once, and simply changing state (like water evaporating then condensing) without forming anything new is a physical change, not a chemical one.

Worked examples

An unlabelled solution turns universal indicator paper orange-red, and a second unlabelled solution turns it dark blue. A student wants to combine small amounts of both to get a roughly neutral mixture. Explain whether this should work.
  1. First, read what each colour means: orange-red on universal indicator corresponds to an acidic solution, dark blue corresponds to an alkaline solution.
  2. So the first solution is acidic and the second is alkaline, they are not two shades of the same type as a quick glance might suggest.
  3. Neutralisation happens when an acid and an alkali react together, forming salt and water, and the mixture becomes closer to neutral.
  4. Since one solution is acidic and the other alkaline, combining appropriate amounts of both can indeed bring the mixture toward neutral, matching the student's plan.
A student claims "all acids are dangerous and all alkalis (bases) are safe to touch," pointing out that acid burns are common. Explain whether this is a scientifically accurate way to think about acids and alkalis.
  1. This claim sounds reasonable at first because acid burns are indeed a well-known hazard.
  2. But it wrongly assumes danger comes from being classified as an acid, rather than from strength and concentration.
  3. Strong alkalis, such as those used in oven cleaners, can cause serious burns just as strong acids can.
  4. Meanwhile weak acids (vinegar) and weak alkalis (soap) are both perfectly safe for everyday handling, so the claim is not accurate: strength matters more than the acid/alkali label.
Solution A has a pH of 3 and Solution B has a pH of 6. Without doing any calculation, explain which is more strongly acidic.
  1. A student might think a smaller number simply means "less" of something and conclude Solution A is less acidic, this is the wrong direction.
  2. On the pH scale, values below 7 are acidic, and the FURTHER below 7 a value is, the more strongly acidic the solution is.
  3. A pH of 3 is farther below 7 than a pH of 6.
  4. So Solution A, with pH 3, is more strongly acidic than Solution B, with pH 6, even though 6 is still on the acidic side, not alkaline.
A student mixes two clear, colourless solutions and observes the mixture instantly turning cloudy yellow, along with the beaker becoming noticeably warmer to the touch. No gas bubbles are seen. Explain whether a chemical reaction has occurred.
  1. A student might assume that without visible gas bubbles, nothing chemical has happened, this overlooks the other valid signs of a reaction.
  2. A new cloudy yellow appearance suggests a new substance (a precipitate) has formed, which did not exist in either starting solution.
  3. A rise in temperature is a separate recognised sign of a chemical reaction taking place.
  4. Together, the colour/cloudiness change and the temperature change are enough to conclude a chemical reaction has occurred, gas bubbles are not required for every reaction.

Mind map

Mind map for Chemical Reactions, Acids and Bases.

  • Acids and alkalis
    • vinegar, lemon juice: acids
    • soap, baking soda: alkalis
  • Indicators
    • litmus: red = acid
    • universal indicator: full colour range
  • pH scale
    • below 7: acidic
    • 7: neutral
    • above 7: alkaline
  • Neutralisation
    • acid + alkali -> salt + water
    • e.g. antacids, liming acidic soil
  • Word equations
    • acid + metal -> salt + hydrogen
    • acid + carbonate -> salt + water + carbon dioxide
  • Signs of a reaction
    • gas bubbles
    • colour change
    • temperature change
    • precipitate
Practise this topic →