What are acids and bases, and how can we tell them apart safely?
Describe the properties of common acids and bases, use indicators and the pH scale to identify them, and give everyday examples
A simple answer to the N(T) Science point on acids and bases. Their properties, everyday examples, how indicators and the pH scale tell them apart, and what neutralisation means.
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What this dot point is asking
This dot point wants you to describe what acids and bases are, give everyday examples of each, and explain how we tell them apart safely using indicators and the pH scale. You should also understand neutralisation, which is what happens when an acid and a base react. The big idea is that acids and bases are opposite kinds of chemical, the pH scale puts a number on how acidic or basic something is, and an indicator changes colour to show you which one you have without tasting or touching it.
The answer
What acids are
An acid is a chemical that tastes sour and can be corrosive, meaning a strong one can attack and damage materials and skin. You should never taste or touch laboratory acids to test them.
Everyday weak acids include lemon juice and vinegar (both sour to taste), and the fizzy acid in soft drinks. Stronger acids found in a laboratory include hydrochloric acid and sulfuric acid, which must be handled carefully. Your own stomach also contains an acid that helps digest food.
What bases are
A base is the opposite of an acid. A base that dissolves in water is also called an alkali. Bases often feel soapy or slippery, and strong ones are also corrosive.
Everyday bases include soap, baking soda and many cleaning products such as oven cleaner and bleach. Toothpaste is mildly basic, which helps cancel out acids on your teeth. Like acids, strong bases should be handled with care and never tasted.
Neutral substances
A neutral substance is neither an acid nor a base. Pure water is the most important example. Neutral substances sit right in the middle between acids and bases.
The pH scale
The pH scale is a way of putting a number on how acidic or basic something is. It runs from 0 to 14.
- A pH less than 7 means the substance is an acid. The lower the number, the stronger the acid.
- A pH of exactly 7 means the substance is neutral, like pure water.
- A pH more than 7 means the substance is a base. The higher the number, the stronger the base.
So pH 1 is a very strong acid, pH 7 is neutral, and pH 14 is a very strong base.
Indicators
An indicator is a chemical that changes colour to show whether something is an acid or a base, so you can test safely without tasting.
The most useful one is universal indicator, which turns a whole range of colours and matches them to the pH scale. Acids turn it red, orange or yellow; neutral turns it green; bases turn it blue or purple. Litmus is a simpler indicator: it turns red in acid and blue in a base.
Neutralisation
When an acid and a base are mixed, they react and cancel each other out. This reaction is called neutralisation. The acid becomes less acidic and the base becomes less basic, so the mixture moves toward neutral (pH 7). Neutralisation is why an indigestion tablet (a base) soothes an acidic stomach.
Examples in context
Example 1. A bee sting and toothpaste. A bee sting is acidic, so dabbing it with a mild base such as baking soda can help neutralise the acid and ease the sting. In the same way, toothpaste is mildly basic and helps neutralise the acids made by bacteria in your mouth, protecting your teeth.
Example 2. Cleaning the kitchen. Many kitchen cleaners are basic (alkaline) because bases are good at breaking down greasy, fatty dirt. Limescale in a kettle, on the other hand, is removed with an acid such as vinegar, which reacts with the limescale. This is why the label warns you to handle cleaning chemicals safely.
Try this
Cue. Vinegar turns universal indicator orange. State whether vinegar is an acid, a base or neutral, and give a rough pH. Orange is on the acid side, so vinegar is an acid with a pH of about 4.
Cue. Explain why you should use an indicator instead of tasting a liquid to find out if it is an acid. Some acids are corrosive and harmful, so tasting is dangerous; an indicator changes colour and tells you safely.
Cue. An acidic soil is treated with a base to help plants grow. Name this kind of reaction and state what happens to the soil's pH. This is neutralisation, and the soil's pH rises toward neutral (7), so it becomes less acidic.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksA student tests three liquids with universal indicator. Lemon juice turns it red, soapy water turns it blue, and pure water stays green. (a) Which liquid is an acid? (b) Which liquid is a base (alkali)? (c) What does the green colour tell you about the pure water? (d) Roughly what pH number matches the green colour?Show worked answer →
(a) Lemon juice is the acid, because it turns universal indicator red. Acids give red, orange or yellow colours.
(b) Soapy water is the base (alkali), because it turns universal indicator blue. Bases give blue or purple colours.
(c) The green colour tells you that the pure water is neutral. It is neither an acid nor a base.
(d) Green matches a pH of about 7, the neutral value in the middle of the scale.
What markers reward: matching red to acid and blue to base, knowing green means neutral, and giving pH 7 for neutral. Use the words "acid", "base/alkali" and "neutral".
Original3 marksA person has a stomach ache caused by too much stomach acid. They take an indigestion tablet that contains a base. (a) Name the type of reaction between the acid and the base. (b) Explain how the tablet helps. (c) State whether the resulting mixture is more or less acidic than before.Show worked answer →
(a) The reaction between an acid and a base is called neutralisation.
(b) The base in the tablet reacts with the extra stomach acid and cancels out some of its acidity, so the stomach is no longer too acidic and the ache eases.
(c) The resulting mixture is less acidic than before, because the base has neutralised some of the acid, moving the pH closer to neutral (pH 7).
What markers reward: naming neutralisation, explaining that the base cancels out the acid, and stating that the mixture becomes less acidic (closer to neutral).
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