Properties of materials

In Grade 7 you learned the periodic table is an organised list of elements. The biggest split on that table is into two families: metals and non-metals.

Metals usually share these properties:

• Shiny when polished, and strong.
• Malleable — they can be bent and hammered into shape without snapping.
• Good conductors of heat and electricity.
• Most are solids at room temperature (mercury is the unusual liquid one).

Non-metals are usually the opposite — dull, brittle (they snap), and poor conductors. Many are gases, like oxygen and nitrogen.

There are always a few exceptions, but knowing the typical properties lets you classify most elements at a glance.

Now for a brand-new way to describe substances — and one of the most important ideas in chemistry. Some substances are acids.

You already know acids without realising it. The sharp, sour taste of a lemon or of vinegar comes from acids — citric acid in the lemon and ethanoic acid in the vinegar.

The properties of an acid:

• They have a sour taste (you must never taste a substance to test it — this is just a property).
• They turn blue litmus paper red.
• They have a pH below 7.
• Stronger acids can be corrosive — they can damage skin and other materials.

Acids you may meet in the lab include hydrochloric acid, sulfuric acid and nitric acid. These are much stronger than the gentle acids in food, so they are handled carefully and you wear eye protection.

The key idea: an acid is not just a dangerous liquid — it is any substance with these shared properties, including weak, safe ones in your kitchen.

The opposite of an acid is an alkali. If acids are at one end of the scale, alkalis are at the other.

The properties of an alkali:

• They feel soapy or slippery to touch.
• They turn red litmus paper blue.
• They have a pH above 7.
• Strong alkalis are also corrosive and must be handled carefully.

Everyday alkalis include soap, toothpaste, baking soda and oven cleaner. In the lab you may use sodium hydroxide or limewater, which are stronger.

A useful pairing to learn: a base is any substance that can cancel out an acid. An alkali is simply a base that dissolves in water. So every alkali is a base, but not every base dissolves.

The two families — acids and alkalis — are chemical opposites. When they meet, something interesting happens, which you will see later in this guide.

How acidic or how alkaline a substance is can be measured on the pH scale — a number line from 0 to 14.

The scale tells you three things:

• pH below 7 — the substance is acidic. The lower the number, the stronger the acid.
• pH exactly 7 — the substance is neutral.
• pH above 7 — the substance is alkaline. The higher the number, the stronger the alkali.

To measure pH you use an indicator — a substance that changes colour depending on the pH. Litmus is a simple indicator: red in acid, blue in alkali. Universal indicator is better — it gives a whole range of colours, so you can read the pH like the coloured strip above.

Right in the middle of the pH scale, at pH 7, are the neutral substances. A neutral substance is neither an acid nor an alkali.

Pure water is the most important neutral substance — it sits exactly at pH 7. A salt solution, such as sodium chloride dissolved in water, is also neutral.

With universal indicator, a neutral substance turns green — the colour right in the middle of the scale.

There is one more important way a neutral substance is made. When an acid and an alkali are mixed in the right amounts, they cancel each other out. This is called neutralisation, and the products are a neutral salt and water. This is why:

• Indigestion tablets (an alkali) neutralise the extra acid in your stomach.
• Lime (an alkali) is spread on soil that is too acidic for crops.
• A bee sting (acidic) can be soothed with a mild alkali.

So 'neutral' is not just the middle of a scale — it is the balanced result when an acid and an alkali meet.

Sorting materials by their properties prepares you for the chemistry ahead:

• Chemical reactions — neutralisation, where an acid reacts with an alkali, is one of the first reactions you will study in detail.
• Metals and reactivity — comparing metals leads into how different metals react with acids, water and air.
• Word equations — you will write acid + alkali reactions using the names from this topic.
• Everyday science — pH matters in cooking, gardening, swimming pools and looking after your teeth.

Whenever you meet a liquid in the lab, ask: is this an acid, an alkali or neutral — and how could I test it? That habit makes chemistry feel far more predictable.