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In Primary School Lower Block Science syllabus, students learn about buoyancy of materials.

However, the syllabus does not tell students why certain materials sink and why other materials float. This presents a challenge to teachers with inquisitive students whose favourite question is, “But cher, cher, why?”

The simple explanation is density. Density is simply mass per unit volume (mass/volume). The idea runs thus: if a material has greater density than water, it sinks; if it has lower density than water, it floats. Other factors such as surface tension and buoyant force play a part, but we’ll leave that for the material engineers. 😅

Below is a table of materials (including water) and their densities that students need to be familiar with for Primary School Science.

You’ll notice that all the materials in the table exhibit a range of densities, for the simple fact that many variations of each material exist.

Take ceramic for example. Its density is always greater than water’s density, so we can confidently say that ceramic sinks in water. In the case of metal, you might be surprised that some metals (lithium, sodium, potassium) are actually less dense than water and will theoretically float (discounting the fact that they will react with water upon contact!). By the same token, while most woods float, there are some that actually sink because they are denser than water.

Do hedgehogs sink or float? There’s only one way to find out…

Now, consider something else: modern ships. Modern ships are made of metals that are denser than water, yet they all float! Though the density of a given material is fixed, the effective density of an object made from that same material may vary. Steel ships float because their effective density has been lowered by encasing air (which has very low density) inside the hull.

You can easily verify this. Remember the swimming test during primary school where you had to blow bubbles into your pyjamas to make a float? The inflated pyjamas could even support our weight in water while the deflated pyjamas sank to the bottom of the pool.

Fortunately, the focus of Primary School Science is on how the properties of materials are used, rather than on identifying which materials sink and which materials float.

So while it is generally true to say that wood floats and metals sink, do remember that there are exceptions to the rule. It is more important to know how to use a material’s properties to decide if it is suitable for making a particular object, than to memorise the properties of various different materials (which is an impossible task for most).

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