Mass: The Amount of Matter

Introduction

A brick feels heavier than a sponge of the same size because it contains more matter. The quantity that tells us "how much matter" is mass . Mass is one of the most important measurements in chemistry: recipes for reactions, medicine doses and industrial production all depend on knowing masses accurately. This page explains what mass is, the units used to measure it and how to convert between them.

Core explanation

What mass means. Mass is the amount of matter in an object. It depends on how many particles the object contains and how heavy each particle is. A gold bar has a large mass because it contains a huge number of heavy gold atoms packed closely together.

Units of mass. The SI base unit of mass is the kilogram (kg) . In chemistry laboratories, smaller units are often more convenient:

- 1 kilogram (kg) = 1000 grams (g) - 1 gram (g) = 1000 milligrams (mg) - 1 tonne (t) = 1000 kilograms (kg)

A paper clip has a mass of about 1 g, a litre of water about 1 kg and a small car about 1 tonne. Medicine doses are often given in milligrams, for example a 500 mg tablet.

Mass stays the same wherever the object is. Moving an object to a different place, turning it over or changing its shape does not change its mass. A lump of modelling clay has the same mass whether it is rolled into a ball or flattened into a sheet, because no matter has been added or removed. Taking the clay to the top of a mountain or even to the Moon still does not change its mass (its weight would change, as the next page explains).

Mass is conserved when things change. If ice melts, the water has exactly the same mass as the ice. If salt dissolves, the solution's mass equals the mass of the water plus the mass of the salt. Mass only changes if matter is added or taken away — for example if a gas escapes from an open container.

Measuring mass. Mass is measured using a balance, which you will study on page 35. Laboratory balances commonly read to 0.01 g or 0.001 g.

Step-by-step reasoning

To convert between mass units:

1. Decide whether you are going to a smaller unit (the number gets bigger) or a larger unit (the number gets smaller). 2. Find the conversion factor (1000 for each step between mg, g, kg and t). 3. Multiply by 1000 for each step to a smaller unit; divide by 1000 for each step to a larger unit. 4. Check the answer makes sense — 2 kg should be a larger number of grams, not smaller.

Visual explanation

A conversion ladder is a helpful picture:

Unit Step down (× 1000) Step up (÷ 1000) --- --- --- tonne (t) → kg — kilogram (kg) → g → t gram (g) → mg → kg milligram (mg) — → g

Moving down the ladder multiplies by 1000; moving up divides by 1000.

Real-world analogy

Mass is like the number of pages in a book. You can carry the book to another room, read it upside down or bend its cover, and it still has the same number of pages. Only tearing pages out or gluing new ones in changes the count. Mass changes only when matter is removed or added.

Real-world example

Pharmacists prepare medicines by mass. A child's dose of a pain reliever might be calculated as a certain number of milligrams for every kilogram of the child's body mass. A mistake in converting between milligrams and grams would mean a dose 1000 times too large or too small, which is why units are always written carefully on prescriptions.

Why?

Why is the kilogram, not the gram, the SI base unit? When the metric system was designed, a gram was too small for trade and everyday life, so the kilogram became the standard. Since 2019 the kilogram has been defined using a fixed value of a fundamental constant of nature (the Planck constant), so it can be reproduced precisely in any well-equipped laboratory.

Common misconception

Students often think that a large object must have a large mass. A beach ball is bigger than a golf ball but has a much smaller mass. Mass depends on the amount of matter, not on size; how tightly the matter is packed is described by density, which you will meet on page 41.

Worked example

Question: A beaker contains 0.250 kg of water. Express this mass in grams and in milligrams.

Reasoning: Kilograms to grams is one step down, so multiply by 1000: 0.250 × 1000 = 250 g. Grams to milligrams is another step down: 250 × 1000 = 250 000 mg.

Answer: 250 g, or 250 000 mg.

Quick check

1. Convert 3500 g into kilograms. Answer: 3500 ÷ 1000 = 3.5 kg.

Exam focus

Always include units with every mass you write, and convert all masses to the same unit before adding, subtracting or comparing them. A frequent error is multiplying when you should divide; check that converting to a larger unit gives a smaller number.

Advanced insight

Chemists also need to count particles, not just weigh them. Because each type of atom has its own characteristic mass, weighing a sample lets chemists work out how many particles it contains. This link between mass and number of particles leads to the mole concept, which you will study in the stoichiometry units.

Summary

Mass is the amount of matter in an object. Its SI unit is the kilogram; grams and milligrams are used for smaller amounts, and tonnes for large ones, with 1000 as the conversion factor between each. Mass does not change when an object moves or changes shape, and it is conserved when substances melt or dissolve.

Practice questions

1. Convert 0.75 kg into grams. Answer: 0.75 × 1000 = 750 g. 2. A tablet contains 250 mg of an active ingredient. What is this in grams? Answer: 250 ÷ 1000 = 0.25 g. 3. A 50 g ice cube melts completely in a sealed bag. What is the mass of the water? Explain. Answer: 50 g, because melting does not add or remove matter, so mass is unchanged. 4. Which has the greater mass: 1.2 kg of sand or 1500 g of feathers? Answer: 1500 g = 1.5 kg of feathers, which is greater than 1.2 kg of sand.