What is sodium silicate? Sodium silicate is a family of compounds made from sodium, silicon and oxygen. It is also called water glass, because some forms dissolve in water to give a clear, glass-like liquid. The simplest member of the family is sodium metasilicate, Na₂SiO₃. Most sodium silicates are white solids or colourless solutions, and they are made from nothing more exotic than sand and an alkali.
This guide explains what sodium silicate is, how its formula and ratio work, how it is made, why its solutions are alkaline and what happens when you add acid. It then covers its uses, from foundry moulds and ceramics to concrete sealing and wartime egg storage, and finishes with safety, revision questions and answers to common questions. The chemistry connects directly to GCSE and A-level ideas about alkalis, neutralisation and giant covalent structures.
What is sodium silicate (water glass)?
Sodium silicate is not one single compound with one universal formula. It is a family of sodium–silicon–oxygen compounds that share the general form (Na₂O)ₓ·(SiO₂)ᵧ. Pure samples are colourless, transparent solids or white powders. Commercial water glass is often tinted green or blue by small amounts of iron impurities.
You will meet sodium silicate in two main forms. The solid is a white powder or glassy lump. The liquid is a thick, syrupy solution of the same material in water. The liquid form is what most people mean by “water glass”, and it is the form sold for most industrial and DIY jobs.
The name makes sense once you see what happens when the solution dries. As the water evaporates, it leaves a hard, glassy film. Water can soften that film again, which is why sodium silicate behaves like a glass that can be rehydrated.
A short history
European alchemists in the 16th century observed soluble alkali silicates, although later historians argued these were not water glass as we know it today. Around 1640, Van Helmont showed that silica could be precipitated out of such a solution by adding acid. In 1646, Johann Rudolf Glauber made potassium silicate, which he called “liquor silicum”. The modern route to water glass came in 1818, when Johann Nepomuk von Fuchs made it by treating silicic acid with alkali.
Sodium is a Group 1 metal whose compounds are usually soluble. If you want to see where it sits in the periodic table, read our guide to sodium (Na). The silicon part of the name comes from the element that sand is built on. Our article on silicon (Si) covers its properties in more detail.
Sodium silicate formula: Na₂SiO₃ and Na₂O·xSiO₂
The formula you will see most often is Na₂SiO₃, called sodium metasilicate. Other members of the family include sodium orthosilicate (Na₄SiO₄) and sodium pyrosilicate (Na₆Si₂O₇). They differ in how much sodium oxide is present compared with silicon dioxide.
| Name | Formula | Moles of Na₂O : SiO₂ |
|---|---|---|
| Sodium metasilicate | Na₂SiO₃ | 1 : 1 |
| Sodium orthosilicate | Na₄SiO₄ | 2 : 1 |
| Sodium pyrosilicate | Na₆Si₂O₇ | 3 : 2 |
The general formula (Na₂O)ₓ·(SiO₂)ᵧ is a way of describing the mixture of oxides that the compound is built from. It does not mean that separate Na₂O and SiO₂ units are present. The sodium is present as Na⁺ ions, and the silicon is bonded to oxygen in the anion.
The structure of the anion is worth knowing. In anhydrous sodium metasilicate, the anion is a polymer: SiO₄ tetrahedra joined corner to corner into chains. It is not a collection of separate SiO₃²⁻ ions, even though the formula is written that way. The hydrates are different. In the hydrates, discrete SiO₂(OH)₂²⁻ ions are present, and the pentahydrate is written Na₂SiO₂(OH)₂·4H₂O.
The silicon–oxygen bond is very strong. That is also why silicon dioxide, the main component of sand and quartz, is a giant covalent structure with a very high melting point. Our overview of silicon, silicone and silica explains how these three similar-sounding names differ.
Calculate the relative formula mass (Mr) of Na₂SiO₃. (Na = 23, Si = 28, O = 16) (2 marks)
Mr = (2 × 23) + 28 + (3 × 16) = 46 + 28 + 48 = 122.
Weight ratio and modulus
Suppliers do not usually quote a formula. They describe a grade of sodium silicate by its weight ratio, which is the mass of SiO₂ divided by the mass of Na₂O. Commercial grades run from about 1 : 2 up to 3.75 : 1. Multiplying the weight ratio by 1.032 gives the molar ratio, also called the modulus. The factor 1.032 comes from the relative formula masses: Na₂O is about 61.98 and SiO₂ is about 60.08.
Grades with a weight ratio below 2.85 : 1 are called alkaline grades. Grades above that are called neutral grades. They are not literally neutral, since their solutions are still alkaline, but they contain more silica and less alkali. A typical liquid grade contains roughly 30 % silica and 9 % sodium oxide, with the rest being water. Try those numbers in the calculator below, then change them to see how the ratio moves.
How sodium silicate is made
The raw materials are cheap and plentiful: silica sand, plus a source of sodium. There are two industrial routes. The second is the simpler one to picture, and the first is the one that makes the high-silica grades.
The furnace (fusion) route
In the conventional route, high-purity sodium carbonate (soda ash) and silica sand are fused in a furnace at around 1,300–1,500 °C. The product is a solid glass. The silica effectively dissolves in the molten sodium carbonate, and carbon dioxide is given off. The liquid form is then made by dissolving that glass in steam and hot water.
Na₂CO₃ + SiO₂ → Na₂SiO₃ + CO₂
Silica on its own melts at 1,713 °C, and sodium carbonate melts at about 851 °C, so the mixture needs a very hot furnace to react fully.
The hydrothermal (wet) route
In the wet route, quartz sand, sodium hydroxide (caustic soda) and water are heated with steam in a reactor. Silica is an acidic oxide, so it reacts with the alkali.
2NaOH + SiO₂ → Na₂SiO₃ + H₂O
A simple boiling process of this kind cannot reach the highest silica ratios. The furnace route is what makes the silica-rich grades.
Alkali metal silicates are made on a large scale. About 4 million tonnes of alkali metal silicates were produced in 1990. Sand is the starting point for many industries. For a very different process that begins with sand, see our article on the Acheson process and silicon carbide production.
Physical properties of sodium silicate
The physical properties of sodium silicate depend on the grade, but a few features are shared by the whole family. The solids are white powders or colourless, glassy lumps. Most sodium silicates dissolve readily in water. The exceptions are the most silica-rich forms. Sodium metasilicate is the best-documented example, so the table below uses it.
| Property | Sodium metasilicate, Na₂SiO₃ |
|---|---|
| Relative formula mass | about 122 |
| Appearance | Colourless transparent solid or white powder |
| Melting point | 1,088 °C |
| Density | 2.61 g/cm³ |
| Solubility in water | 22.2 g per 100 mL at 25 °C; 160.6 g per 100 mL at 80 °C |
| Solubility in alcohol | Insoluble |
| Water uptake | Hygroscopic (absorbs moisture from the air) |
The solubility figures show a strong temperature effect: more than seven times as much dissolves at 80 °C as at 25 °C. That is useful when you want to make a concentrated solution.
The high melting point fits the bonding. Sodium silicate contains Na⁺ ions and a silicate anion held together by strong silicon–oxygen bonds, so a lot of energy is needed to melt it. If you are revising this idea, our guide to the properties of ionic and covalent compounds explains how bonding controls melting point and solubility.
Concentrated solutions are thick and syrupy, and the viscosity rises as the concentration rises. When the water evaporates, the solution dries to a hard, glassy film. That film can be rehydrated by water. When heated, sodium silicate gives a hard, translucent glassy material that is reported to survive very high temperatures.
Liquid sodium silicate (water glass solution)
Liquid sodium silicate is the form most people buy. A typical solution contains 25–50 % sodium silicate in water. It is colourless when pure, though a green or blue tint can come from iron impurities. It mixes with water in all proportions.
Four properties make it so useful:
- It is syrupy. It coats and wets surfaces well, so it spreads easily as an adhesive or sealer.
- It dries to a hard film. Evaporation leaves a glass-like layer that binds particles together.
- It is alkaline. That helps with cleaning and with neutralising acids.
- It reacts with acids. Acid sets it into a gel, which is how it hardens binders and cements.
Ordinary window glass behaves very differently. It is made with lime as well as sand and sodium carbonate, and that stops it dissolving. Our article on the performance and durability of glass covers how glass resists water and weather. Water glass is, in effect, a glass without that protection.
Chemical properties of sodium silicate
The chemical properties of sodium silicate follow from one idea: the silicate ion is the anion of a very weak acid, silicic acid. That single fact explains why the solution is alkaline, why acids set it into a gel, and why carbon dioxide can harden it.
Alkaline solutions
Sodium silicate solutions are alkaline. The ILO safety card describes a 25–50 % solution as a “medium strong base”. A dilute solution of a typical liquid grade is about pH 11. Silicate ions react with water and produce hydroxide ions. In simplified form:
SiO₃²⁻ + H₂O ⇌ HSiO₃⁻ + OH⁻
The hydroxide ions make the solution alkaline. Universal indicator would turn blue or purple. For the general ideas of pH and bases, see acids and bases explained.
Explain why sodium silicate solution turns universal indicator blue/purple. (2 marks)
Silicate ions react with water to form hydroxide ions. The hydroxide ions make the solution alkaline (pH above 7, about 11), so universal indicator turns blue or purple.
Reaction with acids: silica gel
When acid is added to sodium silicate solution, the silicate ions form silicic acids. These break down into hydrated silica, a white gel. In simplified form, with hydrochloric acid:
Na₂SiO₃ + 2HCl → 2NaCl + H₂SiO₃
The salt, sodium chloride, stays dissolved. The silicic acid forms a white gel, which is essentially hydrated SiO₂. Dried carefully, this gel becomes silica gel, the beads you find in small packets of desiccant. This is a neutralisation reaction, so it fits neatly into the GCSE pattern of acid + base → salt.
Write a balanced equation for sodium metasilicate reacting with hydrochloric acid. (2 marks)
Na₂SiO₃ + 2HCl → 2NaCl + H₂SiO₃
Setting with carbon dioxide
Carbon dioxide is an acidic oxide, so it can do the same job as an acid. Bubbling the gas through sodium silicate solution, or passing it over a silicate-bonded sand mould, sets the silicate into silica gel. In simplified form:
Na₂SiO₃ + CO₂ → Na₂CO₃ + SiO₂
This is the basis of the CO₂ process in foundries, described in the next section. For more on the gas itself, see our article on carbon dioxide gas (CO₂).
Reaction with metals such as aluminium and zinc
Because the solution is alkaline, it attacks some metals. Aluminium, tin and zinc are corroded, and hydrogen gas is given off. For this reason, sodium silicate solution should not be stored in aluminium, tin or zinc containers.
Uses of sodium silicate
What is sodium silicate used for? It is a cheap, versatile chemical with a long list of jobs. The main applications are in detergents, paper deinking, water treatment and construction materials. The largest single use is as an adhesive for cardboard. The sections below take the main uses in turn.
Adhesives and cardboard
Sodium silicate is used as a glue, or “cement”, for cardboard and paperboard. It is the largest single application. It is cheap and bonds well as it dries to a hard film, but the joints can crack within a few years. If you are interested in how other glues compare, read about the latest developments in adhesives.
Foundry moulds and the CO₂ process
Metal castings are made by pouring molten metal into a mould made of sand. The sand must hold its shape until the metal has set, so it is mixed with a binder. Sodium silicate is a common binder for moulds and cores.
In the CO₂ process, the sand and silicate mixture is shaped and then gassed with carbon dioxide. The gas sets the silicate into silica gel, and the sand hardens almost immediately. The bond is precipitated silica gel plus dehydrated sodium silicate. The cure needs no heat, and the binder is nitrogen-free, which suits steel castings. Silicate binders can also be hardened with esters or by microwave drying.
Cements, refractories and fire protection
Sodium silicate is used in refractory and insulation boards made with vermiculite or perlite. It is also used in passive fire protection and in high-temperature adhesives. This may seem odd, since a glassy film sounds fragile. Yet the gel it gives on heating is a hard, translucent glassy material that is reported to survive very high temperatures.
A related everyday use is car repair. Sodium silicate is used in head-gasket and cooling-system sealants, which set at about 100–105 °C.
Ceramics: a deflocculant for clay slip
Potters pour liquid clay, called slip, into plaster moulds to make shapes. Slip needs to flow easily, but too much water makes the cast weak and slow to dry. A little sodium silicate acts as a deflocculant. It disperses the clay particles so that they stop clumping, and the slip flows with less water. It is also used for the crackle effect in some pottery.
Sealing concrete
Sodium silicate can be used to treat concrete and masonry. It reacts with calcium hydroxide, Ca(OH)₂, in the concrete to form calcium silicate hydrate gel. The gel reduces porosity and the penetration of water. The treatment is usually applied after about seven days of curing.
The same idea has been used on a large scale. In April 2011, sodium silicate was injected into the ground at Fukushima Daiichi to harden the soil and stop the leakage of radioactive water. For other products used in building and repair, see our guide to hardware, lubricants, sealants and adhesives.
Detergents and water treatment
Sodium silicate is a builder in laundry and dishwasher products. Its alkalinity helps cleaning, and it helps control corrosion, which protects machine metal. In waste-water plants it is used as a coagulant or flocculant to clump fine particles together. Other uses include fixing reactive dyes on cotton, clarifying wine and beer, and stabilising borehole walls in swelling clays in drilling fluids. It is also the starting material for silica gel. Many of these jobs rely on salts, acids and bases working together, as in our article on the uses of acids, bases and salts.
Preserving eggs: a historical use
Before refrigerators were common, water glass solution was used to store eggs. It seals the pores of the shell, which keeps the egg fresh for months. The method was widely used in the UK during the Second World War, when households kept hens but had no fridges.
The chemical garden experiment
Sodium silicate solution produces one of the most striking demonstrations in chemistry. Drop crystals of a coloured metal salt into the solution and tubes begin to grow upwards like plants. The effect was first described by Johann Rudolf Glauber in 1646, who used iron(II) chloride in potassium silicate.
How the tubes grow
- The crystal starts to dissolve. The metal ions meet silicate ions and form an insoluble metal silicate skin around the crystal.
- The skin acts as a semi-permeable membrane. Water enters by osmosis, because the solution inside is more concentrated.
- Pressure builds up until the skin bursts. Metal ions escape, meet more silicate and form new solid at the break.
- The liquid inside is lighter than the surrounding solution, so it rises, and the tube grows upwards.
For copper(II) sulfate, the skin-forming reaction can be written in simplified form as:
CuSO₄ + Na₂SiO₃ → CuSiO₃ + Na₂SO₄
Colours of the growths
| Salt | Colour of growth |
|---|---|
| Copper(II) sulfate | Blue |
| Iron(II) sulfate | Green |
| Iron(III) chloride | Orange |
| Cobalt(II) chloride | Purple |
| Nickel(II) sulfate | Green |
| Manganese(II) chloride | Pink |
| Calcium chloride, zinc sulfate or alum | White |
Method checklist (teacher-led or adult-supervised)
- Put on goggles and gloves.
- Dilute the water glass with water in a clean glass jar or beaker (never aluminium).
- Drop in a few crystals of a coloured salt, such as copper(II) sulfate or an iron salt. Do not stir.
- Leave the jar undisturbed and watch the tubes grow over several hours.
- Record the colours you see.
- Dispose of everything as instructed, then wash your hands.
Advantages and limitations of sodium silicates
Sodium silicate is popular because it is cheap and does a lot of jobs, but it is not perfect. This table sets the strengths against the drawbacks.
| Advantages | Limitations |
|---|---|
| Made from cheap, common raw materials (sand and a sodium compound) | Strongly alkaline, so it can irritate or burn skin and eyes |
| A strong binder and adhesive when it dries | The dried film can crack, as cardboard joints may after a few years |
| Sets without heat when gassed with CO₂ | The dried material can be rehydrated, so it can re-soften in water |
| Used in fire protection and high-temperature jobs | Attacks aluminium, tin and zinc |
| Disperses clay and helps detergents work | Needs care in storage and handling |
Is sodium silicate dangerous? Safety and handling
Sodium silicate is corrosive to skin and eyes, and can be harmful if swallowed. Under the EU harmonised classification, sodium metasilicate carries the signal word Danger. The hazard statements are H314 (causes severe skin burns and eye damage) and H335 (may cause respiratory irritation). Other suppliers’ solutions are labelled only “Warning” or irritant, so the hazard depends on the grade and concentration. Always read the label.
A sensible handling routine is simple:
- Wear goggles, gloves and a lab coat.
- Keep it away from aluminium, tin and zinc.
- Never mix it carelessly with acids, because a gel forms and heat can be produced.
- Keep containers sealed and clearly labelled.
The reason for the corrosive behaviour is the same as the reason for the high pH. The solution is corrosive because silicate ions react with water (hydrolysis) to give hydroxide ions, which makes the solution alkaline, at about pH 11. For the wider pattern of Group 1 chemistry, see patterns seen in the periodic table.
Sodium silicate in GCSE and A-level chemistry
Sodium silicate is a handy case study because it ties together many topics. Here is where it appears:
- Alkalis and pH: a dilute solution is about pH 11.
- Neutralisation and salts: acid + silicate → salt + silicic acid gel.
- Acidic oxides: CO₂ and SiO₂ both react with bases.
- Giant covalent structures: silicon dioxide is the raw material.
- Soda-lime glass: AQA specification 4.10.3.3 — made by heating sand, sodium carbonate and limestone.
- Calculations: relative formula mass of Na₂SiO₃ is 122.
- A-level: hydrolysis of the conjugate base of a weak acid, and SiO₄ tetrahedra.
To see how titration works, read our guide to acid–base titration.
Frequently asked questions about sodium silicate
What is sodium silicate?
Sodium silicate is a family of compounds made of sodium, silicon and oxygen. The simplest is sodium metasilicate, Na₂SiO₃. Pure forms are colourless solids or white powders, and they also exist as syrupy solutions called water glass. They are used in adhesives, detergents, foundries, ceramics and water treatment.
Why is sodium silicate called water glass?
It is called water glass because many sodium silicates dissolve in water to give a clear, glass-like liquid. When the water evaporates, the solution leaves a hard, glassy film. The name describes a glass that can dissolve, unlike ordinary soda-lime glass, which contains lime and does not dissolve in water.
What is the chemical formula of sodium silicate?
There is no single formula. Sodium metasilicate is Na₂SiO₃, sodium orthosilicate is Na₄SiO₄ and sodium pyrosilicate is Na₆Si₂O₇. The family is described by the general formula (Na₂O)ₓ·(SiO₂)ᵧ. Commercial products are graded by the SiO₂ to Na₂O weight ratio, which runs from about 1 : 2 up to 3.75 : 1.
How is sodium silicate made?
The conventional route fuses sand and sodium carbonate at around 1,300–1,500 °C to make a glass, which is then dissolved with steam and hot water. The second route heats sand with sodium hydroxide solution and steam in a reactor. Both give sodium silicate, and carbon dioxide or water is a by-product.
Is sodium silicate solution acidic or alkaline?
It is alkaline. Silicate ions come from a very weak acid, so they react with water and produce hydroxide ions. A dilute solution of a typical liquid grade is about pH 11, and a safety card describes concentrated solutions as a medium strong base. Alkaline grades with more Na₂O are more alkaline still.
What happens when you add acid to sodium silicate?
The acid reacts with the silicate to form silicic acid, which breaks down into a white gel of hydrated silica. A salt is also made. For example, Na₂SiO₃ + 2HCl → 2NaCl + H₂SiO₃. This is how silica gel is made, and it is also how silicate binders harden.
What is sodium silicate used for?
The main uses are detergents, paper deinking, water treatment and construction materials. The largest single use is as an adhesive for cardboard. It also binds foundry sand moulds, acts as a deflocculant in ceramic slip, treats concrete, gives fire protection and is the starting material for silica gel.
Is sodium silicate dangerous to touch?
It can be. Sodium metasilicate is classified as causing severe skin burns and eye damage (H314), and some solutions are similarly corrosive. The hazard depends on the grade and concentration. Wear gloves and goggles, rinse skin with plenty of water for at least 15 minutes if splashed, and seek medical attention.
How does sodium silicate harden?
It hardens in three ways. Drying removes the water and leaves a glassy film. Acids neutralise the alkalinity and turn it into silica gel. Carbon dioxide, an acidic oxide, does the same job: gassing silicate-bonded sand with CO₂ hardens it almost immediately, without any heat.
