Borax dissolves in water, but not as readily as many people expect. At room temperature (20 °C) a saturated solution holds only about 4.7% borax by weight, roughly 49 g of borax per litre of water. Warm the water and solubility climbs steeply: at 50 °C it is around four times higher, and near boiling water will take up more than its own weight. That temperature curve explains most of the problems people meet when they make up a borax solution, from undissolved grit at the bottom of the beaker to crystals that appear overnight. This guide gives the solubility figures, shows how to use them, and sets out how to handle borax powder safely while you do.
New to the chemistry? Our guide What Is Borax? Sodium Tetraborate Explained covers the basics. This article focuses on one practical question: how borax behaves in water.
Borax solubility chart: 0 °C to 100 °C
The table below shows how much borax a saturated solution contains at different water temperatures. The first column of figures is the published value, given as a percentage by weight of the saturated solution (source: Borax, Wikipedia solubility table). The second column is our own conversion into grams of borax per kilogram of water, which is close to grams per litre. The values are rounded and are a guide, not a specification.
| Water temperature | Borax in saturated solution (% w/w) | Approx. g of borax per kg (≈ litre) of water |
|---|---|---|
| 0 °C | 1.99% | 20 g |
| 10 °C | 3.09% | 32 g |
| 20 °C | 4.70% | 49 g |
| 30 °C | 7.20% | 78 g |
| 40 °C | 11.22% | 126 g |
| 50 °C | 17.91% | 218 g |
| 60 °C | 30.32% | 435 g |
| 80 °C | 44.31% | 796 g |
| 100 °C | 65.63% | about 1,900 g |
Published figures vary a little between sources, depending on the hydrate measured and the method used. For critical work, use the data sheet supplied with your batch. The trend is the same everywhere, though: borax is sparingly soluble in cold water and highly soluble in hot water.
How to read the chart
Percent by weight is not grams per litre
A figure of 4.70% means 4.70 g of borax in every 100 g of finished solution, not in 100 g of water. At low concentrations the difference is small. At high temperatures it becomes large, which is why the 100 °C row converts to far more than 656 g per litre. If a method tells you to make a “5% borax solution”, check whether it means weight of borax per weight of solution, or per volume of water.
Which form of borax are you weighing?
Standard borax is the decahydrate, Na₂B₄O₇·10H₂O, and almost half of its weight is water of crystallisation. The pentahydrate and anhydrous forms contain much less water, so the same weight carries more borate. If your method was written for one form and you hold another, the amounts will not match. Our checklist Buying Borax in the UK: How to Read the Label, SDS and Spec Sheet shows how to confirm the form from the CAS number.
Why the curve bends so sharply
Between 0 °C and 20 °C solubility a little more than doubles. Between 40 °C and 60 °C it nearly triples. Borax is also unusual because the solid that settles out of the solution changes with temperature: the pentahydrate crystallises from solutions above about 60.8 °C, while the decahydrate forms below that point. For workshop purposes the lesson is simple. Small changes in temperature make a big difference to how much borax stays in solution.
Dissolving borax powder: a sensible workshop method
Professionals dissolve borax for many reasons: preparing a flux paste for soldering, making up a borate buffer in the laboratory, or mixing a solution for a timber or conservation process under their own method statement. Whatever the purpose, the same handling points apply.
- Work out the target strength first. Decide the temperature the solution will be used and stored at, then check the chart. Stay comfortably below saturation at the lowest temperature it will see.
- Use dedicated equipment. Glass, stainless steel or polypropylene vessels kept for chemical work only. Never use kitchen utensils or food and drink containers.
- Add powder to water, not water to powder. Start with warm water and add the borax gradually while stirring. This limits clumping and keeps dust down.
- Warm gently if needed. Heating speeds things up, but avoid vigorous boiling, which splashes hot alkaline solution.
- Check that it is fully dissolved. Undissolved grains mean either the water is too cold or the solution is already saturated.
- Label the container. Write the substance name, strength, date and hazard information on it, and store it with your other chemicals.
Common problems and what causes them
Crystals forming as the solution cools
This is the most common issue. A solution made close to saturation in hot water will deposit crystals once it cools. For example, water at 50 °C can hold roughly 218 g per litre, but at 20 °C it can only hold about 49 g. Most of the difference comes back out as crystals on the sides and bottom of the vessel. The fix is to make the solution weaker, or to keep it at the temperature you made it at while you use it. Rewarming will normally redissolve the crystals.
Grit that will not dissolve in cold water
Cold tap water in a British winter can be well under 10 °C, where solubility is only around 3%. If powder sits at the bottom, the water is probably already saturated. Warm it or increase the volume of water.
Caked or lumpy powder
According to the producer U.S. Borax, borax decahydrate has the greatest tendency to cake of all the sodium borates, and humid air is the usual cause. Caking does not change the chemistry. Break lumps up inside the closed bag rather than in the open, so you do not create dust, and keep the pack sealed between uses.
Powder that seems to have lost weight
The same producer notes that borax decahydrate slowly loses water of crystallisation in a warm, dry atmosphere, and starts to lose water at about 62 °C in an enclosed space. Stored next to a radiator or kiln, a pack can gradually change composition. Keep it in a cool, dry place with a stable temperature.
Borax solution or borax substitute solution?
A solution of real borax and a solution of a borax substitute are not the same thing. Borax is sodium tetraborate. Our Borax Substitute Powder is a boron-free blend of sodium carbonate and sodium bicarbonate made for household laundry and cleaning, so the borax solubility chart above does not apply to it. If a method calls for borate chemistry, for example a borate buffer or a borax flux, only sodium tetraborate will do. If you need an everyday cleaning product for the home, choose the substitute.
Safety: handling borax and borax solutions
In Great Britain sodium tetraborate is classified as toxic for reproduction, category 1B, with the hazard statement H360FD: may damage fertility; may damage the unborn child. Dissolving borax does not remove that hazard. The solution is still borax. We supply borax for professional, industrial and laboratory use only. It is not a household product, it is not for making slime, and it must never be swallowed or used in anything people or animals eat or drink.
- Read the Safety Data Sheet first. It is supplied with every order and is the authoritative source for your batch.
- Wear protection: chemical-resistant gloves and eye protection when weighing powder or handling solutions, especially hot ones.
- Do not breathe the dust. Weigh and pour gently, ideally with local ventilation, and do not dry-sweep spills. Pick them up so that you do not raise dust.
- Do not swallow. No eating, drinking or smoking where borax is handled, and wash your hands afterwards.
- Keep out of reach of children and away from food, drink and animal feed.
- Store in the original, labelled packaging, sealed, cool and dry. Solutions go in clearly labelled chemical containers, never in drinks bottles.
- Pregnancy and fertility: because of the H360FD classification, workplaces must include borax in their COSHH assessment and consider new and expectant mothers. The Health and Safety Executive’s COSHH guidance explains employers’ duties.
- Disposal: follow section 13 of the SDS and local rules. Do not pour concentrated solutions into drains unless your SDS and waste arrangements allow it.
Buying borax powder in the UK
Our Borax Decahydrate Powder 99.9% is a free-flowing technical grade powder in 900g and 3kg resealable packs, supplied with a Safety Data Sheet and delivered across the UK for professional users. Compare grades and pack sizes on our Borax Powder UK page.
Borax solubility FAQ
How much borax dissolves in cold water?
Very little. A saturated solution holds about 2% borax by weight at 0 °C and about 4.7% at 20 °C, which is roughly 49 g of borax per litre of water at room temperature.
Does borax dissolve better in hot water?
Yes. Solubility rises steeply with temperature, to about 18% by weight at 50 °C and over 65% at 100 °C. Warm water dissolves borax powder much faster and holds much more of it.
Why do crystals form in my borax solution?
The solution was made stronger than cold water can hold. As it cools, the excess borax crystallises out. Make a weaker solution, or keep it at the temperature it was made at; rewarming normally redissolves the crystals.
Does dissolving borax make it safer?
No. A borax solution carries the same Repr. 1B, H360FD hazard as the powder. Wear gloves and eye protection, label the container, keep it away from children and food, and never swallow it.
Does the solubility chart apply to borax substitute?
No. Borax substitute is a boron-free blend of sodium carbonate and sodium bicarbonate, so it dissolves differently. The chart applies to sodium tetraborate only.